Microbiota

From Wikipedia, the free encyclopedia

The predominant bacteria on human skin

A microbiota is an "ecological community of commensal, symbiotic and pathogenic microorganisms" found in and on all multicellular organisms studied to date from plants to animals. A microbiota includes bacteria, archaea, protists, fungi and viruses. Microbiota have been found to be crucial for immunologic, hormonal and metabolic homeostasis of their host. The synonymous term microbiome describes either the collective genomes of the microorganisms that reside in an environmental niche or the microorganisms themselves.

The microbiome and host emerged during evolution as a synergistic unit from epigenetics and genetic characteristics, sometimes collectively referred to as a holobiont.

Introduction

All plants and animals, from simple life forms to humans, live in close association with microbial organisms. Several advances have driven the perception of microbiomes, including:

• the ability to perform genomic and gene expression analyses of single cells and of entire microbial communities in the disciplines of metagenomics and metatranscriptomics
• databases accessible to researchers across multiple disciplines
• methods of mathematical analysis suitable for complex data sets

Biologists have come to appreciate that microbes make up an important part of an organism's phenotype, far beyond the occasional symbiotic case study.

Types of host relationships

Commensalism, a concept developed by Pierre-Joseph van Beneden (1809-1894), a Belgian professor at the University of Louvain during the nineteenth century is central to the microbiome, where microbiota colonize a host in a non-harmful coexistence. The relationship with their host is called mutualistic when organisms perform tasks that are known to be useful for the host, parasitic, when disadvantageous to the host. Other authors define a situation as mutualistic where both benefit, and commensal, where the unaffected host benefits the symbiont. A nutrient exchange may be bidirectional or unidirectional, may be context dependent and may occur in diverse ways. Microbiota that are expected to be present, and that under normal circumstances do not cause disease, are deemed normal flora or normal microbiota.

Acquisition and change

The initial acquisition of microbiota in animals from mammalians to marine sponges is at birth, and may even occur through the germ cell line. In plants, the colonizing process can be initiated below ground in the root zone, around the germinating seed, the spermosphere, or originate from the above ground parts, the phyllosphere and the flower zone or anthosphere. The stability of the rhizosphere microbiota over generations depends upon the plant type but even more on the soil composition, i.e. living and non living environment.

Microbiota by host

Consensus exists among evolutionary biologists that one should not separate an organism's genes from the context of its resident microbes.

Humans

The human microbiota includes bacteria, fungi, archaea and viruses. Micro-animals which live on the human body are excluded. The human microbiome refers to their genomes.

Humans are colonized by many microorganisms; the traditional estimate was that humans live with ten times more non-human cells than human cells; more recent estimates have lowered this to 3:1 and even to about 1:1.

The Human Microbiome Project sequenced the genome of the human microbiota, focusing particularly on the microbiota that normally inhabit the skin, mouth, nose, digestive tract, and vagina. It reached a milestone in 2012 when it published initial results.

Non-human animals

• Amphibians have microbiota on their skin. Some species are able to carry a fungus named Batrachochytrium dendrobatidis, which in others can cause a deadly infection Chytridiomycosis depending on their microbiome, resisting pathogen colonization or inhibiting their growth with antimicrobial skin peptides.
• In mammals, herbivores such as cattle depend on their rumen microbiome to convert cellulose into proteins, short chain fatty acids, and gases. Culture methods cannot provide information on all microorganisms present. Comparative metagenomic studies yielded the surprising result that individual cattle possess markedly different community structures, predicted phenotype, and metabolic potentials, even though they were fed identical diets, were housed together, and were apparently functionally identical in their utilization of plant cell wall resources.
• Mice have become the most studied mammalian regarding their microbiomes. The gut microbiota have been studied in relation to allergic airway disease, obesity, gastrointestinal diseases and diabetes. Perinatal shifting of microbiota through low dose antibiotics can have long-lasting effects on future susceptibility to allergic airway disease. The frequency of certain subsets of microbes has been linked to disease severity. The presence of specific microbes early in postnatal life, instruct future immune responses. In gnotobiotic mice certain gut bacteria were found to transmit a particular phenotype to recipient germ-free mice, that promoted accumulation of colonic regulatory T cells, and strains that modulated mouse adiposity and cecal metabolite concentrations. This combinatorial approach enables a systems-level understanding of microbial contributions to human biology. But also other mucoide tissues as lung and vagina have been studied in relation to diseases such as asthma, allergy and vaginosis.
• Insects have their own microbiomes. For example, leaf-cutter ants form huge underground colonies harvesting hundreds of kilograms of leaves each year and are unable to digest the cellulose in the leaves directly. They maintain fungus gardens as the colony's primary food source. While the fungus itself does not digest cellulose, a microbial community containing a diversity of bacteria is doing so. Analysis of the microbial population's genome revealed many genes with a role in cellulose digestion. This microbiome's predicted carbohydrate-degrading enzyme profile is similar to that of the bovine rumen, but the species composition is almost entirely different. Gut microbiota of the fruit fly can affect the way its gut looks, by impacting epithelial renewal rate, cellular spacing, and the composition of different cell types in the epithelium. When the moth Spodoptera exigua is infected with baculovirus immune-related genes are downregulated and the amount of its gut microbiota increases.

Plants

Light micrograph of a cross section of a coralloid root of a cycad, showing the layer that hosts symbiotic cyanobacteria

• Plants are attractive hosts for microorganisms since they provide a variety of nutrients. Microorganisms on plants can be epiphytes (found on the plants) or endophytes (found inside plant tissue). Oomycetes and fungi have, through convergent evolution, developed similar morphology and occupy similar ecological niches. They develop hyphae, threadlike structures that penetrate the host cell. In mutualistic situations the plant often exchanges hexose sugars for inorganic phosphate from the fungal symbiont. It is speculated that such very ancient associations have aided plants when they first colonized land. Plant-growth promoting bacteria (PGPB) provide the plant with essential services such as nitrogen fixation, solubilization of minerals such as phosphorus, synthesis of plant hormones, direct enhancement of mineral uptake, and protection from pathogens. PGPBs may protect plants from pathogens by competing with the pathogen for an ecological niche or a substrate, producing inhibitory allelochemicals, or inducing systemic resistance in host plants to the pathogen

Functions

Microbiota have been found to be crucial for immunologic, hormonal and metabolic homeostasis of their host.

Immune system

The symbiotic relationship between a host and its microbiota shapes the immune system of mammalians, insects, plants and aquatic organisms. In many animals, the immune system and microbiota engage in "cross-talk", exchanging chemical signals. This allows the immune system to recognize the types of bacteria that are harmful to the host and combat them, while allowing the helpful bacteria to carry out their functions; in turn, the microbiota influence immune reactivity and targeting. Bacteria can be transferred from mother to child through direct contact and after birth, or through indirect contact through eggs, coprophagy, and several other pathways. As the infant microbiome is established, commensal bacteria quickly populate the gut, prompting a range of immune responses and "programming" the immune system with long-lasting effects. The bacteria are able to stimulate lymphoid tissue associated with the gut mucosa, which enables the tissue to produce antibodies for pathogens that may enter the gut.

The human microbiome plays a role in the activation of toll-like receptors (TLRs) in the intestines, a type of pattern recognition receptor host cells use to recognize dangers and repair damage. Pathogens can influence this coexistence leading to immune dysregulation including and susceptibility to diseases, immune tolerance and autoimmune diseases.

Endocrine system

Intestinal microbiota can interact with thyroid-related micronutrients and the metabolism of endogenous iodothyronines such as Triiodothyronine and exogenous iodothyronines, which affects even phyla without thyroid follicles, such as jellyfish, insects, and sea urchins, as iodothyronine-induced metamorphosis is an ancestral feature of all chordates.

Microbiota can affect a fruit fly´s sex preference in mating. Infecting flies with pure cultures of Lactobacillus plantarum established a certain mating preference as L. plantarum can change the fly´s levels of cuticular hydrocarbon sex pheromones.

Metabolism

When adult germ-free mice are colonized with the gut flora of obese mice, they can gain weight dramatically with an increased metabolism of monosaccharides and short-chain fatty acids. The gut flora of obese mice has less Bacteroidetes than Firmicutes and is thought to be more efficient at extracting energy from food.

Co-evolution of microbiota

Bleached branching coral (foreground) and normal branching coral (background). Keppel Islands, Great Barrier Reef

Organisms evolve within eco-systems so that the change of one organism affects the change of others. Co-evolution (also called "hologenome theory") proposes that an object of natural selection is not the individual organism, but the organism together with its associated organisms, including its microbial communities.

Coral reefs. The hologenome theory originated in studies on coral reefs. Coral reefs are the largest structures created by living organisms, and contain abundant and highly complex microbial communities. Over the past several decades, major declines in coral populations have occurred. Climate change, water pollution and over-fishing are three stress factors that have been described as leading to disease susceptibility. Over twenty different coral diseases have been described, but of these, only a handful have had their causative agents isolated and characterized. Coral bleaching is the most serious of these diseases. In the Mediterranean Sea, the bleaching of Oculina patagonica was first described in 1994 and shortly determined to be due to infection by Vibrio shiloi. From 1994 to 2002, bacterial bleaching of O. patagonica occurred every summer in the eastern Mediterranean. Surprisingly, however, after 2003, O. patagonica in the eastern Mediterranean has been resistant to V. shiloi infection, although other diseases still cause bleaching. The surprise stems from the knowledge that corals are long lived, with lifespans on the order of decades, and do not have adaptive immune systems. Their innate immune systems do not produce antibodies, and they should seemingly not be able to respond to new challenges except over evolutionary time scales.

The puzzle of how corals managed to acquire resistance to a specific pathogen led to a 2007 proposal, that a dynamic relationship exists between corals and their symbiotic microbial communities. It is thought that by altering its composition, the holobiont can adapt to changing environmental conditions far more rapidly than by genetic mutation and selection alone. Extrapolating this hypothesis to other organisms, including higher plants and animals, led to the proposal of the "hologenome theory of evolution".

As of 2007 the hologenome theory was still being debated. A major criticism has been the claim that V. shiloi was misidentified as the causative agent of coral bleaching, and that its presence in bleached O. patagonica was simply that of opportunistic colonization. If this is true, the basic observation leading to the theory would be invalid. The theory has gained significant popularity as a way of explaining rapid changes in adaptation that cannot otherwise be explained by traditional mechanisms of natural selection. Within the hologenome theory, the holobiont has not only become the principal unit of natural selection but also the result of other step of integration that it is also observed at the cell (symbiogenesis, endosymbiosis) and genomic levels.

Research methods

Targeted amplicon sequencing

Targeted amplicon sequencing relies on having some expectations about the composition of the community that is being studied. In target amplicon sequencing a phylogenetically informative marker is targeted for sequencing. Such a marker should be present in ideally all the expected organisms. It should also evolve in such a way that it is conserved enough that primers can target genes from a wide range of organisms while evolving quickly enough to allow for finer resolution at the taxonomic level. A common marker for human microbiome studies is the gene for bacterial 16S rRNA (i.e. "16S rDNA", the sequence of DNA which encodes the ribosomal RNA molecule). Since ribosomes are present in all living organisms, using 16S rDNA allows for DNA to be amplified from many more organisms than if another marker were used. The 16S rDNA gene contains both slowly evolving regions and fast evolving regions; the former can be used to design broad primers while the latter allow for finer taxonomic distinction. However, species-level resolution is not typically possible using the 16S rDNA. Primer selection is an important step, as anything that cannot be targeted by the primer will not be amplified and thus will not be detected. Different sets of primers have been shown to amplify different taxonomic groups due to sequence variation.

Targeted studies of eukaryotic and viral communities are limited and subject to the challenge of excluding host DNA from amplification and the reduced eukaryotic and viral biomass in the human microbiome.

After the amplicons are sequenced, molecular phylogenetic methods are used to infer the composition of the microbial community. This is done by clustering the amplicons into operational taxonomic units (OTUs) and inferring phylogenetic relationships between the sequences. Due to the complexity of the data, distance measures such as UniFrac distances are usually defined between microbiome samples, and downstream multivariate methods are carried out on the distance matrices. An important point is that the scale of data is extensive, and further approaches must be taken to identify patterns from the available information. Tools used to analyze the data include VAMPS, QIIME and mothur.

Metagenomic sequencing

Metagenomics is also used extensively for studying microbial communities. In metagenomic sequencing, DNA is recovered directly from environmental samples in an untargeted manner with the goal of obtaining an unbiased sample from all genes of all members of the community. Recent studies use shotgun Sanger sequencing or pyrosequencing to recover the sequences of the reads. The reads can then be assembled into contigs. To determine the phylogenetic identity of a sequence, it is compared to available full genome sequences using methods such as BLAST. One drawback of this approach is that many members of microbial communities do not have a representative sequenced genome.

Despite the fact that metagenomics is limited by the availability of reference sequences, one significant advantage of metagenomics over targeted amplicon sequencing is that metagenomics data can elucidate the functional potential of the community DNA. Targeted gene surveys cannot do this as they only reveal the phylogenetic relationship between the same gene from different organisms. Functional analysis is done by comparing the recovered sequences to databases of metagenomic annotations such as KEGG. The metabolic pathways that these genes are involved in can then be predicted with tools such as MG-RAST, CAMERA, and IMG/M.

RNA and protein-based approaches

Metatranscriptomics studies have been performed to study the gene expression of microbial communities through methods such as the pyrosequencing of extracted RNA. Structure based studies have also identified non-coding RNAs (ncRNAs) such as ribozymes from microbiota.  Metaproteomics is an approach that studies the proteins expressed by microbiota, giving insight into its functional potential.

Projects

The Human Microbiome Project launched in 2008 was a United States National Institutes of Health initiative to identify and characterize microorganisms found in both healthy and diseased humans. The five-year project, best characterized as a feasibility study with a budget of $115 million tested how changes in the human microbiome are associated with human health or disease.

The Earth Microbiome Project (EMP) is an initiative to collect natural samples and analyze the microbial community around the globe. Microbes are highly abundant, diverse and have an important role in the ecological system. Yet as of 2010, it was estimated that the total global environmental DNA sequencing effort had produced less than 1 percent of the total DNA found in a liter of seawater or a gram of soil, and the specific interactions between microbes are largely unknown. The EMP aims to process as many as 200,000 samples in different biomes, generating a complete database of microbes on earth to characterize environments and ecosystems by microbial composition and interaction. Using these data, new ecological and evolutionary theories can be proposed and tested.

The Brazilian Microbiome Project aims to assemble a Brazilian Microbiome Consortium/Database. This is the first attempt to collect and collate information about Brazilian microbial genetic and functional diversity in a systematic and holistic manner. New sequence data have been generated from samples collected in all Brazilian regions.

Privacy issues

Microbial DNA inhabiting a person's human body can uniquely identify the person. A person's privacy may be compromised if the person anonymously donated microbe DNA data. Their medical condition and identity could be revealed.

Starch

From Wikipedia, the free encyclopedia

Starch

Identifiers
CAS Number	9005-25-8
ChemSpider	none
ECHA InfoCard	100.029.696
EC Number	232-679-6
RTECS number	GM5090000
Properties
Chemical formula	(C 6H 10O 5) n - (H 2O)
Molar mass	Variable
Appearance	White powder
Density	Variable
Melting point	decomposes
Solubility in water	insoluble
Thermochemistry
Std enthalpy of combustion (ΔcHo298)	4.1788 kilocalories per gram (17.484 kJ/g)
Hazards
Safety data sheet	ICSC 1553
Autoignition temperature	410 °C (770 °F; 683 K)
US health exposure limits (NIOSH):
PEL (Permissible)	TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Structure of the amylose molecule

 

Structure of the amylopectin molecule

Starch or amylum is a polymeric carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants as energy storage. It is the most common carbohydrate in human diets and is contained in large amounts in staple foods like potatoes, wheat, maize (corn), rice, and cassava.

Pure starch is a white, tasteless and odorless powder that is insoluble in cold water or alcohol. It consists of two types of molecules: the linear and helical amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin by weight. Glycogen, the glucose store of animals, is a more highly branched version of amylopectin.

In industry, starch is converted into sugars, for example by malting, and fermented to produce ethanol in the manufacture of beer, whisky and biofuel. It is processed to produce many of the sugars used in processed foods. Mixing most starches in warm water produces a paste, such as wheatpaste, which can be used as a thickening, stiffening or gluing agent. The biggest industrial non-food use of starch is as an adhesive in the papermaking process. Starch can be applied to parts of some garments before ironing, to stiffen them.

Etymology

The word "starch" is from a Germanic root with the meanings "strong, stiff, strengthen, stiffen". Modern German Stärke (strength) is related. The Greek term for starch, "amylon" (ἄμυλον), is also related. It provides the root amyl, which is used as a prefix in biochemistry for several 5-carbon compounds related to or derived from starch (e.g. amyl alcohol).

History

Starch grains from the rhizomes of Typha (cattails, bullrushes) as flour have been identified from grinding stones in Europe dating back to 30,000 years ago. Starch grains from sorghum were found on grind stones in caves in Ngalue, Mozambique dating up to 100,000 years ago.

Pure extracted wheat starch paste was used in Ancient Egypt possibly to glue papyrus. The extraction of starch is first described in the Natural History of Pliny the Elder around AD 77–79. Romans used it also in cosmetic creams, to powder the hair and to thicken sauces. Persians and Indians used it to make dishes similar to gothumai wheat halva. Rice starch as surface treatment of paper has been used in paper production in China since 700 CE.

In addition to starchy plants consumed directly, 66 million tonnes of starch were being produced per year worldwide by 2008. In the EU this was around 8.5 million tonnes, with around 40% being used for industrial applications and 60% for food uses, most of the latter as glucose syrups.

Energy store of plants

Most green plants use starch as their energy store.The extra glucose is changed into starch which is more complex than glucose(by plants). An exception is the family Asteraceae (asters, daisies and sunflowers), where starch is replaced by the fructan inulin. Inulin-like fructans are also present in grasses such as wheat, in onions and garlic, bananas, and asparagus.

In photosynthesis, plants use light energy to produce glucose from carbon dioxide. The glucose is used to generate the chemical energy required for general metabolism, to make organic compounds such as nucleic acids, lipids, proteins and structural polysaccharides such as cellulose, or is stored in the form of starch granules, in amyloplasts. Toward the end of the growing season, starch accumulates in twigs of trees near the buds. Fruit, seeds, rhizomes, and tubers store starch to prepare for the next growing season.

Glucose is soluble in water, hydrophilic, binds with water and then takes up much space and is osmotically active; glucose in the form of starch, on the other hand, is not soluble, therefore osmotically inactive and can be stored much more compactly.

Glucose molecules are bound in starch by the easily hydrolyzed alpha bonds. The same type of bond is found in the animal reserve polysaccharide glycogen. This is in contrast to many structural polysaccharides such as chitin, cellulose and peptidoglycan, which are bound by beta bonds and are much more resistant to hydrolysis.

Biosynthesis

Plants produce starch by first converting glucose 1-phosphate to ADP-glucose using the enzyme glucose-1-phosphate adenylyltransferase. This step requires energy in the form of ATP. The enzyme starch synthase then adds the ADP-glucose via a 1,4-alpha glycosidic bond to a growing chain of glucose residues, liberating ADP and creating amylose. The ADP-glucose is almost certainly added to the non-reducing end of the amylose polymer, as the UDP-glucose is added to the non-reducing end of glycogen during glycogen synthesis.

Starch branching enzyme introduces 1,6-alpha glycosidic bonds between the amylose chains, creating the branched amylopectin. The starch debranching enzyme isoamylase removes some of these branches. Several isoforms of these enzymes exist, leading to a highly complex synthesis process.

Glycogen and amylopectin have similar structure, but the former has about one branch point per ten 1,4-alpha bonds, compared to about one branch point per thirty 1,4-alpha bonds in amylopectin. Amylopectin is synthesized from ADP-glucose while mammals and fungi synthesize glycogen from UDP-glucose; for most cases, bacteria synthesize glycogen from ADP-glucose (analogous to starch).

In addition to starch synthesis in plants, starch can be synthesized from non-food starch mediated by an enzyme cocktail. In this cell-free biosystem, beta-1,4-glycosidic bond-linked cellulose is partially hydrolyzed to cellobiose. Cellobiose phosphorylase cleaves to glucose 1-phosphate and glucose; the other enzyme—potato alpha-glucan phosphorylase can add a glucose unit from glucose 1-phosphorylase to the non-reducing ends of starch. In it, phosphate is internally recycled. The other product, glucose, can be assimilated by a yeast. This cell-free bioprocessing does not need any costly chemical and energy input, can be conducted in aqueous solution, and does not have sugar losses.

Degradation

Starch is synthesized in plant leaves during the day and stored as granules; it serves as an energy source at night. The insoluble, highly branched starch chains have to be phosphorylated in order to be accessible for degrading enzymes. The enzyme glucan, water dikinase (GWD) phosphorylates at the C-6 position of a glucose molecule, close to the chains 1,6-alpha branching bonds. A second enzyme, phosphoglucan, water dikinase (PWD) phosphorylates the glucose molecule at the C-3 position. A loss of these enzymes, for example a loss of the GWD, leads to a starch excess (sex) phenotype, and because starch cannot be phosphorylated, it accumulates in the plastids.

After the phosphorylation, the first degrading enzyme, beta-amylase (BAM) can attack the glucose chain at its non-reducing end. Maltose is released as the main product of starch degradation. If the glucose chain consists of three or fewer molecules, BAM cannot release maltose. A second enzyme, disproportionating enzyme-1 (DPE1), combines two maltotriose molecules. From this chain, a glucose molecule is released. Now, BAM can release another maltose molecule from the remaining chain. This cycle repeats until starch is degraded completely. If BAM comes close to the phosphorylated branching point of the glucose chain, it can no longer release maltose. In order for the phosphorylated chain to be degraded, the enzyme isoamylase (ISA) is required.

The products of starch degradation are predominantly maltose and smaller amounts of glucose. These molecules are exported from the plastid to the cytosol, maltose via the maltose transporter, which if mutated (MEX1-mutant) results in maltose accumulation in the plastid. Glucose is exported via the plastidic glucose translocator (pGlcT). These two sugars act as a precursor for sucrose synthesis.  Sucrose can then be used in the oxidative pentose phosphate pathway in the mitochondria, to generate ATP at night.

Properties

Structure

Starch, 800x magnified, under polarized light, showing characteristic extinction cross

 

Rice starch seen on light microscope. Characteristic for the rice starch is that starch granules have an angular outline and some of them are attached to each other and form larger granules

While amylose was thought to be completely unbranched, it is now known that some of its molecules contain a few branch points. Amylose is a much smaller molecule than amylopectin. About one quarter of the mass of starch granules in plants consist of amylose, although there are about 150 times more amylose than amylopectin molecules.

Starch molecules arrange themselves in the plant in semi-crystalline granules. Each plant species has a unique starch granular size: rice starch is relatively small (about 2 μm) while potato starches have larger granules (up to 100 μm).

Starch becomes soluble in water when heated. The granules swell and burst, the semi-crystalline structure is lost and the smaller amylose molecules start leaching out of the granule, forming a network that holds water and increasing the mixture's viscosity. This process is called starch gelatinization. During cooking, the starch becomes a paste and increases further in viscosity. During cooling or prolonged storage of the paste, the semi-crystalline structure partially recovers and the starch paste thickens, expelling water. This is mainly caused by retrogradation of the amylose. This process is responsible for the hardening of bread or staling, and for the water layer on top of a starch gel (syneresis).

Some cultivated plant varieties have pure amylopectin starch without amylose, known as waxy starches. The most used is waxy maize, others are glutinous rice and waxy potato starch. Waxy starches have less retrogradation, resulting in a more stable paste. High amylose starch, amylomaize, is cultivated for the use of its gel strength and for use as a resistant starch (a starch that resists digestion) in food products.

Synthetic amylose made from cellulose has a well-controlled degree of polymerization. Therefore, it can be used as a potential drug deliver carrier.

Certain starches, when mixed with water, will produce a non-newtonian fluid sometimes nicknamed "oobleck".

Hydrolysis

The enzymes that break down or hydrolyze starch into the constituent sugars are known as amylases.
Alpha-amylases are found in plants and in animals. Human saliva is rich in amylase, and the pancreas also secretes the enzyme. Individuals from populations with a high-starch diet tend to have more amylase genes than those with low-starch diets.

Beta-amylase cuts starch into maltose units. This process is important in the digestion of starch and is also used in brewing, where amylase from the skin of seed grains is responsible for converting starch to maltose (Malting, Mashing).

Given a heat of combustion of glucose of 2,805 kilojoules per mole (670 kcal/mol) whereas that of starch is 2,835 kJ (678 kcal) per mole of glucose monomer, hydrolysis releases about 30 kJ (7.2 kcal) per mole, or 166 J (40 cal) per gram of glucose product.

Dextrinization

If starch is subjected to dry heat, it breaks down to form dextrins, also called "pyrodextrins" in this context. This break down process is known as dextrinization. (Pyro)dextrins are mainly yellow to brown in color and dextrinization is partially responsible for the browning of toasted bread.

Chemical tests

Granules of wheat starch, stained with iodine, photographed through a light microscope

A triiodide (I₃⁻) solution formed by mixing iodine and iodide (usually from potassium iodide) is used to test for starch; a dark blue color indicates the presence of starch. The details of this reaction are not fully known, but recent scientific work using single crystal x-ray crystallography and comparative Raman spectroscopy suggests that the final starch-iodine structure is similar to an infinite polyiodide chain like one found in a pyrroloperylene-iodine complex. The strength of the resulting blue color depends on the amount of amylose present. Waxy starches with little or no amylose present will color red. Benedict's test and Fehling's test is also done to indicate the presence of starch.

Starch indicator solution consisting of water, starch and iodide is often used in redox titrations: in the presence of an oxidizing agent the solution turns blue, in the presence of reducing agent the blue color disappears because triiodide (I₃⁻) ions break up into three iodide ions, disassembling the starch-iodine complex. Starch solution was used as indicator for visualizing the periodic formation and consumption of triiodide intermediate in the Briggs-Rauscher oscillating reaction. The starch, however, changes the kinetics of the reaction steps involving triiodide ion. A 0.3% w/w solution is the standard concentration for a starch indicator. It is made by adding 3 grams of soluble starch to 1 liter of heated water; the solution is cooled before use (starch-iodine complex becomes unstable at temperatures above 35 °C).

Each species of plant has a unique type of starch granules in granular size, shape and crystallization pattern. Under the microscope, starch grains stained with iodine illuminated from behind with polarized light show a distinctive Maltese cross effect (also known as extinction cross and birefringence).

Food

Starch is the most common carbohydrate in the human diet and is contained in many staple foods. The major sources of starch intake worldwide are the cereals (rice, wheat, and maize) and the root vegetables (potatoes and cassava). Many other starchy foods are grown, some only in specific climates, including acorns, arrowroot, arracacha, bananas, barley, breadfruit, buckwheat, canna, colacasia, katakuri, kudzu, malanga, millet, oats, oca, polynesian arrowroot, sago, sorghum, sweet potatoes, rye, taro, chestnuts, water chestnuts and yams, and many kinds of beans, such as favas, lentils, mung beans, peas, and chickpeas.

Widely used prepared foods containing starch are bread, pancakes, cereals, noodles, pasta, porridge and tortilla.

Digestive enzymes have problems digesting crystalline structures. Raw starch is digested poorly in the duodenum and small intestine, while bacterial degradation takes place mainly in the colon. When starch is cooked, the digestibility is increased.

Starch gelatinization during cake baking can be impaired by sugar competing for water, preventing gelatinization and improving texture.

Before the advent of processed foods, people consumed large amounts of uncooked and unprocessed starch-containing plants, which contained high amounts of resistant starch. Microbes within the large intestine fermented the starch, produced short-chain fatty acids, which are used as energy, and support the maintenance and growth of the microbes. More highly processed foods are more easily digested and release more glucose in the small intestine—less starch reaches the large intestine and more energy is absorbed by the body. It is thought that this shift in energy delivery (as a result of eating more processed foods) may be one of the contributing factors to the development of metabolic disorders of modern life, including obesity and diabetes.

Starch industry

The starch industry extracts and refines starches from seeds, roots and tubers, by wet grinding, washing, sieving and drying. Today, the main commercial refined starches are cornstarch, tapioca, arrowroot, and wheat, rice, and potato starches. To a lesser extent, sources of refined starch are sweet potato, sago and mung bean. To this day, starch is extracted from more than 50 types of plants.

Untreated starch requires heat to thicken or gelatinize. When a starch is pre-cooked, it can then be used to thicken instantly in cold water. This is referred to as a pregelatinized starch.

Starch sugars

Starch can be hydrolyzed into simpler carbohydrates by acids, various enzymes, or a combination of the two. The resulting fragments are known as dextrins. The extent of conversion is typically quantified by dextrose equivalent (DE), which is roughly the fraction of the glycosidic bonds in starch that have been broken.

These starch sugars are by far the most common starch based food ingredient and are used as sweeteners in many drinks and foods. They include:

• Maltodextrin, a lightly hydrolyzed (DE 10–20) starch product used as a bland-tasting filler and thickener.
• Various glucose syrups (DE 30–70), also called corn syrups in the US, viscous solutions used as sweeteners and thickeners in many kinds of processed foods.
• Dextrose (DE 100), commercial glucose, prepared by the complete hydrolysis of starch.
• High fructose syrup, made by treating dextrose solutions with the enzyme glucose isomerase, until a substantial fraction of the glucose has been converted to fructose. In the United States sugar prices are two to three times higher than in the rest of the world; high-fructose corn syrup is significantly cheaper, and is the principal sweetener used in processed foods and beverages. Fructose also has better microbiological stability. One kind of high fructose corn syrup, HFCS-55, is sweeter than sucrose because it is made with more fructose, while the sweetness of HFCS-42 is on par with sucrose.
• Sugar alcohols, such as maltitol, erythritol, sorbitol, mannitol and hydrogenated starch hydrolysate, are sweeteners made by reducing sugars.

Modified starches

A modified starch is a starch that has been chemically modified to allow the starch to function properly under conditions frequently encountered during processing or storage, such as high heat, high shear, low pH, freeze/thaw and cooling.

The modified food starches are E coded according to the International Numbering System for Food Additives (INS):

• 1400 Dextrin
• 1401 Acid-treated starch
• 1402 Alkaline-treated starch
• 1403 Bleached starch
• 1404 Oxidized starch
• 1405 Starches, enzyme-treated
• 1410 Monostarch phosphate
• 1412 Distarch phosphate
• 1413 Phosphated distarch phosphate
• 1414 Acetylated distarch phosphate
• 1420 Starch acetate
• 1422 Acetylated distarch adipate
• 1440 Hydroxypropyl starch
• 1442 Hydroxypropyl distarch phosphate
• 1443 Hydroxypropyl distarch glycerol
• 1450 Starch sodium octenyl succinate
• 1451 Acetylated oxidized starch

INS 1400, 1401, 1402, 1403 and 1405 are in the EU food ingredients without an E-number. Typical modified starches for technical applications are cationic starches, hydroxyethyl starch and carboxymethylated starches.

Use as food additive

As an additive for food processing, food starches are typically used as thickeners and stabilizers in foods such as puddings, custards, soups, sauces, gravies, pie fillings, and salad dressings, and to make noodles and pastas. Function as thickeners, extenders, emulsion stabilizers and are exceptional binders in processed meats.

Gummed sweets such as jelly beans and wine gums are not manufactured using a mold in the conventional sense. A tray is filled with native starch and leveled. A positive mold is then pressed into the starch leaving an impression of 1,000 or so jelly beans. The jelly mix is then poured into the impressions and put onto a stove to set. This method greatly reduces the number of molds that must be manufactured.

Use in pharmaceutical industry

In the pharmaceutical industry, starch is also used as an excipient, as tablet disintegrant, and as binder.

Resistant starch

Resistant starch is starch that escapes digestion in the small intestine of healthy individuals. High amylose starch from corn has a higher gelatinization temperature than other types of starch and retains its resistant starch content through baking, mild extrusion and other food processing techniques. It is used as an insoluble dietary fiber in processed foods such as bread, pasta, cookies, crackers, pretzels and other low moisture foods. It is also utilized as a dietary supplement for its health benefits. Published studies have shown that resistant starch helps to improve insulin sensitivity, increases satiety and improves markers of colonic function. It has been suggested that resistant starch contributes to the health benefits of intact whole grains.

Industrial applications

Starch adhesive

Papermaking

Papermaking is the largest non-food application for starches globally, consuming millions of metric tons annually. In a typical sheet of copy paper for instance, the starch content may be as high as 8%. Both chemically modified and unmodified starches are used in papermaking. In the wet part of the papermaking process, generally called the "wet-end", the starches used are cationic and have a positive charge bound to the starch polymer. These starch derivatives associate with the anionic or negatively charged paper fibers / cellulose and inorganic fillers. Cationic starches together with other retention and internal sizing agents help to give the necessary strength properties to the paper web formed in the papermaking process (wet strength), and to provide strength to the final paper sheet (dry strength).

In the dry end of the papermaking process, the paper web is rewetted with a starch based solution. The process is called surface sizing. Starches used have been chemically, or enzymatically depolymerized at the paper mill or by the starch industry (oxidized starch). The size/starch solutions are applied to the paper web by means of various mechanical presses (size presses). Together with surface sizing agents the surface starches impart additional strength to the paper web and additionally provide water hold out or "size" for superior printing properties. Starch is also used in paper coatings as one of the binders for the coating formulations which include a mixture of pigments, binders and thickeners. Coated paper has improved smoothness, hardness, whiteness and gloss and thus improves printing characteristics.

Corrugated board adhesives

Corrugated board adhesives are the next largest application of non-food starches globally. Starch glues are mostly based on unmodified native starches, plus some additive such as borax and caustic soda. Part of the starch is gelatinized to carry the slurry of uncooked starches and prevent sedimentation. This opaque glue is called a SteinHall adhesive. The glue is applied on tips of the fluting. The fluted paper is pressed to paper called liner. This is then dried under high heat, which causes the rest of the uncooked starch in glue to swell/gelatinize. This gelatinizing makes the glue a fast and strong adhesive for corrugated board production.

Clothing starch

Clothing or laundry starch is a liquid prepared by mixing a vegetable starch in water (earlier preparations also had to be boiled), and is used in the laundering of clothes. Starch was widely used in Europe in the 16th and 17th centuries to stiffen the wide collars and ruffs of fine linen which surrounded the necks of the well-to-do. During the 19th and early 20th century it was stylish to stiffen the collars and sleeves of men's shirts and the ruffles of women's petticoats by applying starch to them as the clean clothes were being ironed. Starch gave clothing smooth, crisp edges, and had an additional practical purpose: dirt and sweat from a person's neck and wrists would stick to the starch rather than to the fibers of the clothing. The dirt would wash away along with the starch; after laundering, the starch would be reapplied. Today, starch is sold in aerosol cans for home use.

Other

Another large non-food starch application is in the construction industry, where starch is used in the gypsum wall board manufacturing process. Chemically modified or unmodified starches are added to the stucco containing primarily gypsum. Top and bottom heavyweight sheets of paper are applied to the formulation, and the process is allowed to heat and cure to form the eventual rigid wall board. The starches act as a glue for the cured gypsum rock with the paper covering, and also provide rigidity to the board.

Starch is used in the manufacture of various adhesives or glues for book-binding, wallpaper adhesives, paper sack production, tube winding, gummed paper, envelope adhesives, school glues and bottle labeling. Starch derivatives, such as yellow dextrins, can be modified by addition of some chemicals to form a hard glue for paper work; some of those forms use borax or soda ash, which are mixed with the starch solution at 50–70 °C (122–158 °F) to create a very good adhesive. Sodium silicate can be added to reinforce these formula.

• Textile chemicals from starch: warp sizing agents are used to reduce breaking of yarns during weaving. Starch is mainly used to size cotton based yarns. Modified starch is also used as textile printing thickener.
• In oil exploration, starch is used to adjust the viscosity of drilling fluid, which is used to lubricate the drill head and suspend the grinding residue in petroleum extraction.
• Starch is also used to make some packing peanuts, and some drop ceiling tiles.
• In the printing industry, food grade starch is used in the manufacture of anti-set-off spray powder used to separate printed sheets of paper to avoid wet ink being set off.
• For body powder, powdered corn starch is used as a substitute for talcum powder, and similarly in other health and beauty products.
• Starch is used to produce various bioplastics, synthetic polymers that are biodegradable. An example is polylactic acid based on glucose from starch.
• Glucose from starch can be further fermented to biofuel corn ethanol using the so-called wet milling process. Today most bioethanol production plants use the dry milling process to ferment corn or other feedstock directly to ethanol.
• Hydrogen production could use glucose from starch as the raw material, using enzymes.

Occupational safety and health

The Occupational Safety and Health Administration (OSHA) has set the legal limit (Permissible exposure limit) for starch exposure in the workplace as 15 mg/m³ total exposure and 5 mg/m³ respiratory exposure over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a Recommended exposure limit (REL) of 10 mg/m³ total exposure and 5 mg/m³ respiratory exposure over an 8-hour workday.

Biodegradable polymer

From Wikipedia, the free encyclopedia

Example of cutlery made from biodegradable plastic

Biodegradable polymers are a specific type of polymer that breaks down after its intended purpose to result in natural byproducts such as gases (CO₂, N₂), water, biomass, and inorganic salts. These polymers are found both naturally and synthetically made, and largely consist of ester, amide, and ether functional groups. Their properties and breakdown mechanism are determined by their exact structure. These polymers are often synthesized by condensation reactions, ring opening polymerization, and metal catalysts. There are vast examples and applications of biodegradable polymers.

Bio-based packaging materials have been introduced as a green alternative in the past decades, among which, edible films have gained more attention due to their environmentally-friendly characteristics, vast variety and availability, non-toxicity, and low cost.

History

Biodegradable polymers have a long history, and since many are natural products, the precise timeline of their discovery and use cannot be accurately traced. One of the first medicinal uses of a biodegradable polymer was the catgut suture, which dates back to at least 100 AD. The first catgut sutures were made from the intestines of sheep, but modern catgut sutures are made from purified collagen extracted from the small intestines of cattle, sheep, or goats.

The concept of synthetic biodegradable plastics and polymers was first introduced in the 1980s. In 1992, an international meeting was called where leaders in biodegradable polymers met to discuss a definition, standard, and testing protocol for biodegradable polymers. Also, oversight organizations such as American Society for Testing of Materials (ASTM) and the International Standards Organization (ISO) were created. Large clothing and grocery store chains have pushed to utilize biodegradable bags in the late 2010s. Biodegradable polymers also received notice from various fields in 2012 when Professor Geoffrey Coates of Cornell University received the Presidential Green Chemistry Challenge Award. As of 2013, 5-10% of the plastic market focused on biodegradable polymer derived plastics.

Structure and properties

The structure of biodegradable polymers is instrumental in their properties. While there are innumerable biodegradable polymers, both synthetic and natural, there are a few commonalities among them.

Structure

Biodegradable polymers tend to consist of ester, amide, or ether bonds. In general, biodegradable polymers can be grouped into two large groups based on their structure and synthesis. One of these groups is agro-polymers, or those derived from biomass. The other consists of biopolyesters, which are those derived from microorganisms or synthetically made from either naturally or synthetic monomers.

Biodegradable polymers based on structure and occurrence

Agro-polymers include polysaccharides, like starches found in potatoes or wood, and proteins, such as animal based whey or plant derived gluten. Polysacharides consist of glycosidic bonds, which take a hemiacetal of a saccharide and binds it to a alcohol via loss of water. Proteins are made from amino acids, which contain various functional groups. These amino acids come together again through condensation reactions to form peptide bonds, which consist of amide functional groups. Examples of biopolyesters include polyhydroxybutyrate and polylactic acid.

Properties

Even though biodegradable polymers have numerous applications, there are properties that tend to be common among them. All biodegradable polymers should be stable and durable enough for use in their particular application, but upon disposal they should easily break down. Polymers, specifically biodegradable polymers, have extremely strong carbon backbones that are difficult to break, such that degradation often starts from the end-groups. Since the degradation begins at the end, a high surface area is common as it allows easy access for either the chemical, light, or organism. Biodegradable polymers also tend to have minimal chain branching as this cross linking often decreases the number of end groups per unit weight. Crystallinity is often low as it also inhibits access to end groups. A low degree of polymerization is normally seen, as hinted at above, as doing so allows for more accessible end groups for reaction with the degradation initiator. Another commonality of these polymers is their hydrophillicity. Hydrophobic polymers and end groups will prevent an enzyme from easily interacting if the water-soluble enzyme cannot easily get in contact with the polymer.

Other properties of biodegradable polymers that are common among those used for medicinal usages include being:

• non-toxic
• capable of maintaining good mechanical integrity until degraded
• capable of controlled rates of degradation

A goal is not to elicit the immune response, and the products of degradation also need not to be toxic. These are important as biodegradable polymers are used for drug delivery where it is critical to slowly release the drug into the body over time instead of all at once and that the pill is stable in the bottle until ready to be taken. Factors controlling the rate of degradation include percent crystallinity, molecular weight, and hydrophobicity. The degradation rate depends on the location in the body, which influences the environment surrounding the polymer such as pH, enzymes concentration, and amount of water, among others. These are rapidly decomposed.

Synthesis

One of the most important and most studied groups of biodegradable polymers are polyesters. Polyesters can be synthesized in a number of ways including direct condensation of alcohols and acids, ring opening polymerizations (ROP), and metal-catalyzed polymerization reactions. A great disadvantage of the step-wise polymerization via condensation of an acid and an alcohol is the need to continuously remove water from this system in order to drive the equilibrium of the reaction forward. This can necessitate harsh reaction conditions and long reaction times, resulting in a wide dispersity. A wide variety of starting materials can be used to synthesize polyesters, and each monomer type endows the final polymer chain with different characteristics and properties. The ROP of cyclic dimeric glycolic or lactic acid forms α-hydroxy acids which then polymerize into poly-(α-esters). A variety of organometallic initiators can be used to start the polymerization of polyesters, including tin, zinc, and aluminum complexes. The most common is tin(II)octanoate and has been approved as a food additive by the U.S. FDA, but there are still concerns about using the tin catalysts in the synthesis of biodegradable polymers for biomedical uses. The synthesis of poly(β-esters) and poly(γ-esters) can be carried out by similar ROP or condensation methods as with poly(γ-esters). Development of metal-free process that involve the use of bacterial or enzymatic catalysis in polyester formation is also being explored. These reactions have the benefit of generally being regioselective and stereospecific but suffer from the high cost of bacteria and enzymes, long reaction times, and products of low molecular weight.

Example of routes to polyester formation using lactic acid.

a) Condensation of lactic acid into dimeric lactide followed by

ring-opening polymerization of to form poly(lactic acid);

b) Direct condensation of lactic acid, demonstrating the need to

continuously remove water from the system in order to drive

the reaction forward.

 

While polyesters dominate both the research and industrial focus on synthetic biodegradable polymers, other classes of polymers are also of interest. Polyanhydrides are an active area of research in drug delivery because they only degrade from the surface and so are able to release the drug they carry at a constant rate. Polyanhydrides can be made via a variety of methods also used in the synthesis of other polymers, including condensation, dehydrochlorination, dehydrative coupling, and ROP. Polyurethanes and poly(ester amide)s are used in biomaterials. Polyurethanes were initially used for their biocompatibility, durability, resilience, but are more recently being investigated for their biodegradability. Polyurethanes are typically synthesized using a diisocyanate, a diol, and a polymer chain extender. The initial reaction is carried out between the diisocyanate and the diol, with the diisocyanate in excess to ensure that the ends of the new polymer chain are isocyanate groups. This polymer can then be reacted with either a diol or a diamine to form urethane or urethane-urea end groups, respectively. The choice of terminal groups affects the properties of the resulting polymer. Additionally, the use of vegetable oil and biomass in the formation of polyurethanes, as well as the conversion of polyurethanes to polyols, is an active area of research.

Synthesis of polyurethane from a diisocyanate and a diol. To

cap this polymer, chain extenders of either diols or diamines

can be added in order to tailor the properties.

Mechanism of breakdown

In general, biodegradable polymers break down to form gases, salts, and biomass. Complete biodegradation is said to occur when there are no oligomers or monomers left. The breakdown of these polymers depend on a variety of factors including the polymer and also, the environment the polymer is in. Polymer properties that influence degradation are bond type, solubility, and copolymers among others. The surrounding environment of the polymer is just as important as the polymer structure itself. These factors included items such as the pH, temperature, micoorganisms present, and water are just a few examples.

There are two primary mechanisms through which biodegradation can occur. One is through physical decomposition through reactions such as hydrolysis and photodegradation, which can lead to partial or complete degradation. The second mechanistic route is through biological processes which can be further broken down into aerobic and anaerobic processes. The first involves aerobic biodegradition, where oxygen is present and important. In this case, the general equation seen below where C_residue represents smaller fragments of the initial polymer such as oligomers.

General equation for aerobic biodegradition

 

The second mechanism of biodegradation is by anaerobic processes, where oxygen is not present.

General equation for anaerobic biodegradition

There are numerous organisms that have the ability to break down natural polymers. There are also synthetic polymers that have only been around for a hundred years with new features that microorganisms do not have the capability to break down. It will take millions of years before organisms can adapt to degrade all of these new synthetic polymers. Typically, after physical processes carry out the initial breakdown of the polymer, microorganisms will then take what is left and break down the components into even simpler units. These microorganisms normally take polymer fragments, such as oligomers or monomers, into the cell where enzymes work to make adenosine triphosphate (ATP) and polymer end products carbon dioxide, nitrogen gas, methane, water, minerals, and biomass. These enzymes act in a variety of ways to break down polymers including through oxidation or hydrolysis. Examples of key enzymes include proteases, esterases, glycosidases, and manganese peroxidases.

Applications and uses

Biodegradable polymers are of significant interest to a variety of fields including medicine, agriculture, and packaging. One of the most active areas of research in biodegradable polymer is in controlled drug delivery and release.

Medical

Biodegradable polymers have an innumerable uses in the biomedical field, particularly in the fields of tissue engineering and drug delivery. In order for a biodegradable polymer to be used as a therapeutic, it must meet several criteria: 1) be non-toxic in order to eliminate foreign body response; 2) the time it takes for the polymer to degrade is proportional to the time required for therapy; 3) the products resulting from biodegredation are not cytotoxic and are readily eliminated from the body; 4) the material must be easily processed in order to tailor the mechanical properties for the required task; 5) be easily sterilized; and 6) have acceptable shelf life.

Biodegradable polymers are of great interest in the field of drug delivery and nanomedicine. The great benefit of a biodegradable drug delivery system is the ability of the drug carrier to target the release of its payload to a specific site in the body and then degrade into nontoxic materials that are then eliminated from the body via natural metabolic pathways. The polymer slowly degrades into smaller fragments, releasing a natural product, and there is controlled ability to release a drug. The drug slowly releases as polymer degrades. For example, polylactic acid, poly(lactic-co-glycolic) acid, and poly(caprolactone), all of which are biodegradable, have been used to carry anti-cancer drugs. Encapsulating the therapeutic in a polymer and adding targeting agents decreases the toxicity of the drug to healthy cells.

Sutures made from polyglycolic acid. These sutures are absorbable and will be degraded by the body over time.

Biodegradable polymers and biomaterials are also of significant interest for tissue engineering and regeneration. Tissue engineering is the ability to regenerate tissue with the help of artificial materials. The perfection of such systems can be used to grow tissues and cells in vitro or use a biodegradable scaffold to construct new structures and organs in vitro. For these uses, a biodegradable scaffold is obviously preferred as it reduces the risk of immunological reaction and rejection of the foreign object. While many of the more advanced systems are not ready for human therapeutics, there is significant positive research in animal studies. For example, it was possible to successfully grow rat smooth muscle tissue on a polycaprolactone/polylactide scaffold. Further research and development may allow for this technology to be used for tissue replacement, support, or enhancement in humans. One of the ultimate goals of tissue engineering is the creation of organs, such as the kidney, from basic constituents. A scaffolding is necessary to grow the entity into a functioning organ, after which the polymer scaffold would degrade and be safely eliminated from the body. There are reports of using polyglycolic acid and polylactic acid to engineer vascular tissue for heart repair. The scaffold can be used to help create undamaged arteries and vessels.

In addition to tissue engineering, biodegradable polymers are being used orthopedic applications, such as bone and joint replacement. A wide variety of non-biodegradable polymers have been used for orthopedic applications including silicone rubber, polyethylene, acrylic resins, polyurethane, polypropylene, and polymethylmethacrylate. The primary role of many of these polymers was to act as a biocompatible cement in the fixation of prostheses and in the replacement of joints. Newer biologically compatible synthetic and natural biodegradable polymers have been developed; these include polyglycolide, polylactide, polyhydroxobutyrate, chitosan, hyaluronic acid, and hydrogels. In particular, poly(2-hydroxyethyl-methacrylate), poly(ethylene glycol), chitosan, and hyaluronic acid have been used extensively in the repair of cartilage, ligaments, and tendons. For example, poly(L-lactide) (PLA), is used to make screws and darts for meniscal repair and is marketed under the trade name Clearfix Mensical Dart/Screw. PLA is a slow degrading polymer and requires times greater than two years to degrade and be absorbed by the body.

Packaging and materials

A trash bag made of a poly(lactic acid) blend, marketed under the brand Bio-Flex® 

 

In addition to medicine, biodegradable polymers are often used to reduce the volume of waste in packaging materials. There is also significant effort to replace materials derived from petrochemicals with those that can be made from biodegradable components. One of the most commonly used polymers for packaging purposes is polylactic acid, PLA. The production of PLA has several advantages, the most important of which is the ability to tailor the physical properties of the polymer through processing methods. PLA is used for a variety of films, wrappings, and containers (including bottles and cups). In 2002, FDA ruled that PLA was safe to use in all food packaging. BASF markets a product called ecovio® which is a biobased blend of the company's certified compostable and biodegradable co-polyester ecoflex® and PLA. An application for this certified compostable and bio-based material is for any kind of plastic films such as shopping bags or organic waste bags. ecovio® can also be used in other applications, like thermoformed and injection moulded articles. Even paper-coating or particle foamed products can be produced by this very versatile biopolymer.

Notable examples

2012 Presidential Green Chemistry Challenge

Carbon dioxide directly used in a polymer backbone

Each year hundreds of millions of tons of plastics are produced from petroleum. Most of these plastics will remain in landfills for years to come or litter the environment posing significant health risks to animals; however, the average person's lifestyle would be impractical without them. One solution to this conundrum lies in biodegradable polymers. These polymers have the distinct advantage that over time they will break down. Dr. Geoffrey Coates headed research to create catalysts that can not only efficiently create these biodegradable polymers, but the polymers also incorporate the greenhouse gas and global warming contributor, CO₂, and, environmentally present ground-ozone producer, CO. These two gases can be found or produced in high concentrations from agricultural waste, coal, and industrial applications as byproducts. Not only do the catalysts utilize these normally wasted and environmentally unfriendly gases, but they also do it extremely efficiently with high turnover numbers and frequencies in addition to good selectivity. These catalysts have been actively used by Novomer Inc to make polycarbonates that can replace the current coating bisphenol A (BPA) found in many food and drink packaging. Novomer's analysis shows that if used in all cases, these biodegradable polymer coatings could not only sequester, but also avoid further production of CO₂ in hundreds of millions of metric tons in just a single year.