In this article we will discuss about:- 1. Introduction to Yogurt 2. Commercial Production of Yogurt 3. Microorganisms in Yogurt and their Specific Role 4. Yogurt Starter Culture 5. Defects.
Introduction to Yogurt:
Yogurt is one of the oldest fermented milk-product and has been an important food item of peoples in the Middle East since early times, especially in those countries bordering the eastern Mediterranean coast. Yogurt is a quick-curdled, decidedly acid preparation with little or no alcohol.
It is similar to fermented milks called “leben” in Egypt, “lebeny” in Syria, “dadhi” in India, and “mazun” in Armenia. Production of yogurt at commercial level increased rapidly in Europe early in the 20th century after Metchnikoff’s publications advocated consumption of sour milk to prolong life. Attempts to popularize yogurt in the United States and Canada were first successful in the 1940’s.
Traditionally, yogurt was prepared from milk boiled for a considerable period of time to evaporate off part of the water and thus concentrate the solids. At present, a similar effect is achieved by removing 1/4 to 1/3 of the water in a vacuum pan, by adding 4 to 5 per cent of dry skim milk solids to whole milk or by mixing appropriate quantities of whole or skim milk with ordinary condensed milk.
In some countries the product made from these concentrated milk preparations is called “Bulgarian yogurt” to distinguish it from the “milk yogurt” prepared from un-concentrated milk.
Yogurt normally has a heavy consistency and resembles to custard. Ideally the curd is smooth and firm. In addition to being acid, yogurt is said to have a mildly nut-like flavour and a typical “yogurt” aroma.
Commercial Production of Yogurt:
Commercial production methods of yogurt vary considerably in certain details, the basic process is essentially similar in all dairy plants. Good quality milk is heated to reduce its microbial content as well as to improve it generally for growth of the yogurt microorganisms.
After the heat treatment, the milk is cooled to about 48°C (118.4°F) and inoculated with 2 to 3 per cent of microbial culture. The inoculum is mixed well with the milk, the mixture is dispensed in the final retail containers, usually jars or cartons, and these are incubated at 45°C (113°F).
The final acidity desired in the yogurt depends on the tastes of the consumer, but most seem to prefer a product with a titratable acidity of 0.85 to 0.90 per cent.
To achieve this range of acidity many producers remove the yogurt from the incubator when the acidity reaches 0.65 to 0.70 per cent. Further acid formation takes place while the product is being cooled. Under the conditions described here an active microbial culture requires only 2.5 to 3.5 hours to produce the desired amount of acid.
The yogurt is cooled to about 5°C (41°F) and held at this temperature until distributed to the consumer. Under these conditions the product can be kept satisfactorily for 1 to 2 weeks.
Microorganisms in Yogurt and their Specific Role:
Streptococcus thermophilus and Lactobacillus bulgaricus are the microorganisms essential for yogurt preparation. For best results these microorganisms should be in approximately equal numbers in the culture; otherwise the yogurt will lack the most desirable consistency, flavour, and odor.
When the mixture of yogurt microorganisms is inoculated into milk, the streptococci grow much more fastly than do the lactobacilli, often outnumbering the latter by 3 or 4 to 1 at the end of the first hour of incubation at 45°C (113°F). Thus the initial acid production is due largely to the activity of Streptococcus thermophilus.
Gradually, the lactobacilli increase in numbers until at the end of the incubation period they again approximate the streptococci in numbers. Acid production during the latter part of the incubation period is accomplished by Lactobacillus.
In the microbial culture for yogurt, the function of Streptococcus thermophilus is not completely understood beyond the fact that it reproduces and grows more rapidly than Lactobacillus bulgaricus and thus initiates acid production. Some microbiologists are of the opinion that the streptococci contribute to the flavour and aroma of the final product.
Other workers, on the other hand, attribute all of the yogurt aroma and most of the flavour development to lactobacilli. Streptococcus thermophilus is considered to improve the body of yogurt by reducing the viscosity characteristic of milk cultures of Lactobacillus bulgaricus. Whatever its specific role may be, Streptococcus thermophilus is necessary for production of good quality yogurt.
The function of Lactobacillus bulgaricus is better understood than is that of the Streptococcus thermophilus. It has been shown that the bacilli stimulate growth of Streptococcus thermophilus by releasing essential amino acids, especially valine, from the milk proteins.
It has been further shown that Streptococcus thermophilus grown in association with Lactobacillus bulgaricus produces considerably more acid per cell than does the same culture when grown alone. The culture of lactobacilli produces enough acid to give the product its desired final characteristics and in addition releases the volatile products responsible for the typical yogurt flavour and aroma.
Yogurt Starter Culture:
Dairy microbiologists suggest that yogurt producers should obtain pure cultures of Streptococcus thermophilus and Lactobacillus bulgaricus, carry them separately, and then mix the two for preparation of the final yogurt starter culture.
The separate pure cultures can be obtained either from stock culture collections or by isolating them from a good yogurt culture. But most commercial yogurt starter cultures are supplied as mixtures of the desired microorganisms.
Distributors of these cultures usually advise Yogurt producers to obtain new starter cultures each week or two, rather than attempt to carry the cultures in the dairy plant and run the risk of changes in the characteristics of the starter culture.
Yogurt starter cultures should be carried in sterile skim or whole milk. To maintain the optimum 1: 1 ratio between the two bacterial species, inoculation should be at the rate of 2 to 3 per cent, incubation should be at 45°C (113°F), and the final acidity should be 0.85 to 0.90 per cent. Preferably the culture should be transferred daily. Bulk starter culture is prepared in the same way, using larger quantities of milk.
Defects that Creep in Yogurt:
1. Perhaps the most common defect that creeps in the yogurt is the absence of typical yogurt flavour and aroma. Assuming that the starter culture contains the desired balance of streptococci and lactobacilli, insufficient flavour development in the final product usually results from inadequate acid formation.
Optimum flavour development takes place only after the acidity reaches about 0.85 per cent, but ripening much beyond 0.95 per cent yields a product that is too sour. Aroma compounds are formed over a considerably wider range of acidity. Absence of typical yogurt flavour and aroma also can result from use of such strains of Lactobacillus bulgaricus which produce little of the flavour and aroma substances.
2. Weak curd formation is a serious problem with yogurt made from milk of normal solids content. As per the results obtained, a weak curd is most likely to result if the solids content of the milk is low or if a significant amount of the milk is from cows early in the lactation cycle. Also, some cows give milk that inherently forms a weak curd.
Firmness of the curd from “weak curd” milks can be increased by adding 1 to 2 per cent of dry milk solids or by homogenizing the milk at about 54°C (129.2°F) and a pressure of 3,000 pounds per square inch just before inoculation. Addition of a small amount of rennet also increases firmness, but it harms the flavour and texture of the yogurt.
3. Undesired bitter flavours in yogurt sometimes result from the use of poor quality milk or contaminated starter culture. Certain strains of Lactobacillus bulgaricus can also cause bitter taste in yogurt.
4. Slow acid formation by yogurt cultures has been traced to bacteriophage-attack to the cells of Streptococcus thermophilus. Although phage-resistant cultures have been developed, but the body of yogurt produced with these cultures is not as firm as is desired.