This image shows the best before
date on a packaged food item, showing that the consumer should consume
the product before this time in order to reduce chance of consuming
spoiled food.
Food spoilage is the process where a food product becomes
unsuitable to ingest by the consumer. The cause of such a process is due
to many outside factors as a side-effect of the type of product it is,
as well as how the product is packaged and stored. Due to food spoilage, one-third of the world's food produced for the consumption of humans is lost every year.
Bacteria and various fungi are the cause of spoilage and can create
serious consequences for the consumers, but there are preventive
measures that can be taken.
Bacteria
Bacteria are responsible for the spoilage of food. When bacteria breaks down the food, acids and other waste products are generated in the process.
While the bacteria itself may or may not be harmful, the waste products
may be unpleasant to taste or may even be harmful to one's health.
There are two types of pathogenic bacteria that target different categories of food. The first type is called Clostridium botulinum and targets foods such as meat and poultry, and Bacillus cereus,
which targets milk and cream. When stored or subjected to unruly
conditions, the organisms will begin to breed apace, releasing harmful
toxins that can cause severe illness, even when cooked safely.
Fungi
This image shows a bowl of white rice with mold growing over it.
Fungi has been seen as a method of food spoilage, causing only
an undesirable appearance to food, however, there has been significant
evidence of various fungi being a cause of death of many people spanning
across hundreds of years in many places through the world. Fungi are
caused by acidifying, fermenting, discoloring and disintegrating
processes and can create fuzz, powder and slimes of many different
colors, including black, white, red, brown and green.
Mold is a type of fungus, but the two terms are not
reciprocal of each other; they have their own defining features and
perform their own tasks.
Very well known types of mold are Aspergillus and Penicillium, and,
like regular fungi, create a fuzz, powder and slime of various colors.
Yeast is also a type of fungus that grows vegetatively via
single cells that either bud or divide by way of fission, allowing for
yeast to multiply in liquid environments favoring the dissemination of
single celled microorganisms. Yeast forms mainly in liquid environments
and anaerobic conditions, but being single celled, it oftentimes cannot
spread on or into solid surfaces where other fungus flourish. Yeast also
produces at a slower rate than bacteria, therefore being at a
disadvantage in environments where bacteria are. Yeasts
can be responsible for the decomposition of food with a high sugar
content. The same effect is useful in the production of various types of
food and beverages, such as bread, yogurt, cider, and alcoholic beverages.
This image depicts the process of decomposition beyond the point of human appeal.
Signs
Signs of
food spoilage may include an appearance different from the food in its
fresh form, such as a change in color, a change in texture, an unpleasant odour, or an undesirable taste. The item may become softer than normal. If mold occurs, it is often visible externally on the item.
Consequences
Spoilage bacteria do not normally cause "food poisoning"; typically, the microorganisms that cause foodborne illnesses are odorless and flavourless, and otherwise undetectable outside the lab.Eating deteriorated food could not be considered safe due to mycotoxins or microbial wastes. Some pathogenic bacteria, such as Clostridium perfringens and Bacillus cereus, are capable of causing spoilage.
Issues of food spoilage do not necessarily have to do with the
quality of the food, but more so with the safety of consuming said food.
However, there are cases where food has been proven to contain toxic
ingredients. 200 years ago, Claviceps purpurea, a type of fungus, was
linked to human diseases and 100 years ago in Japan, yellow rice was
found to contain toxic ingredients.
Prevention
A number of methods of prevention can be used that can either totally prevent, delay, or otherwise reduce food spoilage. A food rotation system uses the first in first out method (FIFO), which ensures that the first item purchased is the first item consumed.
Preservatives can expand the shelf life
of food and can lengthen the time long enough for it to be harvested,
processed, sold, and kept in the consumer's home for a reasonable length
of time. One of the age old techniques for food preservation, to avoid
mold and fungus growth, is the process of drying out the food or
dehydrating it. While there is a chance of it developing a fungus
targeted towards dried food products, the chances are quite low.
Other than drying, other methods include salting, curing,
canning, refrigeration, freezing, preservatives, irradiation, and high
hydrostatic pressure: Refrigeration
can increase the shelf life of certain foods and beverages, though with
most items, it does not indefinitely expand it. Freezing can preserve
food even longer, though even freezing has limitations. Canning of food can preserve food for a particularly long period of time, whether done at home or commercially. Canned food is vacuum packed
in order to keep oxygen, which is needed by bacteria in aerobic
spoilage, out of the can. Canning does have limitations, and does not
preserve the food indefinitely. Lactic acid fermentation also preserves food and prevents spoilage.
Food like meat, poultry, milk and cream should be kept out of the
Danger Zone (between 40°F to 140°F). Anything between that range is
considered dangerous and can cause pathogenic toxins to be emitted,
resulting in severe illness in the consumer.
Another way to keep your food from spoiling is by following a four step
system: Clean, Separate, Cook, Chill. This will reduce any risks.
Food preservation prevents the growth of microorganisms (such as yeasts), or other microorganisms (although some methods work by introducing benign bacteria or fungi to the food), as well as slowing the oxidation of fats that cause rancidity. Food preservation may also include processes that inhibit visual deterioration, such as the enzymatic browning reaction in apples after they are cut during food preparation.
Many processes designed to preserve food involve more than one
food preservation method. Preserving fruit by turning it into jam, for
example, involves boiling (to reduce the fruit's moisture content and to
kill bacteria, etc.), sugaring (to prevent their re-growth) and sealing
within an airtight jar (to prevent recontamination). Some traditional
methods of preserving food have been shown to have a lower energy input and carbon footprint, when compared to modern methods.
Maintaining or creating nutritional value, texture and flavor is an important aspect of food preservation.
Traditional techniques
New techniques of food preservation became available to the home chef from the dawn of agriculture until the Industrial Revolution.
Curing
Bag of Prague powder#1, also known as "curing salt"
or "pink salt". It is typically a combination of salt and sodium
nitrite, with the pink color added to distinguish it from ordinary salt.
The earliest form of curing was dehydration or drying, used as early as 12,000 BC. Smoking and salting
techniques improve on the drying process and add antimicrobial agents
that aid in preservation. Smoke deposits a number of pyrolysis products
onto the food, including the phenolssyringol, guaiacol and catechol. Salt accelerates the drying process using osmosis and also inhibits the growth of several common strains of bacteria. More recently nitrites have been used to cure meat, contributing a characteristic pink colour.
Cooling
Cooling
preserves food by slowing down the growth and reproduction of
microorganisms and the action of enzymes that causes the food to rot.
The introduction of commercial and domestic refrigerators drastically
improved the diets of many in the Western world
by allowing food such as fresh fruit, salads and dairy products to be
stored safely for longer periods, particularly during warm weather.
Freezing
is also one of the most commonly used processes, both commercially and
domestically, for preserving a very wide range of foods, including
prepared foods that would not have required freezing in their unprepared
state. For example, potato waffles are stored in the freezer, but
potatoes themselves require only a cool dark place to ensure many
months' storage. Cold stores provide large-volume, long-term storage for
strategic food stocks held in case of national emergency in many
countries.
Boiling
Boiling liquid food items can kill any existing microbes. Milk and
water are often boiled to kill any harmful microbes that may be present
in them.
Heating
Heating to temperatures which are sufficient to kill microorganisms inside the food is a method used with perpetual stews. Milk is also boiled before storing to kill many microorganisms.
Sugaring
The earliest cultures have used sugar as a preservative, and it was commonplace to store fruit in honey. Similar to pickled foods, sugar cane was brought to Europe through the trade routes. In northern climates without sufficient sun to dry foods, preserves are made by heating the fruit with sugar.
"Sugar tends to draw water from the microbes (plasmolysis). This
process leaves the microbial cells dehydrated, thus killing them. In
this way, the food will remain safe from microbial spoilage." Sugar is used to preserve fruits, either in an antimicrobial syrup with fruit such as apples, pears, peaches, apricots, and plums,
or in crystallized form where the preserved material is cooked in sugar
to the point of crystallization and the resultant product is then
stored dry. This method is used for the skins of citrus fruit (candied peel), angelica, and ginger. Also, sugaring can be used in the production of jam and jelly.
Pickling
Pickling is a method of preserving food in an edible, antimicrobial
liquid. Pickling can be broadly classified into two categories: chemical
pickling and fermentation pickling.
In chemical pickling, the food is placed in an edible liquid that
inhibits or kills bacteria and other microorganisms. Typical pickling
agents include brine (high in salt), vinegar, alcohol, and vegetable oil.
Many chemical pickling processes also involve heating or boiling so
that the food being preserved becomes saturated with the pickling agent.
Common chemically pickled foods include cucumbers, peppers, corned beef, herring, and eggs, as well as mixed vegetables such as piccalilli.
Sodium hydroxide (lye) makes food too alkaline for bacterial growth. Lye will saponify fats in the food, which will change its flavor and texture. Lutefisk uses lye in its preparation, as do some olive recipes. Modern recipes for century eggs also call for lye.
Canning
Preserved food
Canning involves cooking food, sealing it in sterilized cans or jars, and boiling the containers to kill or weaken any remaining bacteria as a form of sterilization. It was invented by the French confectioner Nicolas Appert.
By 1806, this process was used by the French Navy to preserve meat,
fruit, vegetables, and even milk. Although Appert had discovered a new
way of preservation, it wasn't understood until 1864 when Louis Pasteur found the relationship between microorganisms, food spoilage, and illness.
Foods have varying degrees of natural protection against spoilage and may require that the final step occur in a pressure cooker. High-acid fruits like strawberries require no preservatives to can and only a short boiling cycle, whereas marginal vegetables such as carrots
require longer boiling and addition of other acidic elements. Low-acid
foods, such as vegetables and meats, require pressure canning. Food
preserved by canning or bottling is at immediate risk of spoilage once
the can or bottle has been opened.
Lack of quality control in the canning process may allow ingress
of water or micro-organisms. Most such failures are rapidly detected as
decomposition within the can causes gas production and the can will
swell or burst. However, there have been examples of poor manufacture
(underprocessing) and poor hygiene allowing contamination of canned food by the obligate anaerobeClostridium botulinum,
which produces an acute toxin within the food, leading to severe
illness or death. This organism produces no gas or obvious taste and
remains undetected by taste or smell. Its toxin is denatured by cooking,
however. Cooked mushrooms, handled poorly and then canned, can support the growth of Staphylococcus aureus, which produces a toxin that is not destroyed by canning or subsequent reheating.
Jellying
Food may be preserved by cooking in a material that solidifies to form a gel. Such materials include gelatin, agar, maize flour, and arrowroot flour. Some foods naturally form a protein gel when cooked, such as eels and elvers, and sipunculid worms, which are a delicacy in Xiamen, in the Fujian province of the People's Republic of China. Jellied eels are a delicacy in the East End of London, where they are eaten with mashed potatoes. Potted meats in aspic (a gel made from gelatin and clarified meat broth) were a common way of serving meat off-cuts in the UK until the 1950s. Many jugged meats are also jellied. A traditional British way of preserving meat (particularly shrimp)
is by setting it in a pot and sealing it with a layer of fat. Also
common is potted chicken liver; jellying is one of the steps in
producing traditional pâtés.
Besides jellying of meat and seafood, a widely known type of
Jellying is fruit preserves which are preparations of fruits, vegetables
and sugar, often stored in glass jam jars and Mason jars. Many
varieties of fruit preserves are made globally, including sweet fruit
preserves, such as those made from strawberry or apricot, and savory
preserves, such as those made from tomatoes or squash. The ingredients
used and how they are prepared determine the type of preserves; jams,
jellies, and marmalades are all examples of different styles of fruit
preserves that vary based upon the fruit used. In English, the word, in
plural form, "preserves" is used to describe all types of jams and
jellies.
Jugging
Meat can be preserved by jugging. Jugging is the process of stewing the meat (commonly game or fish) in a covered earthenware jug or casserole. The animal to be jugged is usually cut into pieces, placed into a tightly-sealed jug with brine or gravy, and stewed. Red wine
and/or the animal's own blood is sometimes added to the cooking liquid.
Jugging was a popular method of preserving meat up until the middle of
the 20th century.
Burial
Burial of food can preserve it due to a variety of factors: lack of light, lack of oxygen, cool temperatures, pH level, or desiccants
in the soil. Burial may be combined with other methods such as salting
or fermentation. Most foods can be preserved in soil that is very dry
and salty (thus a desiccant) such as sand, or soil that is frozen.
Many root vegetables
are very resistant to spoilage and require no other preservation than
storage in cool dark conditions, for example by burial in the ground,
such as in a storage clamp. Century eggs
are traditionally created by placing eggs in alkaline mud (or other
alkaline substance), resulting in their "inorganic" fermentation through
raised pH instead of spoiling. The fermentation preserves them and
breaks down some of the complex, less flavorful proteins and fats into
simpler, more flavorful ones. Cabbage was traditionally buried during Autumn in northern US farms for preservation. Some methods keep it crispy while other methods produce sauerkraut. A similar process is used in the traditional production of kimchi.
Sometimes meat is buried under conditions that cause preservation. If
buried on hot coals or ashes, the heat can kill pathogens, the dry ash
can desiccate, and the earth can block oxygen and further contamination.
If buried where the earth is very cold, the earth acts like a
refrigerator.
In Orissa, India,
it is practical to store rice by burying it underground. This method
helps to store for three to six months during the dry season.
Butter and similar substances have been preserved as bog butter in Irish peat bogs for centuries.
Confit
Meat can be preserved by salting it, cooking it at or near 100 °C in some kind of fat (such as lard or tallow),
and then storing it immersed in the fat. These preparations were
popular in Europe before refrigerators became ubiquitous. They are
still popular in France, where they are called confit. The preparation will keep longer if stored in a cold cellar or buried in cold ground.
Fermentation
Some foods, such as many cheeses, wines, and beers,
use specific micro-organisms that combat spoilage from other
less-benign organisms. These micro-organisms keep pathogens in check by
creating an environment toxic for themselves and other micro-organisms
by producing acid or alcohol. Methods of fermentation include, but are
not limited to, starter micro-organisms, salt, hops, controlled (usually
cool) temperatures and controlled (usually low) levels of oxygen. These
methods are used to create the specific controlled conditions that will
support the desirable organisms that produce food fit for human
consumption.
Fermentation is the microbial conversion of starch and sugars
into alcohol. Not only can fermentation produce alcohol, but it can also
be a valuable preservation technique. Fermentation can also make foods
more nutritious and palatable. For example, drinking water in the Middle
Ages was dangerous because it often contained pathogens that could
spread disease. When the water is made into beer, the boiling during the
brewing process kills any bacteria in the water that could make people
sick. Additionally, the water now has the nutrients from the barley and
other ingredients, and the microorganisms can also produce vitamins as
they ferment.
Modern industrial techniques
Techniques of food preservation were developed in research laboratories for commercial applications.
Pasteurization
Pasteurization is a process for preservation of liquid food. It was
originally applied to combat the souring of young local wines. Today,
the process is mainly applied to dairy products. In this method, milk is
heated at about 70 °C (158 °F) for 15–30 seconds to kill the bacteria
present in it and cooling it quickly to 10 °C (50 °F) to prevent the
remaining bacteria from growing. The milk is then stored in sterilized
bottles or pouches in cold places. This method was invented by Louis Pasteur, a French chemist, in 1862.
Vacuum packing
Vacuum-packing stores food in a vacuum environment, usually in an air-tight bag or bottle. The vacuum environment strips bacteria of oxygen needed for survival. Vacuum-packing is commonly used for storing nuts
to reduce loss of flavor from oxidization. A major drawback to vacuum
packaging, at the consumer level, is that vacuum sealing can deform
contents and rob certain foods, such as cheese, of its flavor.
Irradiation of food is the exposure of food to ionizing radiation. Multiple types of ionizing radiation can be used, including beta particles (high-energy electrons) and gamma rays (emitted from radioactive sources such as cobalt-60 or cesium-137).
Irradiation can kill bacteria, molds, and insect pests, reduce the
ripening and spoiling of fruits, and at higher doses induce sterility.
The technology may be compared to pasteurization;
it is sometimes called "cold pasteurization", as the product is not
heated. Irradiation may allow lower-quality or contaminated foods to be
rendered marketable.
National and international expert bodies have declared food irradiation as "wholesome"; organizations of the United Nations, such as the World Health Organization and Food and Agriculture Organization, endorse food irradiation. Consumers may have a negative view of irradiated food based on the misconception that such food is radioactive;
in fact, irradiated food does not and cannot become radioactive.
Activists have also opposed food irradiation for other reasons, for
example, arguing that irradiation can be used to sterilize contaminated
food without resolving the underlying cause of the contamination. International legislation on whether food may be irradiated or not varies worldwide from no regulation to a full ban.
Approximately 500,000 tons of food items are irradiated per year worldwide in over 40 countries. These are mainly spices and condiments, with an increasing segment of fresh fruit irradiated for fruit fly quarantine.
Pulsed electric field electroporation
Pulsed electric field (PEF) electroporation is a method for
processing cells by means of brief pulses of a strong electric field.
PEF holds potential as a type of low-temperature alternative
pasteurization process for sterilizing food products. In PEF processing,
a substance is placed between two electrodes, then the pulsed electric
field is applied. The electric field enlarges the pores of the cell
membranes, which kills the cells and releases their contents. PEF for
food processing is a developing technology still being researched. There
have been limited industrial applications of PEF processing for the
pasteurization of fruit juices. To date, several PEF treated juices are
available on the market in Europe. Furthermore, for several years a
juice pasteurization application in the US has used PEF. For cell
disintegration purposes especially potato processors show great interest
in PEF technology as an efficient alternative for their preheaters.
Potato applications are already operational in the US and Canada. There
are also commercial PEF potato applications in various countries in
Europe, as well as in Australia, India, and China.
Modified atmosphere
Modifying atmosphere is a way to preserve food by operating on the
atmosphere around it. Salad crops that are notoriously difficult to
preserve are now being packaged in sealed bags with an atmosphere
modified to reduce the oxygen (O2) concentration and increase the carbon dioxide (CO2)
concentration. There is concern that, although salad vegetables retain
their appearance and texture in such conditions, this method of
preservation may not retain nutrients, especially vitamins.
There are two methods for preserving grains with carbon dioxide. One method is placing a block of dry ice
in the bottom and filling the can with the grain. Another method is
purging the container from the bottom by gaseous carbon dioxide from a
cylinder or bulk supply vessel.
Carbon dioxide prevents insects and, depending on concentration, mold and oxidation from damaging the grain. Grain stored in this way can remain edible for approximately five years.
Nitrogen gas (N2) at concentrations of 98% or higher is also used effectively to kill insects in the grain through hypoxia. However, carbon dioxide has an advantage in this respect, as it kills organisms through hypercarbia and hypoxia (depending on concentration), but it requires concentrations of above 35%, or so. This makes carbon dioxide preferable for fumigation in situations where a hermetic seal cannot be maintained.
Controlled Atmospheric Storage (CA): "CA storage is a
non-chemical process. Oxygen levels in the sealed rooms are reduced,
usually by the infusion of nitrogen gas, from the approximate 21 percent
in the air we breathe to 1 percent or 2 percent. Temperatures are kept
at a constant 0–2 °C (32–36 °F). Humidity is maintained at 95 percent
and carbon dioxide levels are also controlled. Exact conditions in the
rooms are set according to the apple variety. Researchers develop
specific regimens for each variety to achieve the best quality.
Computers help keep conditions constant."
"Eastern Washington, where most of Washington’s apples are grown, has
enough warehouse storage for 181 million boxes of fruit, according to a
report done in 1997 by managers for the Washington State Department of
Agriculture Plant Services Division. The storage capacity study shows
that 67 percent of that space—enough for 121,008,000 boxes of apples—is
CA storage."
Air-tight storage of grains (sometimes called hermetic storage)
relies on the respiration of grain, insects, and fungi that can modify
the enclosed atmosphere sufficiently to control insect pests. This is a
method of great antiquity,
as well as having modern equivalents. The success of the method relies
on having the correct mix of sealing, grain moisture, and temperature.
A patented process uses fuel cells to exhaust and automatically maintain the exhaustion of oxygen in a shipping container, containing, for example, fresh fish.
Nonthermal plasma
This process subjects the surface of food to a "flame" of ionized gas
molecules, such as helium or nitrogen. This causes micro-organisms to
die off on the surface.
High-pressure food preservation
High-pressure food preservation or pascalization refers to the use of a food preservation technique that makes use of high pressure.
"Pressed inside a vessel exerting 70,000 pounds per square inch
(480 MPa) or more, food can be processed so that it retains its fresh
appearance, flavor, texture and nutrients while disabling harmful
microorganisms and slowing spoilage." By 2005, the process was being
used for products ranging from orange juice to guacamole to deli meats and widely sold.
Biopreservation
3D stick model of nisin. Some lactic acid bacteria manufacture nisin. It is a particularly effective preservative.
Biopreservation is the use of natural or controlled microbiota or antimicrobials as a way of preserving food and extending its shelf life. Beneficial bacteria or the fermentation products produced by these bacteria are used in biopreservation to control spoilage and render pathogens inactive in food. It is a benign ecological approach which is gaining increasing attention.
Of special interest are lactic acid bacteria
(LAB). Lactic acid bacteria have antagonistic properties that make them
particularly useful as biopreservatives. When LABs compete for
nutrients, their metabolites often include active antimicrobials such as lactic acid, acetic acid, hydrogen peroxide, and peptidebacteriocins. Some LABs produce the antimicrobial nisin, which is a particularly effective preservative.
These days, LAB bacteriocins are used as an integral part of hurdle technology.
Using them in combination with other preservative techniques can
effectively control spoilage bacteria and other pathogens, and can
inhibit the activities of a wide spectrum of organisms, including
inherently resistant Gram-negative bacteria.
Hurdle technology
Hurdle technology is a method of ensuring that pathogens in food products
can be eliminated or controlled by combining more than one approach.
These approaches can be thought of as "hurdles" the pathogen has to
overcome if it is to remain active in the food. The right combination of
hurdles can ensure all pathogens are eliminated or rendered harmless in
the final product.
Hurdle technology has been defined by Leistner (2000) as an intelligent combination of hurdles that secures the microbial safety and stability as well as the organoleptic and nutritional quality and the economic viability of food products. The organoleptic quality of the food refers to its sensory properties, that is its look, taste, smell, and texture.
Examples of hurdles in a food system are high temperature during processing, low temperature during storage, increasing the acidity, lowering the water activity or redox potential, and the presence of preservatives or biopreservatives.
According to the type of pathogens and how risky they are, the
intensity of the hurdles can be adjusted individually to meet consumer
preferences in an economical way, without sacrificing the safety of the
product.
Principal hurdles used for food preservation (after Leistner, 1995)
With respect to cultural heritage, conservation science
is the interdisciplinary study of the conservation of art,
architecture, technical art history and other cultural works through the
use of scientific inquiry. General areas of research include the
technology and structure of artistic and historic works. In other words,
the materials and techniques from which cultural, artistic and historic
objects are made. There are three broad categories of conservation
science with respect to cultural heritage: 1) understanding the
materials and techniques used by artists, 2) study of the causes of
deterioration, and 3) improving methods/techniques and materials for
examination and treatment. Conservation science includes aspects of chemistry, physics and biology, engineering, as well as art history and anthropology. Institutions such as the Getty Conservation Institute
specialize in publishing and disseminating information relating to both
tools used for and outcomes of conservation science research, as well
as recent discoveries in the field.
Introduction
Prior
to thorough scientific analysis, a detailed visual assessment of the
object, heritage site, or artwork is necessary in addition to gathering
all relevant historic and current documentation. Diagnosing the current state in a non-invasive way allows both conservators and conservation scientists
to determine exactly what further analysis would be required and
whether the subject of the study will be able to withstand more rigorous
examination. Additionally, since the goal of conservation-restoration
is to only do the minimum required for preservation, this initial
assessment falls in line with the American Institute for Conservation (AIC) Code of Ethics which outlines best practice for conservators and scientists alike.
Along with assessing the current state and potential risk of
future deterioration of artworks and objects, scientific study may be
necessary to determine if there is risk to the conservators themselves.
For example, some pigments used in paintings contain highly toxic
elements such as arsenic or lead and could be hazardous to those working
with them.
Alternatively, previous restoration efforts may have involved chemicals
that are now known to have dangerous side affects with prolonged
exposure.
In these cases, conservation science may reveal the nature of these
hazards as well as present solutions for how to prevent current and
future exposure.
Material properties
Research
into the chemical and physical properties intrinsic to the materials
used to create cultural heritage objects is a large part of the study of
conservation science. Materials science,
in conjunction with the broader field of restoration and preservation,
has resulted in what is now recognized as modern conservation.
Using analytical techniques and tools, conservation scientists are able
to determine what makes up a particular object or artwork. In turn,
this knowledge informs how deterioration is likely to occur due to both
environmental effects and the inherent traits of that given material.
The necessary environment to maintain or prolong the current state of
that material, and which treatments will have the least amount of
reaction and impact on the materials of the objects being studied, are
the primary goals of conservation research. Conservation treatments fall under four broad categories including cleaning, desalination, consolidation, and deinfestation.
Knowledge of the material properties of cultural heritage and how they
deteriorate over time helps conservators formulate actions to preserve
and conserve cultural heritage.
In many countries, including the United Kingdom and Italy, conservation science is considered part of the broader field called 'Heritage Science' which also encompasses scientific aspects less directly related to cultural heritage conservation, as well its management and interpretation.
Paper
The majority of paper is made up of cellulose fibers.
The deterioration of paper may be the result of pests such as vermin,
insects, and microbes, or by theft, fire, and flood. More specifically,
paper deteriorates from two mechanisms that alter its hue and weaken its
fibers: acid-catalyzed hydrolysis and oxidation. Treatment for paper includes deacidification, bleaching and washing.
Safe environments for the storage and display of paper artifacts include having a relative humidity (RH) of below 65% and above 40% and an ideal temperature between 18-20 °C (64-68 °F).
Textiles
Textiles
are woven fabrics or cloth that represent culture, material legacy of
international trade, social history, agricultural development, artistic
trends, and technological progress. There are four main material sources: animal, plant, mineral, and synthetic. Deterioration of textiles can be caused by exposure to ultraviolet (UV) or infrared light (IR), incorrect relative humidity and temperature, pests, pollutants, and physical forces such as fire and water.
Textiles may be treated in a number of ways including vacuuming, wet
cleaning, dry cleaning, steaming, and ironing. To preserve the integrity
of textiles, storage and display environments result in as little light
exposure as possible. Safe environments for textiles include those with
a temperature of around 21 °C (70 °F) and relative humidity of 50%.
Leather
Leather is a manufactured product made from the skin of animals. Leather can deteriorate from red rot,
excessive dryness resulting in cracking and breakage, fading from
exposure to light, mold resulting in odors, stains, and distortion, and
insects and dust, both of which can cause holes and abrasions. Corrosion can also occur when leather comes into contact with metals.
There are two primary methods for leather conservation: application of
dressings or treatments to prolong the life of the leather and improving
the means by which leather is stored. The second method is a preventive
approach while the first, an older method, is an interventive approach. Leather artifacts are best stored with relative humidity between 45% to 55% and a temperature of 18-20 °C (64-68 °F).
Glass and ceramics
Glass and ceramics
can be maintained for much longer periods of time and are two of the
most durable materials. The biggest risk to glass and ceramics is
breakage, however improper display and storage can lead to stains and
discoloration. Ceramics can become stained from inappropriate cleaning
and repair while porous or cracked ceramics can develop stains from
being soaked in water during cleaning. Increased temperatures can cause
darkening of already existing stains and can lead to cracks. Glass can
become damaged from 'weeping glass' wherein droplets of moisture form on
glass surfaces. This can lead to a leaching out of unstable components
that produce an alkaline solution. If allowed to remain on the glass for an extended period of time, this solution can produce fine cracks known as crizzling.
Careful handling and storage is the surest means to preventing damage
to glass and ceramics. The below table displays recommended storage
conditions for damaged and unstable objects:
Weeping glass
Temperature and relative humidity
18-21 °C (65-70 °F), 40%
Crizzling glass
Temperature and relative humidity
18-21 °C (65-70 °F), 55%
Archaeological ceramics
Temperature and relative humidity
18-21 °C (65-70 °F), 45%
Metals
Metals are produced from ores that are found naturally in the environment. Most metal objects are made from a combination of individual metals called alloys
and exhibit different strengths and colors based on their composition.
Metals and alloys commonly found in cultural objects include gold, silver, copper, pewter, tin, and iron. The most common form of deterioration for metal is corrosion. Corrosion occurs when metals come into contact with water, acids, bases, salts, oils, polishes, pollutants and chemicals.
Mechanical damage, breakage, dents, and scratches can occur from
mishandling metal objects and result in damage to the metal object. Over
polishing can lead to deterioration and potentially misidentification
by removing plating, decoration, makers' marks, or engravings.
Mechanical, electrical, and chemical interventions are often used in the
treatment of metals. Appropriate storage of metal objects helps to
increase their longevity; it is recommended that metal objects be stored
in closed systems with well-sealed doors and drawers with relative
humidity between 35 and 55%.
Plastics
Plastics experience degradation from several factors including light, ultraviolet radiation,
oxygen, water, heat, and pollutants. There are no international
standards for the storage of plastics so it is common for museums to
employ similar methods to those used to preserve paper and other organic
materials. A wide range of instruments and techniques can be used in
the treatment of plastics including 3-D scanning and printing
technologies as a means of reproducing broken or missing parts.
Recommended relative humidity for plastics is 50% along with a
temperature of 18–20 °C (64-68 °F).
Stone
Stone
objects take on many forms including sculpture, architecture, ornamental
decoration, or functional pieces. Deterioration of stone depends on
several factors such as the type of stone, geographical or physical
location, and maintenance. Stone is subject to a number of decay
mechanisms that include environmental, mechanical, and applied decay. Erosion
from air, water, and physical touch can wear away surface texture.
Carved stone should not be regularly cleaned as cleaning can cause
deterioration by opening its pores as well as removing surface features
such as engravings, artists' tools, and historical marks. Dirt, moss,
and lichen do not usually cause decay to stone but may add to its patina.
Wood
Wood is a biodegradable,
organic material that is susceptible to deterioration from both living
organisms and environmental factors. Some ancient wood is recognized for
its archaeological value and falls into two categories: dry and
waterlogged.
The recommended temperature for storage and display of wooden artifacts
is 21 °C (70 °F) during the winter months and 21-24 °C (70-75 °F)
during the summer months. The recommended relative humidity for storage
and display of wooden artifacts during the winter months is 35%-45% and
55%-65% during the summer months. Effective cleaning of wooden artifacts includes waxing, polishing, dusting, and buffing.
Paintings
Painting materials include acrylic paint, oil paint, egg tempera, lacquer, water color, and gouache. Conservation techniques for paintings include dirt and varnish removal, consolidation, structural treatments, in-painting, in-filling, and retouching of losses. It is recommended that paintings be stored with other heritage and art collections.
Mechanisms of deterioration
Conservation science studies the process by which the various mechanisms of deterioration cause changes to material culture that affect their longevity for future generations. These mechanisms may produce chemical, physical, or biological changes and differ based on the material properties of the subject at hand.
A large portion of conservation science research is the study of the
behavior of different materials under a range of environmental
conditions. One method used by scientists is to artificially age objects in order to study what conditions cause or mitigate deterioration.
The results of these investigations informs the field on the major risk
factors as well as the strategies to control and monitor environmental
conditions to aid in long term preservation. Further, scientific inquiry
has led to the development of more stable and long-term treatment
methods and techniques for the types of damages that do occur.
Fire
Fire is caused by chemical reactions resulting in combustion. Organic material such as paper, textiles, and wood are especially susceptible to combustion. Inorganic material, while less susceptible, may still suffer damage if exposed to fire for any period of time.
The materials used to extinguish fires, such as chemical retardants or
water, can also result in further damage to material culture.
Water
Water
primarily causes physical changes such as warping, stains,
discoloration, and other weakening to both inorganic and organic
materials. Water can come from natural sources such as flooding, mechanical/technological failures, or human error.
Water damage to organic material may lead to the growth of other pests
such as mold. In addition to the physical effects of water directly on
an object or artwork, moisture in the air directly affects relative
humidity which can in turn exacerbate deterioration and damage.
Light
Light causes cumulative and irreversible damage to light-sensitive objects.
The energy from light interacts with objects at the molecular level and
can lead to both physical and chemical damage such as fading,
darkening, yellowing, embrittlement, and stiffening. Ultraviolet radiation and Infrared radiation, in addition to visible light,
can be emitted from light sources and can also be damaging to material
culture. Cultural institutions are tasked with finding the balance
between needing light for patrons and guests and exposure to the
collection. Any amount of light can be damaging to a variety of objects
and artworks and the effects are cumulative and irreversible.
Conservation science has helped establish 50 Lux
as the benchmark level of light intensity that allows the human eye to
operate within the full range the visible light spectrum.
While this is a baseline for many museums, adjustments are often needed
for based on specific situations. Conservation science has informed the
industry on the levels of light sensitivity of common materials used in
material culture and the length of time permissible before
deterioration is likely to occur.
Control strategies must be considered on an item by item basis. Light,
ultraviolet, and thermometers for infrared radiation are some of the
tools used to detect when levels fall outside of an acceptable range.
Incorrect relative humidity
Relative humidity (RH) is the measure of the humidity, or the water vapor content, in relation to the atmosphere and ranges from damp to dry.
Material properties determine the affect that different levels of RH
can have on any particular item. Organic materials like wood, paper, and
leather, as well as some inorganic material like metals are susceptible
to damage from incorrect RH.
Damage ranges from physical changes like cracking and warping of
organic materials to chemical reactions like corrosion of metals.
Temperature has a direct effect on relative humidity: as warm air
cools, relative humidity increases and as cool air warms up, relative
humidity falls.
Dampness can cause the growth of mold which has its own damaging
properties. Research in the field has determined the various ranges and
fluctuations of incorrect humidity, the sensitivity of various objects
to each one, and has helped establish guidelines for proper
environmental conditions specific to the objects in question.
Incorrect temperature
Material properties directly determine the appropriate temperature
needed to preserve that item. Incorrect temperatures, whether too high,
too low, or fluctuating between the two, can cause varying levels of
deterioration for objects. Temperatures that are too high can lead to chemical and physical damage such as embrittlement,
cracking, fading, and disintegration. Too high temperatures can also
promote biological reactions like mold growth. Temperatures that are too
low can also result in physical damages such as embrittlement and
cracking. Temperature fluctuations can cause materials to expand and contract rapidly which causes stress to build up within the material and eventual deterioration over time.
Pests
Pests include microorganisms, insects, and rodents and are able to disfigure, damage, and destroy material culture.
Both organic material and inorganic material are highly susceptible.
Damage can occur from pests consuming, burrowing into, and excreting on
material.
The presence of pests can be the result of other deterioration
mechanisms such as incorrect temperature, incorrect relative humidity,
and the presence of water. Fumigation and pesticides
may also be damaging to certain materials and requires careful
consideration. Conservation science has aided in the development of
thermal control methods to eradicate pests.
Pollutants
Pollutants consist of a wide range of compounds that can have detrimental chemical reactions with objects. Pollutants can be gases, aerosols, liquids, or solids
and are able to reach objects from transference from other objects,
dissipation in the air, or intrinsically as part of the object's makeup.
They all have the potential to cause adverse reactions with material
culture.
Conservation science aids in identifying both material and pollutant
properties and the types of reactions that will occur. Reactions range
from discoloration and stains, to acidification and structural weakening. Dust is one of the most common airborne pollutants and its presence can attract pests as well as alter the object's surface.
Research in the field informs conservators on how to properly manage
damage that occurs as well as means to monitor and control pollutant
levels.
Physical forces
Physical forces
are any interaction with an object that changes its current state of
motion. Physical forces can cause a range of damage from small cracks
and fissures to complete destruction or disintegration of material. The level of damage is dependent on the fragility, brittleness, or hardness
of object's material and the magnitude of the force being inflicted.
Impact, shock, vibration, pressure, and abrasion are a few examples of
physical forces that can have adverse effects on material culture.
Physical forces can occur from natural disasters like earthquakes,
working forces like handling, cumulative forces like gravity, or
low-level forces like building vibrations. During an object's risk assessment,
the material properties of the object will inform the necessary steps
(i.e. building, housing, and handling) that need to take place to
mitigate the effects of physical forces.
Theft and vandalism
Theft, the removal of an asset, and vandalism,
the deliberate destruction or disfigurement of an asset, are directly
controlled and limited by the security measures put in place at a
cultural institution. Conservation science can aid in the authentication
or identification of stolen objects. In addition, the research of the
field can help inform decisions as to the best course of action repair,
minimize, or mitigate damage from vandalism.
Dissociation
Dissociation is the loss of an object, its associated data, or its value due to outside influence.
Adherence to proper policies and procedures is the best defense against
dissociation and as such, meticulous record keeping is the basis for
all good practice. Conservation science aids in the authentication or
identification of misplaced objects and detailed records of all past,
present, and future study is necessary for the prevention of
dissociation.
Methods
Optical
microscope used to visually study very small paint fragments (mounted
in epoxy) as a means of identifying paints used by artists.
Prior to any type of scientific analysis, detailed documentation
of the initial state of the object and justification for all proposed
examinations is required to avoid unnecessary or potentially damaging
study and keep the amount of handling to a minimum. Processes such as stereomicroscopy can reveal surface features such as the weave of parchment paper, whether a print was done in relief or in intaglio, and even what kind of tools an artist may have used to create their works.
While there are many different specialized and generic tools used for
conservation science studies, some of the most common are listed below.
Scientific equipment
X-ray fluorescence spectroscopy (XRF) of the wooden, painted portrait of a Roman portrait mummy.
The portable tool is hooked up to a rig that allows it to pan left and
right, up and down, so as to scan the entire surface of the portrait.
The height can also be manually adjusted to ensure focus is maintained.
This technique provides information on the paints used which aids in provenance and compositional studies.
Method of surface imaging whereby the location of the light
source can be changed to image so an object or artwork is illuminated
from a variety of directions
Method for identifying materials in works of art based on the
fact that each compound or element has a specific combination of atoms,
each of which will have a unique peak in the resultant spectra
Non-invasive and non-destructive method for chemical analysis that
requires very small quantities of sample from inconspicuous locations on
artworks and objects
The type of material present will be the deciding factor in what method will be appropriate for study.
For example, organic materials are likely to be destroyed if exposed to
too much radiation, a concern when doing X-ray and electron-based
imaging. Conservation scientists may specialize with specific materials and work closely with conservators and curators in order to determine appropriate analysis and treatment methods.
Electron Backscatter Diffraction
(EBSD) can provide better contrast within the microscope in order to
better visualize different phases, materials, and compounds present to
identify composition
Can help to determine paint composition (specific type of paint used) in art works and compounds that may aid in provenance queries
Allows scientists to analyze whether the object's appearance merits preservation or if there are products of deterioration and decay that ought to be removed or cleaned prior to preservation
Destructive/invasive method - requires obtaining a sample from an object or artwork and exposing it to X-Ray radiation
