FOOD SPOILAGE MICROORGANISMS

Chapter 16

• What are spoilage bacteria?Spoilage bacteria are microorganisms too small to be seen without a microscope that cause food to deteriorate and develop unpleasant odours, tastes, and textures. These one-celled microorganisms can cause fruits and vegetables to get mushy or slimy, or meat to develop a bad odour.

•Do spoilage bacteria make people sick?Most people would not choose to eat spoiled food. However, if they did, they probably would not get sick.

Pathogenic bacteria cause illness. They grow rapidly in the "Danger Zone" – the temperatures between 40 and 140 °F – and do not generally affect the taste, smell, or appearance of food. Food that is left too long at unsafe temperatures could be dangerous to eat, but smell and look just fine. E. coli O157:H7, Campylobacter, and Salmonella are examples of pathogenic bacteria.

• How do bacteria spoil food?There are different spoilage bacteria and each reproduces at specific temperatures. Some can grow at the low temperatures in the refrigerator or freezer. Others grow well at room temperature and in the "Danger Zone." Bacteria will grow anywhere they have access to nutrients and water. Under the correct conditions, spoilage bacteria reproduce rapidly and the populations can grow very large. In some cases, they can double their numbers in as little as 20 minutes. The large number of microorganisms and their waste products cause the objectionable changes in odour, taste, and texture.

• Food can deteriorate as a result of two main factors:

• 1) growth of micro-organisms - usually from surface contamination - especially important in processed food - see below2) action of enzymes - from within cells - part of normal life processes, (responsible for respiration, for instance). It is important to note that many plants - fresh vegetables and fruit - are still alive when bought and even when eaten raw, and meat from animals undergoes gradual chemical changes after slaughter.

• Micro-organisms involved are normally bacteria and fungi (including yeasts)Various members of these groups may cause changes in the character of food, which may be classed as "positive" or "negative".

Products of "positive" microbial transformations include cheese, yoghurt, and wine, which can be seen as increasing the nutritional value or keeping quality of products with a short shelf life.

• "Negative" aspects of microbial growth include food deterioration and spoilage by decay, and food poisoning, mainly caused by different and less widespread bacteria. As they grow, micro-organisms release their own enzymes into the liquid surrounding them, and absorb the products of external digestion. This is the main basis of microbial food spoilage, which lowers its nutritional value. Bacteria and moulds may also produce waste products which act as poisons or toxins, thus causing the renowned ill-effects.

• Conditions for microbial growth (and also for enzyme action)

•1) organic food (proteins, carbohydrates, fats) - same as humans!2) suitable temperature3) moisture (water)4) [in some cases] air5) suitable pH

• For numbers 2-5, this (scientific) knowledge can be applied to (technological) processes in order to control food spoilage.

1.Temperature treatments

• For each 10 °C rise in temperature, the activity of micro-organisms and enzymes increases by at least 2x, in the range 0-60 °C. Above this, heat quickly destroys enzymes and stops living cells from working.

• Decreased temperatures therefore work by slowing down these changes. The technology involved may be fairly expensive, and is needed continuously, but it does not change the flavour of food.

• In a refrigerator (about 4 °C) food keeps longer because it decays more slowly. Bacteria are not killed, but merely less active.

• In a freezer (about -15 °C) bacteria are completely inactive, but internal enzymes are still active. For this reason, frozen vegetables such as peas are blanched by treatment with boiling water before freezing.

• The cooling process may also have the effect of removing water from food, so it is necessary to wrap it to prevent dehydration.

• Increased temperatures can have a more permanent preservative effect, and only require a fairly brief treatment. They may also alter the flavour of food.

• Gentle heating (about 60-70 °C, up to boiling, 100 °C) kills most bacterial cells in a few minutes, but does not affect some species which form spores.

• It also denatures proteins, so it deactivates enzymes. Most cooking does this, and pasteurisation of milk is carefully controlled (63-66 °C for 30 minutes) to prevent flavour changes, although it does not quite kill all contaminating bacteria.

• Pressure cooking raises the temperature (usually to 120 °C or more), which kills bacterial cells in seconds, although spores need about 15-20 minutes. This is called sterilization.

• Apparatus called an autoclave is used commercially, and for preparing sterile media for growth of micro-organisms in the laboratory.Ultra-high temperatures (UHT) - 132+ °C for at least 1 second) are at the basis of treatment of liquids like milk, and fruit juices, so that these have a "long life".

• After heat treatment, it is essential to ensure that foods cannot become contaminated by contact with raw food, because they are now more easily colonised by bacteria.

2.Removal of waterMany foods can be preserved by being simply dried (dehydrated, desiccated), perhaps in the sun . Water may be added to rehydrate it before consumption, or if eaten dry, digestive juices moisten it and make it available to our bodies.

• Salted foods, and foods with added sugar are also effectively using the same technique, since the water they contain is unavailable for microbial growth. Indeed, cells of micro-organisms become plasmolysed when they come into contact with the surfaces of these foods.

• Smoking foods, as well as drying, covers the outside (most exposed to microbial contamination) with a thin film of antimicrobial chemicals.

3.Removal of airThis technique is not usually used on its own - in fact some of the worst food poisoning bacteria thrive in its absence.Vacuum packing is, however, often used in conjunction with other techniques.

4.Alteration of pHPickling, usually in vinegar or other acids, lowers the pH so that bacterial enzymes cannot operate.

• Combined treatments - provide extremely long keeping qualities.

• Freeze drying is a relatively recent method of preservation involving the removal of water (as vapour) from frozen food under reduced pressure. advantages of the product of the process.> lighter, and does not need refrigeration - lower transport & storage costs

• Canning is heat-treatment in an autoclave, together with sealing of the food in an air-tight container.

• What is food spoilage? • Food spoilage means the original nutritional value,

texture, flavour of the food are damaged, the food become harmful to people and unsuitable to eat.

• Causes of the spoilage of food

1.Microbial spoilage

2.Autolysis

3.Other factors

1.Microbial spoilage• There are three types of microorganisms that cause

food spoilage -- yeasts, moulds and bacteria. • Yeasts growth causes fermentation which is the

result of yeast metabolism. There are two types of yeasts true yeast and false yeast. True yeast metabolizes sugar producing alcohol and carbon dioxide gas. This is known as fermentation. False yeast grows as a dry film on a food surface, such as on pickle brine. False yeast occurs in foods that have a high sugar or high acid environment.

• Moulds grow in filaments forming a tough mass which is visible as `mould growth'. Moulds form spores which, when dry, float through the air to find suitable conditions where they can start the growth cycle again.

• Mould can cause illness, especially if the person is allergic to molds. Usually though, the main symptoms from eating mouldy food will be nausea or vomiting from the bad taste and smell of the mouldy food.

• Both yeasts and moulds can thrive in high acid foods like fruit, tomatoes, jams, jellies and pickles. Both are easily destroyed by heat. Processing high acid foods at a temperature of 100°C (212°F) in a boiling water canner for the appropriate length of time destroys yeasts and moulds.

• Common mould on bread• Rhizopus stolonifer • Penicillin expansum • Aspergillus niger • Chrysonilia sitophila

Rhizopus stolonifer

Penicillin expansum

Aspergillus niger

Chrysonilia sitophila

Canned food spoilage Severe swell due to entensive gas production. Note the great deformation of the can. This can is potentially dangerous, and could explode if dropped or hit!

The can shown here was dropped and the gas pressure resulted in a violent explosion. Note that the lid has been torn apart.

Fungal Spoilage

Storage rot in grapes caused by Botrytis cinerea.

Storage rot in strawberry caused by Botrytis cinerea.

Blue mould rot in tomato caused by Penicilliumi spp. (also by Fusarium spp.

Blue mould on oranges caused by Penicillium digitatum.

• Bacteria are round, rod or spiral shaped microorganisms. Bacteria may grow under a wide variety of conditions.

• There are many types of bacteria that cause spoilage. They can be divided into: spore-forming and nonspore-forming.

• Bacteria generally prefer low acid foods like vegetables and meat. In order to destroy bacteria spores in a relatively short period of time, low acid foods must be processed for the appropriate length of time at 116°C (240°F) in a pressure canner. (Temperatures higher than 100°C [212°F] can be obtained only by pressure canning.)

• Eatting spoiled food caused by bacteria can cause food poisoning.

• Soft rot in tomato caused by Erwinia carotovora.

2.AutolysisEnzymes Enzymes are proteins found in all plants and animals.

If uncooked foods are not used while fresh, enzymes cause undesirable changes in colour, texture and flavour. Enzymes are destroyed easily by heat processing.

b) Oxidation by air

Atmospheric oxygen can react with some food components which may cause rancidity or color changes.

3.Other factorsInfestations (invasions) by insects and rodents, which

account for huge losses in food stocks. Low temperature injury - the internal structures of the

food are damaged by very low temperature.

Chilling injury in cucumber caused by low temperature. Note the watery surface.

Internal mahogany browning of potato caused by low temperature injury.

Types of food decay• There are three types of food decay: • Putrefaction • Fermentation • Rancidity

• Example of food spoilage• Microorganisms involved in food spoilage • Common mould on bread • Canned food spoilage

A, Fermentation , chemical changes in organic substances produced by the action of enzymes. This general definition includes virtually all chemical reactions of physiological importance, and scientists today often restrict the term to the action of specific enzymes, called ferments, produced by minute organisms such as molds, bacteria, and yeasts. For example, lactase, a ferment produced by bacteria usually found in milk, causes the milk to sour by changing lactose (milk sugar) into lactic acid.

• Formula of fermentation:

b. Putrefaction• Biological decomposition of organic matter,

with the production of ill-smelling and tasting products, associated with anaerobic (no oxygen present) conditions.

• Formula of putrefaction:

•

• (1)Microbial rancidity• Like all food components, fats undergo

deteriorative changes with time, which result in undesirable flavors and odorus. These changes in fats are given the term "rancidity".

• Formula of microbial rancidity:

(2)Hydrolytic rancidity

• Fatty acids formed through hydrolysis of the lipid(fat) by the water which it contains. Some of the liberated fatty acids are volatile, and some have very unpleasant odours and flavours.

• Formula of hydrolytic rancidity

(3)Oxidative rancidity

• The oxidation of acylglycerols which occurs in air, without the presence of enzymes, is called autoxidation.

• Among the products of autoxidation are hydroperoxides, ROOH.

• These have no taste, but they decompose easily to form aldehydes, ketones and acids, which give oxidised fats and oils their rancid flavours.

• It can be slow down by addition of antioxidants.

The need to preserve food

• Food Preservation , processes involved in protecting food against microbes and other spoilage agents to permit its future consumption.

• The preserved food should retain a palatable appearance, flavor, and texture, as well as its original nutritional value.

• The following points are the main reasons of food presevation:

a) To protect food against microbes and other spoilage agents

b) To ensure that food is safe for future consumption c) To prolong food storage time d) To allow many foods to be available year-round, in

great quantity and the best-quality

Principles of preservations

• Micro-organisms, enzyme, chemical reaction of food components are the main causes of food spoilage.

• So the principles of preservations are: • Killing of micro-organisms • Inhibition of microbial growth • Removing micro-organisms • Destroying enzyme • Retardation of chemical changes

Conditions for the growth of micro-organismsa) Suitable Temperature : Some micro-organisms can

grow at 50 - 60oC ; others have optimum growth temperatures of 25 - 40oC , and others grow well at 10 - 20oC, and survive at 0oC ;

b) pH value : Most micro-organisms grow best at a pH of 6.6 to 7.5. Some micro-organisms grow at lower pH values. The spoilage of fruits is usually caused by yeasts and moulds, which can tolerate low pH. The spoilage of meat and fish is usually caused by bacteria ;

c) Moisture ( water activity of the food ) : The water activity of a food describes the amount of available water in a food. It is given by: water activity = (water vapour pressure above the food at a certain temperature) / (water vapour pressure above pure water at the same temperature)

• The micro-organisms cannot grow with the water activity less than 0.6 ;

d) Nutrients : to supply energy, to supply nitrogen for protein synthesis and minerals ;

e) Time : Some bacteria divide every 20 minutes. If the food supply were adequate, a single bacterium could produce 2 million offspring in 7 hours. The growth rate declines as the food supply is exhausted ;

f) A suitable atmosphere : Some micro-organisms such as mould and some bacteria cannot thrive in absence of oxygen. Yeast and and few bacteria can thrive in either the presence or the absence of oxygen.

Ways of the prevention and their side effects1. Heat treatment 2. Irradiation 3. Smoking 4. Drying and dehydration 5. Refrigeration 6. Freezing 7. Canning 8. Sugaring and Salting 9. Meat curing 10. Pickling in vinegar 11. Use of food additives 12. Filtration

1. Heat treatment

Principle• Food is heated up or cooked.• Heat kills micro-organisms and their spores,

alters protein structure, destroys enzyme activity of micro-organisms in food .

Methods of heating• Variation in temperature and treatment time

i) Blanching • a. Mild heat treatment, usually applied to fruits

and vegetables to denature enzymes • b. Often used before freezing of fruits and

vegetables

ii. Pasteurization The process is named after the French chemist Louis Pasteur, who devised it in 1865 to inhibit fermentation of wine and milk.

• a. Destroys pathogenic microorganisms and extends the shelf life of a food

• b. Pasteurized products still contain many viable organisms capable of growing and causing spoilage defects

  • c. Usually pasteurization is combined with another

means of preservation like refrigeration 　

• d. Levels of pasteurization used to thermally process milk:

– (1) Low Temperature Long Time (LTLT): 63° C (145° F) for 30 min

　 – (2) High Temperature Short Time (HTST): 72°

C(161° F) for 15 s (LTLT and HTST are equivalent thermal processes)

　 – (3) Ultra High Temperature(UHT): 138° C

(280 ° F) (or higher) for 2 s

• e. Concept of temperature-time processing conditions that give equal destruction of microorganisms

iii. Commercial sterilization • a. All pathogenic and toxin-forming organisms

are destroyed, as well as other types of organisms, which if present could grow in the food and cause spoilage under normal handling and storage conditions

• b. These foods may contain a small number of heat resistant bacterial spores, but they will not multiply under normal handling and storage conditions

　 • c.Types of commercially sterile processes:

canning, bottling, and aseptic processing – 　

• d. Most commercially sterile food products have a shelf life of 2 years or longer

Sterilization • a. Complete destruction of all microorganisms,

including both vegetative cells and spores

Side effect• High temperatures can diminish product

appearance, texture, and nutrient quality.• Examples: All forms of cooked food, milk

sterilized by UHT (ultra high temperature),beer, wine

•   Milk is pasteurized by heating at a temperature of 63° C (145° F) for 30 minutes, rapidly cooling it, and then storing it at a temperature below 10° C (50° F). Beer and wine are pasteurized by being heated at about 60° C (about 140° F) for about 20 minutes; a newer method involves heating at 70° C (158° F) for about 30 seconds and filling the container under sterile conditions.

2. Irradiation • Principle• Exposing food to ionizing radiation • Ionizing radiation or irradiation is used as a method

to destroy enzymes and micro-organisms in food; kill mould; delay ripening of fruits and vegetables; inhibit sprouting in bulbs and tubers; remove insects from grain, cereal products, fresh and dried fruits, and vegetables; and destroy bacteria in fresh meats, all with minimal effect on the nutritive value of food.

• Irradiated foods are not radioactive • a. Radiant energy disappears from the food once it

is removed from the source of ionizing radiation because the food itself never comes into direct contact with the radiation source.

• b. There is no radiation hazard from consuming irradiated foods

• Type of radiation used for food preservation is ionizing radiation

• a. Wavelengths of 2000 Angstroms (Å) (10-10) or less

• b. Includes gamma rays, x-rays, and electrons • Gamma rays: radiation emitted from the excited

nucleus of radioactive elements, such as cobalt-60 and cesium-137

• X-rays: produced by bombardment of heavy-metal targets with high-velocity electrons

• Electrons: produced by linear accelerators

The three basic techniques for radiation processing; interactions of electrons, X rays, and gamma rays in the medium

Effect on nutritional quality • Nutritional losses resulting from irradiation are

considerably less than with other food processing techniques

• The chemical reactions associated with food irradiation do, in fact, produce very small amounts of new compounds called radiolytic products

Disadvantage• If irradiated food becomes contaminated, new

pathogens have little competition and can multiply more rapidly. Therefore, strict sanitation and cooking standards, and limits on the amount of bacteria allowed in food, remain essential to ensure safe irradiated food.

• Commercially available irradiated foods must be clearly labeled

• The international symbol used to label irradiated foods is called the radura

• There are three components of the radura symbol • The solid circle represents the energy source • The clam shell figure represents the food • The five openings in the outer circle represent rays

from the energy source

• Examples: Some fruit (peaches, strawberries, tomatoes ), mushrooms, vegetables and potatoes (prevents sprouting)

3. Smoking • Principle• The smoke is obtained by burning hickory or a

similar wood under low breeze/wind at about 93oC to 104oC.

• Preservative action is provided by such bactericidal chemicals in the smoke as formaldehyde(HCHO) and creosote(antiseptic obtained from wood tar) , and by the dehydration that occurs in the smokehouse.

Side effects• When foods are smoked they absorb various

chemicals from the smoke including aldehydes and acids.

• The aldehydes causes many microbes to death and the acids, lowers the pH of the meat.

• Aldehydes are carcinogenic and people who eat a heavy diet of smoked foods suffer disproportionately from cancer of the mouth, stomach and esophagus.

• Examples: Fish (Smoked salmon), ham, and sausage

Smoked salmon

Smoked sausage

4. Drying and Dehydration • Principle• Drying removes the moisture from the food so that

bacteria, yeasts and moulds cannot grow and spoil the food. It also slows down the action of enzymes, but does not inactivate them. Dried food items can be kept almost indefinitely, as long as they are not rehydrated.

• The process of drying foods removes roughly 80 to 90 percent of the water content of fruits and vegetables. Drying food is a combination of continuous mild heat with air circulation that will carry the moisture off.

• Because drying removes moisture, the food becomes smaller and lighter in weight. When the food is ready for use, the water is added back and the food returns to its original shape.

Drying methods • Air • Oven • Microwave oven

Advantage of drying• Drying food loss only a little nutrition in the food.• Nutritional Losses by Selected Preservation

ProcessesPreservation Normal loss Processing process

of nutrition method

• Canning     60-80%     high temperatures Freezing    40-60%         water-logging and rupturing of cells

• Dehydrating 5-15%     moderate heat and

moderate air flowIn-home• Drying 3-5% low heat and gentle air

flow

Side effects• The disadvantages of this preservation method

include the time and labour involved in rehydrating the food before eating. Moreover, rehydrated food typically absorbs only about two-thirds of its original water content, making the texture tough and chewy.

Dried fruits • Dried fruits are unique, tasty and nutritious. • It might be argued that dried fruits are even

tastier than fresh fruits. • They have been called nature's candy. • Dried fruit tastes sweeter because the water has

been removed thus concentrating the fruit's flavor.

• Dried fruit can be eaten as a snack or added to cereals, muffins or ice cream.

Drying tomatos

Dried tomatos

5. Refrigeration • Principle• Refrigeration , process of lowering the temperature

and maintaining it in a given space for the purpose of chilling foods, preserving certain substances, or providing an atmosphere conducive to bodily comfort.

• Storing perishable foods, furs, pharmaceuticals, or other items under refrigeration is commonly known as cold storage.

• Chilling slows down microbial activities and chemical changes resulting in spoilage .

• In chilling, food is kept at 0oC - 4oC.

Disadvantage• Some spoilage microorganisms (psychrophiles)

may still alive, so the food cannot be stored for long time.

Differences between refrigeration and freezing • A. Refrigeration

– Provides cool storage of foods at temperatures ranging from 0oC to 4oC.

– Water in the food is not frozen • a. Consequently, the shelf life of perishable

products is extended only for days or a few weeks – Growth of nearly all pathogenic

microorganisms is prevented • b. However, some spoilage microorganisms

(psychrophiles) may thrive – 　

• B. Freezing – Provides cold storage of foods at

temperatures lower than -20oC  – Most of the water in the food

(about 95%) is frozen • a. Consequently, the shelf life of perishable

products is extended for months and, in some cases, years

• b. The rate of freezing is affected by several factors that may be controlled by the food manufacturer

• Examples: Chilled meat, fruit, vegetables, etc. in a refrigerator.

6. Freezing • Principle• Freezing turns water in food to ice. Water is

unavailable for reactions to occur, and for micro-organisms to grow .

• Freezing preserves food by preventing microorganisms from multiplying.

• Because the process does not kill all types of bacteria, however, those that survive reanimate in thawing food and often grow more rapidly than before freezing.

• In freezing, food temperature is reduced to about -17oC .However, the freezing compartments of some home refrigerators are not designed to give a temperature of -17°C, the temperature needed for prolonged storage of frozen foods. Hold frozen foods in these compartments only a few days.

Chemical changes during freezing• Enzymes are proteins present in plants and

animals. While the plant or animal lives, enzymes help speed up the ripening and maturing processes.

• Even after we harvest plants or slaughter animals, enzyme reactions can continue and result in undesirable color, flavor and texture changes in the food.

• Freezing slows down, but does not destroy, enzymes in fruits and vegetables. That is why it is important to stop enzyme activity before freezing. The two methods you can use are blanching and adding chemical compounds such as ascorbic acid.

• Freezer

A bag of frozen vegetables will last many months without spoiling.

Changes in food texture during freezing

• Freezing involves the change of water contained in the food from a liquid to a solid (ice). When water freezes it expands, and the ice crystals formed cause cell walls of food to rupture. As a result, the texture of the product will be much softer when it thaws.

• These textural changes are most noticeable in fruits and vegetables that have a high water content. For example, when frozen lettuce thaws, it turns limp and wilted. This is the reason vegetables with a high water content, such as celery and salad greens, are not usually frozen . It is also the reason why many frozen fruits are best served while they still contain a few ice crystals. The effect of freezing on fruit tissue is less noticeable when fruit is still partially frozen.

• Textural changes due to freezing are not as apparent in products that are cooked before eating because cooking also softens cell walls. Textural changes are also less noticeable in high-starch vegetables, such as peas, corn and lima beans.

Side effect• Freezing would cause low temperature injury.

• Chilling injury in cucumber caused by low temperature. Note the watery surface.

Examples: Frozen meat, peas, vegetables, ice-cream

7. Canning

Principle• Canning, process of preserving food by heating and

sealing it in airtight containers. The process was invented (1809) by Nicolas Appert, a French confectioner.

• Heating destroys enzymes and micro-organisms. The sealing of cans ensures no micro-organism and oxygen can get in. Airtight containers make sure that no oxygen in the containers for bacteria to live and chemical changes.

Vacuum Sealing • Vacuum seal prevents air from getting back into the

product carrying with it naturally occurring microorganisms to recontaminate the food.

Advantage• Steel can can be recycled to protect the

environment.

Disadvantage• Food are not thoroughly heated or the containers

are not properly canned may result in botulism or spoilage of canned food.

• The steel can recycling process

Examples: All kinds of tinned foods, such as soup, meat, beans

8. Sugaring and Salting

Principle• Food is treated with salt, strong salt solution or

strong sugar solution• After adding salt or sugar, the water potential

outside the micro-organisms is higher than that inside the micro-organisms. As a result water essential for enzyme action and microbial growth is removed by osmosis, the microbial can't continue to live. 

Disadvantage• High concentration of salt and sugar makes the

food very salty and sweety respectively.

• Examples: Bacon, salted fish, soy sauce • Examples: Jam, fruits in heavy sugar syrup

• soy sauce

Jam

fruits in heavy sugar syrup

9. Meat curing Principle• Meat is treated with salt or strong salt solution

containing NaNO3, KNO3 and which may contain sugar and spices .

• Salt and sugar both cure meat by osmosis. In addition to drawing the water from the food, they dehydrate and kill the bacteria that make food spoil.

• Nitrite or nitrate are used for retarding rancidity, curing-pink color, killing bacterial

Two curing methods• Wet (or brine) curing and dry curing. In the wet

curing, the curing ingredients were mixed with boiling water to form a "pickling" brine. But in dry curing, the ingredients were simply rubbed into the meat several times over the period of the cure.

Side effects• The excessive intake of nitrites causes a fall in

the level of haemoglobin in the blood. In long term this leads to malnutrition and reduced lifespan. Nitrates are harmless, but when nitrates are ingested in the diet they are reduced to nitrites in the body. Then nitrites may react to form nitrosoamines.

• In the stomach, nitrites are converted into nitrous acid:

• NaNO2(aq) + HCl(aq) --> HNO2 (aq) + NaCl(aq)• Amines are formed during the digestion of

proteins. Under certain conditions, nitrousacid reacts with secondary amines to form nitrosoamine:

• HNO2 + R2NH --> R2N-N=O + H2O• Many nitrosoamines have been shown to cause

cancer in animals.

• Examples: Salted fish, pork shoulder, ham, corned beef, bacon, sausages, luncheon meat, Chinese-style preserved meats (sausage, duck and pork)

10. Pickling in vinegar Principle• Food is kept in vinegar since micro-organisms

can not grow well in low pH value solutions.• Vinegar• Ordinary vinegar containing 5 percent acetic

acid. Vinegar also contains small quantities of ash, sugars, phosphoric acid, alcohol, and glycerol.

• Acetic acid is a clear, colorless liquid with a pugnent vinegary odor.

Disadvantage • Vinegar slows the growth of spoilage bacteria,

gives flavor and softens bones. Vinegar, however, is only a temporary preservative, because enzymes continue to act, softening and spoiling the product.

• Some kind of vinegar such as apple cider vinegar will darken most vegetables and fruits.

• Examples: Sauces, pickled onions and cucumbers

Sauces

11. Use of food additives Principle• Food Additives , natural and synthetic compounds

added to food to supply nutrients, to enhance color, flavor, or texture, and to prevent or delay spoilage. Some additives can inactivates or kill micro-organisms, retard chemical spoilage.

Types of additives a) Natural additives • Some additives are manufactured from natural

sources such as soybeans and corn, which provide lecithin to maintain product consistency, or beets, which provide beet powder used as food coloring.

b) Artificial additives • Additives are not found in nature and must be man-

made. Artificial additives can be produced more economically, with greater purity and more consistent quality than some of their natural counterparts.

Functions of food additives • Preserve food • Do not preserve food

E-number• It is the reference number of food additives

• Side effect of food additives• Additives can cause allergic reactions in some

people. Colourings, in particular are thought to cause hyperactivity or asthmatic symptoms especially in children and young people.

12. Filtration

Principle• Filtration can remove microorganisms from the

liquid.• Drinks such as beer can be sterilized by filtration to

avoid quality changes which may impose from treatments such as heat sterilization.

• The fluid is filtered through millipore membrane which has pore diameter (e.g. 2 um) smaller then that of bacteria. Microorganisms can be effectively eliminated with this preservation procedure.

Food additives

•It is sometimes wrongly thought that food additives are a recent development, but there has certainly been an increase in public interest in the topic. Not all of this has been well-informed, and there are signs that commercial interests have been influenced by consumer pressure, as well as food producers manipulating the situation by marketing techniques.

• Various labelling regulations have been put into effect to ensure that contents of (processed) foods are known to consumers, and to ensure that food is fresh - important in unprocessed foods and probably important even if preservatives are used.

• E numbers are a short method of declaring some common food additives (not actual ingredients of food as such), which have been cleared as probably harmless to most people by EEC bureaucrats. However, some authorities disagree about their methods, and say that some iniduals may be susceptible to ill effects. Others distrust the system, and believe it is used to mislead. It is not universal, and other names can be substituted. Terms like natural are certainly used in a cynical way by some manufacturers. The same system covers both artificial (synthetic) and natural substances, which are not by definition without risk!Flavourings are not covered by the system, either.

• Preservatives(E nos in range 200- )

• Colourings(E nos in range 100- )

• n.b. some are "natural", e.g.>

• Many are synthetic dyes derived originally from coal tar - "azo dyes" (originally recommended because they are so stable)e.g.>

Emulsifiers and anti-oxidants(E 300- )

Microorganisms involved in food spoilage(Other than Canned Foods) with some examples of causative organisms

Food Type of Spoilage Microorganisms involved

Bread Mouldy

Rhizopus nigricans Penicillium

Aspergillus niger

Ropy Bacillus subtilis

Maple sap and syrup

Ropy Enterobacter aerogenes

Yeasty Saccharomyces

Zygosaccharomyces

Pink Micrococcus roseus

Mouldy Aspergillus Penicillium

Fresh fruits and vegetables

Soft rot Rhizopus Erwinia

Gray mold rot Botrytis

Black mold rot A. niger

Pickles, sauerkraut Film yeasts, pink yeasts Rhodotorula

Fresh meat Putrefaction

Alcaligenes Clostridium

Proteus vulgaris Pseudomonas fluorescens

Cured meat

Mouldy Aspergillus Rhizopus

Penicillium

Souring Pseudomonas Micrococcus

Greening, slime Lactobacillus Leuconostoc

Fish

Discoloration Pseudomonas

Putrefaction Alcaligenes

Flavobacterium

Eggs

Green rot P. fluorescens

Colorless rots Pseudomonas

Alcaligenes

Black rots Proteus

Concentrated orange juice

"Off" flavor Lactobacillus Leuconostoc Acetobacter

Poultry Slime, odor Pseudomonas

Alcaligenes

What is food poisoning? • Food poisoning (or food borne illness) is an illness

that you may get after eating food contaminated by certain bacteria, parasites, viruses, or chemicals.

Common causes of food poisoning• Food poisoning is commonly caused by certain

bacteria or their toxins, which are poisonous proteins produced by these bacteria.

• Example of food poisoning caused by bacteria• Botulism • Botulism is an often fatal disease that results from

eating improperly canned foods( spoiled canned food) , improperly processed, low-acid foods such as green beans, mush-rooms, spinach, olives and beef or fish contaminated with toxins released by the bacteria Clostridium botulinum.

• Staphylococcus• Staphylococcus is a bacterium which causes

pimples, boils and septic wounds. It also causes food poisoning. Staphylococcus produces a toxin in infected food. It does not form spores, and it is quickly killed by boiling. The toxin is more persistent, and food must be kept at 100°C for 30 minutes to destroy all the toxin. Many foods cannot be heated to such a high temperature, such as cream-filled pastries, and these may be a source of food-poisoning by Staphylococcus.

• Clostridium botulinum causes Botulism • Clostridium botulinum is the spore-forming bacteria

that is the cause of concern when home canning low acid foods. While the bacteria is destroyed at 100°C, it has the ability to form hardy spores that can survive boiling water processing. These spores can germinate and grow in a low acid environment in the absence of air. As they grow they give off a deadly toxin. Clostridium botulinum spores can be destroyed by heat processing at a temperature of 116°C. Foods that are low in acid such as meat, poultry, seafood or vegetables must be processed in a pressure canner in order to obtain temperatures of 116°C.

• Clostridium botulinum is not a spoilage concern in high acid foods. The spores cannot grow or produce the deadly toxin in high acid foods. High acid foods can be safely heat processed in a boiling water canner.

The symptoms of botulism • Generally, victims experience symptoms 12-36

hours after eating, although symptoms have started as long as 96 hours after an exposure. The effects of botulism include blurred or double vision, dilated pupils, droopy eyelids that are difficult to open, dry, sore mouth and throat, a weak grip, and muscle weakness. Other symptoms can include difficulty swallowing and speaking, difficult breathing, major muscle weakness and paralysis.