Biology

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Food Microbiology of Curd and its Uses

Curd is a dairy product obtained by curdling or coagulating milk with rennet or an edible acidity substance such as lemon juice or vinegar and then draining off the liquid portion called whey milk that has been left to sour (raw) milk alone or pasteurized milk with added lactic acid bacteria or yeast (Example: Lactobacillus acidophilus) will also naturally produce curds and sour milk cheese is produced this way.

The increased acidity causes the milk protein (casein) to tangle into solid masses or curds in cow’s milk, 80% of the protein and caseins (Figure 5.8).

Uses

• Enhances healthy digestion
• improves immunity
• For stronger bones and teeth
• Helps to lose weight
• Beauty benefits of curd – for healthy and Radiant skin, prevent premature wrinkles remove dark spots and dandruff.

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Food Microbiology of Yogurt – An Overview

Yoghurt or Bulgarian Milk

Yoghurt is derived from a Turkish word ‘Jugurt’ which is the most popular fermented milk in the world now – a – days. It is made from milk, skimmed milk or flavoured milk. For the preparation of yoghurt, the milk should be free from contamination. The solid content (not fat should be between 11 – 15% which can be obtained by adding skin or whole milk powder in fresh milk that normally contains 8% solids.

The product can be further improved by adding small amount of modified gums which bind water and impart thickening to the product. At this stage the size of the fat particles in the milk should be around 2µm because this improves the milk’s viscosity, product’s stability. The milk is then heated at 80-90ºC for 30 min., starter culture is added to it.

Heating improves the milk by inactivating immunoglobulins, remove excessive oxygen to produce micro aerophilic environment which support the growth of starter culture. Besides, heating also induce the interactions between whey or serum proteins and casein which increase yoghurt viscosity.

The milk is now cooled to 40-43ºC so as to allow fermentation using starter organisms such as Streptococcus salivarius sub sp. thermophilus and Lactobacillus delbruckii sub sp. bulgaricus together at a level of 2% by volume (10⁶ – 10⁷ cfu/ml).

It is to be carried out for about 4h during which lactose is converted into lactic acid, pH decreases to a level of 6.3 – 6.5 to 4.6 – 4.7. The flavour in yoghurt is due to acetaldehyde which should be present at 23 – 41 mg/kg (Figure 5.7).

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Food Microbiology of Cheese

There are about 2000 varieties of cheese made from mammalian milk. Cheese is thought to have originated in south western Asia some 8000 years ago. The Romans encouraged technical improvements and stimulated the development of new varieties during their invasion in Europe between 60 B.C and A.D. 300. The cheese name is derived from Latin name caseus (Figure 5.5).

They are two groups of cheese, fresh cheese and ripened cheese. The fresh cheese are made up of milk coagulated by acid or high heat. Example: cottage cheese. Ripened cheese are made through lactic acid bacterial fermentation and coagulated by an enzyme preparation. The curd is removed and salted and whey
is separated. The salted curd is held in controlled environment.

During this process, various physical and chemical changes occur to give a characteristic flavour and texture. So the mammalian origin of milk influences the flavour and aroma of a natural ripened cheese.

Microbiology of cheese

A large number of microorganisms plays a role in the ripening process. On the first day of cheese making process, the microbial number in the starting material ranges from one to two billion. Therefore, the production declines because of insufficient oxygen, high acidity and the presence of inhibitory
compounds that are produced as the cheese ripens.

It is mainly the action of their cellular enzymes on lactose, fat and proteins that creates the ripened cheese flavour. The gas forming culture of Propionibacterium shermanii is essential for giving swiss cheese its eye, or holes and flavour (Figure 5.6).

The specificity of cheese depends upon the varieties of microorganisms used. The process of cheese making, involves nine steps:

a. Preparing the milk
b. Forming a curd.
c. Cutting
d. Cooking
e. Separating the whey
f. Salting the residue
g. Applying microbes
h. Pressing the curd
i. Ripening the young cheese

Types of Cheese

Cheese can be divided among different categories or types, according to their firmness. There are various system for classifying cheese and there are variations within each system (Table 5.6).

Types of Cheese

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Dairy Microbiology

The area of dairy microbiology is large and diverse. The bacteria in dairy products may cause disease or spoilage. Some bacteria may be specifically added to milk for fermentation to produce products like yoghurts and cheese (Figure 5.3).

MILK

Milk is the fluid, secreted by mammals for the nourishment of their young ones. It is in liquid form without having any colostrum. The milk contains water, fat, protein and lactose. About 80 – 85% of the protein is casein. Due to moderate pH (6.4 – 6.6), good quantity of nutrients and high water content, milk an excellent nutrient for the microbial growth. (Flowchart 5.2).

Flowchart 5.2: Various products obtained from raw milk.

pH – Hydrogen ion concentration
T – Elevated temperature
H – Reduced water pressure
aw – water activity diet. It is an extremely complex mixture and usally contains (Table 5.3).

Complex mixture

Sources of Microorganisms in Milk

Three sources contribute to the microorganism found in milk the udder interior, the teat exterior and its immediate surroundings, and the milking and milk handling equipment.

Bacteria that get on to the outside of the teat may be able to invade the opening and hence the udder interior. The organisms most commonly isolated are Micrococcus, Streptococci and the diptheroid Corynebacterium bovis. Aseptically taken milk from a healthy cow normally contains low number of organisms, typically fewer than 10² – 10³ cfu ml^-1.

The udder exterior and its immediate environment can be contaminated with organisms from the cow’s general environment.

Heavily contaminated teats have been reported to contribute up to 10⁵ cfu ml^-1 in the milk. Contamination
from bedding and manure can be source of human pathogens such as E.Coli, Campylobacter, Salmonella, Bacillus spp. and Clostridia spp.

Milk – handling equipment such as teat cups, pipe work, milk holders and storage tanks is the principal source of the microorganisms found in raw milk. Micrococcus and Enterococcus.

Microbiological Standard and Grading of Milk

In India, raw milk is graded by Bureau of Indian standards (BIS) 1977. The Indian standard institute (ISI) has prescribed microbiological standard for quality of milk.

1. Coliforms count in raw milk is satisfactory if, coliforms are absent in 1:100 dilution.
2. Coliforms count in pasteurized milk is satisfactory is coliforms are absent in 1:10 dilution (Table 5.4).

Microbiological Standard and Grading of Milk

Grading of milk

The quality of milk is judged by certain standards and it is known as grading milk. Grading of milk is based upon regulations pertaining to production, processing and distribution.

This includes sanitation, pasteurization, holding conditions and microbiological standards. The U.S public health secrine publication “Milk ordinance and code” shows the following chemical, bacteriological and temperature standards for grade A milk and milk products.

Methylene Blue dye Reduction Test (MBRT)

Methylene blue dye reduction test commonly known as MBRT test is used as a quick method to access the microbiological quality of raw and pasteurized milk. This test is based on the fact that the blue colour of the dye solution added to the milk get decolorized when the oxygen present in the milk get exhausted due to microbial activity.

The sooner the de colorization, more inferior is the bacteriological quality of milk assumed to be MBRT test may be utilized for grading of milk which may be useful for the milk processor to take a decision on further processing of milk.

Procedure

The test has to be done under sterile conditions. Take 10ml milk sample in sterile MBRT test tube. Add 1 ml Methylene Blue dye solution (dye concentration 0.005%). Stopper the tubes with sterilized rubber stopper and carefully place them in a test tube stand dipped in a serological water bath maintained at 37°C, records this time as the beginning of the incubation period. Decolourization is considered complete when only a faint blue ring (about 5mm) persists at the top (Figure 5.4).

Recording of Results – During incubation, observe colour changes as follows:

a. If any sample is decolourized on incubation for 30 minutes, record the reduction time as MBRT 30 minutes.

b. Record such readings as, reduction times in whole hours. For example, if the colour disappears between 0.5 and 1.5 hour readings, record the result as MBRT 1 hour, similarly, if between 1.5 and 2.5 hours as MBRT-2 hour and so on.

c. Immediately after each, reading, remove and record all the decolourized samples and then gently invert the remaining tubes if the decolourization has not yet begun (Table 5.5).

Microbilogical Quality of Milk

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Food Preservation Methods – An Overview

Foods can be preserved by a variety of methods. It is vital to eliminate or reduce the populations of spoilage and disease – causing microorganisms and to maintain the microbiological quality of a food with proper storage and packaging. Contamination often occurs after a package or can is opened and just before
the food is served.

This can proved an ideal opportunity for growth and transmission of pathogens, if care is not taken. Preservation of food is the process by which food is stored by special methods. Cooked or uncooked food can be preserved in different ways to be used later Table 5.2. Some methods of preservation are:

Basic Approaches to Food Preservation

1. Freezing

Food kept in a refrigerator remains fresh for some day. Germs do not grow easily in cool places. We preserve food items, like milk, fruit, vegetable and cooked food by keeping them in a refrigerator.

2. Boiling

By this method, we can preserve food for a short period of time. Germs in milk are killed by pasteurization. It is done by boiling milk for sometimes and then cooling it quickly.

3. Salting

Add salt to preserve pickles and fish.

4. Sweetening

Sugar act as a preservative when added in large quantities. For example, food can be stored for a long time in the form of jams, jellies and murabbas (Figure 5.2) by adding sugar.

5. Drying

In this method, the food items are dried in sun to stop the growth of bacteria in them. Certain foods, like raw mangoes, fishes, potato chips and papads are preserved by this method.

6. Canning

In this method, food is processed and sealed in airtight cans. Food items like vegetables, seafood, and dairy product are preserved through this method.

Advantages of food preservation:-

• Germs do not grow easily in preserved food and make it safe to eat.
• Preservation enables us to enjoy seasonal fruits like strawberries and mangoes even during the off-season.

Disadvantages:-

• Excess salt and sugar are used in the preservation of food which is not good for health.
• Some methods of food preservation may lead to loss of nutrients.

Principles of Food preservation

In accomplishing the preservation of foods by the various methods, the following principles are involved.

1. Prevention or delay of microbial decomposition.

• By keeping out microorganism (asepsis)
• By removal of microorganism. Example: Filtration
• By hindering the growth and activity of microorganism Example: Low temperature, drying, anaerobic conditions or chemicals.
• By killing the microorganism Example: Heat or radiation

2. Prevention or delay of self – decomposition of the food.

• By destruction or inactivation of food enzymes Example: Blanching
• By prevention or delay of purely chemical reactions Example: Prevention of oxidation by means of antioxidants.

3. Prevention of damage because of insects, animals, mechanical causes, etc.