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