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Future Of The Dairy Industry

Starter cultures are food grade microorganisms, which upon culturing in milk or milk based mixes, impart desirable and predictable characteristics of flavour and textures attributable to a certain fermented milk product. The microorganisms coming under the group starter culture may consist of lactic acid bacteria (LAB), propionibacteria, surface ripening bacteria and yeasts and moulds.


These microorganisms have multifunctional role in the fermentation sector of the dairy industry. The rapidity with which the starter cultures produce lactic acid, helps in curd separation from whey during the cheese making process, modifies the texture of the cheeses and fermented milks, and improves preservation. Production of compounds such as diacetyl, acetoin and 2,3 butylene glycol, which are low in molecular weight, adds to the flavour and aroma of the fermented products.


In some special varieties of cheese, such as Swiss cheese, gas production is used for eye formation, which is regarded as desirable. The enzymes originating from the bacterial and fungal cultures add to the flavour development and texture changes during ripening of cheese.


Starter cultures may be classified depending on their temperature of growth as follows.


1. Mesophilic bacteria - they require optimal growth temperatures between 20 - 30C.


2. Thermophilic bacteria - they require optimal growth temperatures between 40 - 45C.


IMPORTANT CHARACTERISTICS OF COMMON LACTIC ACID BACTERIA


The important lactic acid bacteria can be classified in to four genus as follows.


I. Lactococcus


Formerly known as streptococci, the lactococci form the majority of mesophilic organisms (optimum temperature of growth between 25 and 30C) which are used mainly for acid production in the dairy industry. Among all the bacteria in this group, Lactococcus lactis subspecies lactis is an important organism known for its versatile applications. It is gram positive, cocci, usually occur in chains, though single and paired occurrence is not uncommon. They are homofermentative in nature and when grown in milk, lactic acid (L form) forms 95% of the end product.


There are two subspecies available viz. Lactococcus lactis subspecies lactis and Lactococcus lactis subspecies cremoris, the former being more heat resistant and salt tolerant than the latter. There is another variant of Lactococcus lactis subspecies lactis known as Lactococcus lactis subspecies lactis biovar diacetylactis which converts citrate present in milk in to diacetyl, acetoin and 2,3 butylene glycol along with carbon dioxide and other compounds, responsible for the characteristic aroma.


Diacetyl is the principal flavour compound in cultured buttermilk, curd or dahi, butter, cottage cheese and it is commonly referred to as butter flavour. Lactococcus lactis subspecies cremoris is mainly used as Cheddar cheese starters.


II. Streptococcus


The only species of importance to the dairy industry under the genus Streptococcus is Streptococcus thermophilus. It is relatively heat resistant and grows well at high temperatures up to 52C. It is generally used in combination with Lactobacillus bulgaricus in the preparation of yoghurt, mozzarella and emmental cheese where the temperature of incubation generally exceeds 40C. Known for its ability to have associative interaction with Lactobacillus bulgaricus, the extracellular polysaccharide synthesizing species are used in the manufacture of high viscosity stirred type of yoghurt.


III. Leuconostoc


Two species of leuconostocs are important as dairy starters. They are Leuconostoc mesenteroides subspecies cremoris and Leuconostoc mesenteroides subspecies lactis. These organisms are heterofermentative in nature and produce carbon dioxide and ethanol in addition to lactic acid as end products. The problem with leuconostoc is that they are slow growers when cultured alone in milk. Their growth can be influenced by the metabolic end products of other bacteria when co cultured along with them. They are usually grown along with lactococci resulting in the production of diacetyl, carbon dioxide in addition to lactic acid.


IV. Lactobacillus


These are organisms consisting of physiologically and genetically diversified group, gram positive and rod shaped in nature. Majority of the organisms falling under the category are homofermentative (converting lactose in to lactic acid). E.g. Lactobacillus delbrueckii subspecies bulgaricus and Lactobacillus delbrueckii subspecies helveticus. They are called thermodurics since they are capable of growing at higher temperatures (40C) and considered the most acid tolerant in the lactic acid bacteria. The growth of these bacteria is generally initiated at a pH of 5.5 to 6.2 and the pH is lowered to below 4.0 once when they establish in the substrate. In general, the lactobacilli are slow growers when cultured alone and hence they are usually co cultured with streptococci especially, Streptococcus thermophilus. Thus the lactic acid bacteria play a great role in the fermentation process of the dairy industry.


The author is a dairy expert, specializing in the technology and microbiology of dairy foods and holds a doctoral degree in Dairy Science; for more info on milk and dairy products please visit her site A Professional Dairy Site.


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