UNIT 3 - FOOD SPOILAGE AND FOOD PRESERVATION

Foods undergo deterioration or spoilage from the time they are harvested, slaughtered or manufactured. Foods undergo physical, chemical and biological changes and make them unfit for human consumption.

Number of causes are responsible for food deterioration. These include:

1. Micro-organisms
2. Activities of enzymes present in food
3. Insects
4. parasites
5. rodents
6. temperature
7. moisture
8. Oxygen, light and time.

These factors are not isolated in nature. At any one time, many forms of spoilage may take place depending upon the food and environmental conditions

TYPES AND CAUSES OF SPOILAGE

1. Micro-organism:

 Bacteria, yeasts & molds spoil food after harvesting, during handling, processing & storage. The micro­ organisms are found everywhere & are always present to invade the flesh of animals & plants, when there is a cut in their skin or if the skin is weakened by disease or death.

2. Food enzymes:

Enzymes present in plant & animal foods continue to be present and are even intensified after harvest & slaughter.
Enzymes are responsible for facilitating many changes during storage such as changes in colour, texture and flavor e.g. ripening of tomatoes, tenderizing of meat on ageing are desirable, but if proceeded too far can result in food spoilage if not halted at the appropriate time.
The enzymes need to be inactivated by suitable method at appropriate time to prevent food spoilage.

3. Insects, parasites & rodents:

Insects are destructive to cereal grains, fruits & vegetables. The loss of food due to insects destruction varies from 5-50% depending upon the care taken in the field & storage. Insects are generally controlled by fumigation with ethylene oxide & propylene oxide.
Parasitic food spoilage occurs in some foods. Pigs eat uncooked food waste, the parasitic nematode penetrates the pig's intestine finds its way into pork. The live worms can infect man if the pork is not thoroughly cooked.
Infected water and poor hygiene also spread the parasites. Cooking kills most of these parasites.
Rodents contribute substantially to food spoilage. Rats, cockroaches, rodents urine and drippings harbour several kinds of disease producing bacteria and rats spreads such human disease as typhus fever, plague, typhoid fever etc.

5. Temperature

Heat and cold contribute to food spoilage if not controlled. The rate of chemical reaction doubles itself for every 10° C rise in temperature. Excessive heat brings about protein denaturation, destroy vitamins, break emulsions and dries out food by removing moisture.
Freezing and thawing of fruits and vegetables destroy their structure.

6. Moisture

Foods with high % water spoil fast. Perishable foods have a high water content. Control of moisture in foods is thus very important from the point of view of their preservation.

7. Oxygen,  Light and Time

Air and oxygen bring about a number of changes in food components such as destruction of food colour, flavor, vitamin A & C and other food constituents .
Oxygen is to be excluded from in the course of processing while deareat ion, vacuum packing or flushing containers with nitrogen or carbon dioxide.
Light destroys vitamin B2, A and C.
It also deteriorates many food colours. Foods may be protected from light by impervious packing or keeping them in containers that screen out specific wavelengths. Foods spoilage is time dependent. The larger the time. the greater the destructive influences .

SOURCES OF CONTAMINATION

1. Contamination from the Source

Meat, raw veggies, eggs and milk can be contaminated with e-coli, salmonella or other food borne pathogens before they even reach the processing plant. These products are exposed while in or on the ground through contact with animal feces, bacterial infection, and contaminated ground water. According to the Center for Disease Control and Prevention, eggs are often contaminated before they are deposited by the hen, through salmonella present in the hen’s ovaries.

2. Contact in Pre-Processing

Slaughterhouses are notorious for co-mingling meats before and after slaughter, thus causing contamination. Contact with pathogens can also occur during transit, when the food is sitting idle in the back of a hot truck. Improperly cooled or heated food only heightens the problem. Food that only contains a few microbes can become a teaming piece of bacterial heaven after sitting in transit for a few hours.

3. Problems During Processing

Contamination during processing involves environmental contaminants like metal shavings, pesticides and chemicals used to operate and maintain the machinery. Contamination can occur from improper maintenance, improper use of the machines and breaches in the ventilation system that leads to the processing area.
Most contaminants can be defeated after processing with high heat or freezing. Other types of contamination's requires immediate disposal of the food.

Spoilage of different food products

Spoilage of Cereals and Cereal Products

The exterior of harvested grains retain some of the natural flora plus contamination from soil, insects & other sources e.g.
Washing & milling reduces microorganisms.
Blending & conditioning increases contamination.
Cereal products
Wheat flour - bacteria
Corn meal - molds
Bread - a freshly baked loaf is practically free of viable microorganisms, but mold spores contaminate during cooking & before wrapping slicing by knives also contaminates.

MILK AND MILK PRODUCTS

Milk contains few bacteria when it leaves the udder of healthy cow.
Contamination starts from the animal especially the exterior of the adjacent areas. Bacteria found in manure, soil & water may enter from this source .
Microorganisms from milking machine, when milking by hand.
Contamination from dairy utensils & milk contact surfaces like milk oil or milking machines. bulk milk cooler .
MILK PRODUCTS
BUTTER: - microorganisms from churner, from water used in its washing, old cream & packaging material.
Dry milk, evaporated milk & sweetened condensed milk may be contaminated from special equipments used in their preparation.
Cheese - it is contaminated from air, brine, tanks, shelves & packaging material.
Ice cream - organisms may be added to ice cream in the ingredients.

MEAT

The healthy inner flesh meat contains few or no microorganisms although they have been found in lymph nodes, bone marrow & even flesh. Normal slaughtering practices would remove the lymph nodes from edible parts. Contamination comes from external sources during bleeding, handling and processing. During bleeding, skinning and cutting the main sources of microbes is the exterior of the animals (hide, hoofs and hair) and the intestinal tract.
Knives, clothes, air, hands and clothing of the workers can serve as intermediate source of containments.
During handling contamination comes from cart, boxes, and contaminated meat, from air and from personals.
Grinders ,sausages stutters, slicing, casing and ingredients are the sources.

EGGS

Most freshly laid eggs are sterile but the shells of some become contaminated by faecal material from the hen, by the lining of the nest, by wash water, by handling the materials in which eggs are packed.

FRUITS AND VEGETABLES

Spoilage occur during storage, transportation while waiting to be processed, washing, mechanical damage, processes such as trimming, peeling, cutting, coring add to contamination .

CANNED  PRODUCT

Spoilage occurs by chemical, biological or both.
CHEMICAL: - by hydrogen swell resulting from the pressure of hydrogen gas released by action of acid of goods on the iron of the can, time, temperature of storage, tinning imperfection, poor exhaust etc.
BIOLOGICAL: - by microorganisms, survival of organisms after administration of the heat treatment, leakage of the container after the process permitting the entrance of microorganisms.

Basic principles of food preservation

In accomplishing the preservation of foods by the various methods the following principles are involved: -
Prevention or delay of microbial decomposition.

• By keeping out microorganisms.
• By removal of microorganisms.
• By hindering growth activities of microorganisms.
• By killing of microorganisms i.e. by heat or radiation.

Prevention or delay of self decomposition of the food:-

• By destruction or inactivation of food enzymes e.g. by blanching
• By prevention or delay of chemical reaction

Prevention of damage because of insects animals and mechanical causes

Methods of food preservation

1. Asepsis or keeping out microorganisms.
2. Removal of microorganisms.
3. Maintenance of anaerobic conditions.
4. Use of high temperature.
5. Use of low temperature.
6. Drying
7. Use of chemical preservatives
8.  Irradiation
9. Mechanical destruction of microorganisms by grinding or high pressure.
10. Combination of two or more of above methods.

1. ASEPSIS:  - keeping out the microorganisms by adopting good hygienic  practices  and sanitized environment.
2. REMOVAL OF MICRO-OGANISMS: - can be done by filtration, centrifugation (sedimentation or clarification) washing or trimming.
3. MAINTENANCE  OF ANAEROBIC CONDITION: - a complete fill, evacuation of unfilled space or replacement of air by carbon dioxide and by inert gas nitrogen.

PRESERVATION BY USE OF HIGH TEMPERATURE

• High temperatures denature proteins and inactivate enzymes required for metabolisms of the microorganisms.
• The treatment varies with microorganisms. Depending on the heat treatment employed some of the vegetative cells, most of the cells are part of bacterial spores, or all of them may be killed.
• Yeast and yeast spores: are killed at 60°C for 10 to 15 minutes
• Mold & mold spores: Are killed at 60°C in 5 to 10 minutes
• Bacteria: salmonella - 60°C for 4 minute
• E.coli - 57.3°C for 20 to 30 minutes, streptococcus thermophiles - 70°C to 75°C for 15 minutes, clostridium botulinium -   100°C for 100 to 330 minutes.

PRESERVATION BY USE OF LOW TEMPERATURE

• Low temperatures are used to retard chemical reactions and action of food enzymes and to slow down or stop the growth and activity of microorganisms in foods.
• The lower the temperature the slower the chemical reaction, enzyme action and microbial growth.
• A low enough temperature will prevent the growth of any microorganisms.
• Commercial refrigeration temperatures i.e. lower than 0°C to 7°C effectively retard growth of many food borne pathogens.
• Cold storage: - most commercial lstorage freezers are at or below - 18°C.
• Common or cellar storage lower than 15°C (for root crops,potatoes. cabbage, apples)
• CHILLING STORAGE: - it involves cooling by ice or by mechanical refrigeration. Most perishable foods including eggs,dairy products, meat, sea food,vegetables & fruits are kept at chilling storage. Temperature varies from – 0.5°C to 10°C.
• FROZEN STORAGE:  -
• Sharp freezing:  -15°C to -29°C & may take 3 - 72 hours.
• Quick freezing:
• Direct immersion of the food or the package food in a refrigerant as in the freezing of fish in brine.
• Indirect contact with the refrigerant at -17.8°C to -45.6°C.
• Air blast freezing where air at - 17 .8°C to -34.4°C is blown across the materials being frozen.

PRESERVATION BY DRYING

• Drying usually is accomplished by the removal of water but any method that reduces the amount of available moisture in a food is a form of drying.
• Method of drying includes
• Solar drying ( raisins, figs, pears, fish, rice)
• Mechanical dryers - it involves passage of heated air with controlled relative humidity over the food to be dried or the passage of the food through such air. E.g. evaporator or kiln, drum dry, spray drying, freeze dryers & vacuum dryers.

PRESERVATION BY FOOD ADDITIVES
A food additive is a substance or mixture of substances, other than the basic food stuff, which is present in food as a result of any aspect of production, processing, storage or packaging.
Those food additives which are specifically added to prevent the deterioration or decomposition of a food are referred as chemical preservatives, their enzymes activity or their genetic mechanisms. E.g. propeonic acid,

PRESERVATION BY RADIATION
Use of UV radiation
Ionizing radiation which include x rays, gamma rays, beta rays and cathode rays and microwave heating which have electromagnetic waves between infrared  and radio waves.

UNIT 2 - Micro-organisms In Food

Characteristics of micro organisms

A microorganism is an organism so small that it can only be viewed with the aid of a microscope. The use of the microscope and other laboratory techniques in the study of microorganisms has evolved as a branch of science called microbiology.
Microorganisms include:

• Fungi
• Bacteria
• Protozoa
• Viruses 
• Algae.

BACTERIA

• Bacteria are single-celled microorganisms with prokaryotic cells, which are single cells that do not have organelles or a true nucleus and are less complex than eukaryotic cells. 
• Bacteria come in a variety of shapes. The three main shapes of bacteria are coccus, spiral, and bacillus.
• Cocci are bacteria that are spherical or ovoid in shape. 
• Spiral bacteria are, as the name suggests, spiral-shaped. Spirillums are thick, tough spirals. 
• Bacilli are rod-shaped bacteria. Bacilli can be solitary or arranged together.
• Bacteria can also be other shapes such as filamentous (long and thin), square, star-shaped, and stalked. 
• Bacterial spores are seed like and they are more resistant to most processing conditions than yeast or mould spores.
• Bacteria with few exceptions, cannot grew in media as acid as those in which yeast and mold thrive. 
• Under favorable conditions bacteria can double their number every 30 min.
•  Some bacteria cannot tolerate oxygen (anaerobes) and some require oxygen for growth (aerobes). 
• Some can grow in an atmosphere devoid of oxygen but manage also in air (facultative anaerobes).

FUNGI

YEAST

• Yeast are unicellular plants(fungi) widely distributed in nature  and they grow well in slightly acidic medium in the presence of sugar and water. They are found in fruits, cereals and other food containing sugar. 
• They are also found in soil, air, on the skins and in the intestine of animals.
• They are larger than bacteria. 
• The individual cell length is about O micron .
• Most yeast is spherical and ellipsoidal. They have been used for centuries for leavening of bread and to bring about fermentation of food .
• They can be harmful to foods if they bring about undesired fermentation .

MOULD

• Moulds are multicellular filamentous fungi having a fuzzy or cottony appearance as they grow in foods. They are larger than yeast.
• They are strictly aerobes and require oxygen for growth and multiplication. 
• They grow slower than bacteria. 
• Moulds frequently grow under conditions of acidity .
• That is why they are found on jams and jellies .
• Moulds require less free moisture for growth than yeast and bacteria the absence of bright light and presence of stagnant air favour there rapid development.

VIRUS

• The viruses are extremely small ranging from 25-250 micron in size.
• Although they share some characteristics with living organisms, they are not truly alive. 
• They replicate inside a living cell using its host cells metabolism.
• There are no. of ways by which viruses may be transmitted most common of this is via direct or distance contact i.e. from host to host by touching or through short distance in air.
• Indirect transmission may occur. Also via contact with inanimate objects. By the bite of certain infected insects , by water and food .
• There are variety of ways in which viruses may enter food.
• Primary when the food product already contain virus at the time of slaughter or harvest or
• Secondary when it occurs during processing. storage or distribution of a food.

PARASITES

• An organism which lives in or on another organism (its host) and benefits by deriving nutrients at the other's expense.
• There are three types of parasites that feast on humans.
• Protozoa - one-celled organisms that live and multiply in the blood or tissue of humans. They infect the body via mosquitoes and flies and are found in soil and water.
• Helminths - parasitic flatworms, flukes, tapeworms, thorny-headed worms, roundworms, and pinworms. They live in the gastrointestinal tract, blood, lymphatic system, and other tissues.
• Ectoparasites - ticks, fleas, lice, and mites that live on the surface of a human host and attach or burrow into the skin.
• Ingestion of raw or semi-cooked food by humans can lead to infection by parasites.
•  Food like pork, beef, lamb, shellfish, vegetables act as vehicles for infection.
• The organism contaminates food when raw human excreta are used as manure for crops. Infected water poor hygiene also spread parasite.
• Cooking kills most of these parasites.

Factors affecting their growth in food (intrinsic and extrinsic)

In most cases, micro-organism utilizes our food supply as a source of nutrient for their growth. This course can result in deterioration(decay)of food. The organism not only deteriorates the food but may also pose risks of disease to the human being on consumption of such contaminated food. However, the growth of microorganisms in food may be affected by several factors like physical, chemical and biological.
These factors can broadly divide into two categories i.e.

• Intrinsic parameters or intrinsic factors
• Extrinsic parameters or extrinsic factors

Intrinsic Factors

Intrinsic parameters are natural or inherent properties of food. These parameters greatly affect the number and types of microorganism that will colonize the food and food product. Intrinsic parameters affect only microorganisms, not to the food. Intrinsic parameters of food include:-

1. pH value
2. Moisture contains(water activity)
3. Nutrients contain
4. Anti-microbial constituents
5. Biological structure

pH Value

• Every organism has a minimal, maximal and optimal pH for growth. 
• Some organism can grow better at low pH or acidic pH, some can grow in alkaline pH and while other grow at somewhat neutral pH. 
• pH influence both the growth rate and types of organism that will predominant the food.
•  In general yeast and mould are more acid tolerant than bacteria.
• Moisture content or water activity(aw)
• Microorganisms grow in aqueous solutions.
•  A term, "water activity (aw)" express the degree of availability of water in foods . 
• Foods with high water content deteriorate fast. Leafy vegetables, fruits, meat, milk deteriorate rapidly 
• Fruits & vegetables can give of moisture from respiration & transpiration even when packed in a moisture free package.
• This moisture is enough for microorganisms to grow.

Nutrients Contained

• The kinds and proportional of nutrient in food are all important in determining which micro-organism(microorganisms) is most likely to grow. 
• In general, simple compound is utilized first by the multiplying microorganisms. Carbohydrate(simple sugar) is most commonly utilized as an energy source. Protein-rich food like meat, egg, fish etc. are always spoiled by proteolytic organism because they can utilize protein as a source of energy if sugar is not available.
•  In fact, protein-rich food promotes more growth of bacteria then yeast and mould . 
• Similarlly, in general mould can grow in the higher concentration of sugar, yeast in fairly high concentration but most bacteria grow best in the low concentration of sugar.

Anti-microbial Constituents

Some foods possess naturally occurring substances which influence the activity of invading microorganisms, for example:-
In Plant
Clove
Garlic
Mustard oil
In Animal
Cow’s milk
Eggs

Biological Structure

The natural covering of some foods provides excellent protection against the energy of micro-organism and spoilage of food by such microorganisms. Natural covering of food like,
Testa of seeds
Shell of egg/nuts
Peel of fruits/vegetable
Hide of animal may limit the entry of microorganisms

Extrinsic Factors

Extrinsic parameters are environmental factors, in which food and food products are kept. Extrinsic parameters affect both micro-organism as well as food. Unlike intrinsic parameters, extrinsic parameters can be maintained and regulated well. The extrinsic parameters include:-

1. Temperature of storage
2. Relative humidity of the environment
3. Presence and concentration of gases
4. Presence and activities of micro-organism

Temperature of storage

• Temperature of storage is a highly important parameter that affects the spoilage of highly perishable food. 
• Micro-Organisms are reported to grow between -340c to 1000c and each organism exhibit a minimum, optimum and maximum temperature for growth and these are known as cardinal temperature. 
• Yeast and mould can grow at the temperature range of 20 to 300c.
• Most bacteria can grow well at ordinary temperature(370c) ,however, some(thermophiles) grow at high temperature and other(psychrophilic) grow at low temperature.
• Microorganisms have optimum growth temperature .They do not grow above or below a specific range of temperature. Bacteria can grow and survive under more extreme conditions than those tolerated by any of the molds or yeasts. Bacteria's are classified as:-
• Psychrophiles - (0-20°C) 
• Mesophiles - (20-45°C) 
• Thermophiles - (45-60°C)
• Moulds can grow and can survive under more extreme conditions than can the yeasts.

Relative humidity

• Humidity is the concentration of water vapour in the atmosphere.
• Relative humidity is the ratio expressed as the percentage of moisture in the air to the moisture present in food.
• Relative humidity and water activity are inter-related i.e. when food with low water activity are stored in the environment of high humidity, water will transfer from gas phase (air) to the food and thus increased water activity of the food ,leading to spoilage by viable micro-organisms.

Presence and concentration of gases

• Presence of different gases and its varying concentration may significantly affect the colonizing micro organisms on the food i.e. surface spoilage is prevented by altering the gaseous composition.
• Oxygen is one of the most important gases which affects both food products as well as micro organisms. Oxygen gas when comes in contact with food, influence redox potential of food and finally the microbial growth.
• Ozone added to food as a preservative action on certain food. Ozone has GRAS(generally recognized as safe) status in the US, effective range is 1-5 ppm. 
• However, it has some demerits like strong oxidizing agents, causes the rancidity of high lipid-containing food.

Presence and activity of micro-organism

Inhibition or destruction of one population of micro-organism by the presence of other population of micro-organism present in the same habitat is the microbial interference.
Some Micro-organisms produced substances/metabolites (like secondary metabolites), that are either lethal or inhibitory to others.

MICROBIAL GROWTH CURVE

Microbial growth curve  - A curve on a graph that shows the changes in size of a bacterial population over time.

The curve is divided into phase as indicated in the figure

• The initial lag phase, during which there is no growth or even a decline in numbers. The phase of positive acceleration the rate of growth is continuously increasing.
• The log or exponential phase of growth . during which the rate of multiplication is most rapid and is constant. 
• The maximal stationary phase  where numbers remain constant.
• The death phase or phase of decline, during which numbers decrease at a faster rates then new cells are formed.

Application of Growth Curve

Foods preservation is best by lengthening lag phase & the phase of positive.

acceleration. This can be done by: 

1. By introducing as few spoilage organisms as possible i.e. by reducing the amount of contamination, the fewer organisms present, the longer the lag phase
2. By avoiding the addition of activity growing organisms
3. By one or more unfavorable environmental conditions .
4. By actual damage to organism by processing methods such as heating or irradiation