Paper 6 Food Biotechnology F06FB24 Fermented Milk Products Curd (Indian Dahi), Buttermilk and Cheese 6.24.1 Introduction This module deals with the variety of fermented milk products like curd (Indian Dahi), buttermilk and cheese. In this module the learner will be introduced the technologies used to prepare these fermented dairy products and their functional role in daily life. 6.24.2 Objectives After completing this module, the learner will be able to know the way of preparing different fermented milk products. learn the characteristics of fermented milk products like curd (Indian Dahi), buttermilk and cheese. comprehend the functional properties of these dairy products. 6.24.3 Overview The module is a part of the Food Biotechnology Paper. It is designed to give an overview of various fermented milk products. The preparation of these products using fermentation techniques are presented and explained. The student will be able to explain the techniques in preparing the fermented dairy products, their characteristics and functional roles. 6.24.4 Fermentation Fermentation is a metabolic process which converts sugars to acids, gases or alcohol. It mainly occurs in yeast and bacteria; however it may also take place in oxygen starved muscle cells, as in the case of lactic acid fermentation. Fermentation is a process by which cells growing anaerobically can be able to generate ATP. Thus fermentation is defined as the breakdown of glucose (or other sugars) in which the terminal hydrogen acceptor is an organic molecule (carbon containing). During the breakdown of glucose known as glycolysis, excess hydrogen atoms are generated and must be deposited somewhere. In lactic acid bacteria, they dump excess hydrogen on to pyruvic acid, end product of glucose. This in turn converts pyruvic acid to lactic acid. In all fermentation, NADH gives up its hydrogen to produce NAD, which is required for further glycolysis (Fig.1). Yeast too performs fermentation, but with different terminal hydrogen acceptors (acetaldehyde) and products (CO2 and ethanol). In food processing, fermentation means the conversion of carbohydrates to alcohols and carbon dioxide or organic acid using yeasts, bacteria or a combination thereof under anaerobic conditions. Fermentation exists since time immemorial and must have been in existence over before humans managed upon it. The primary benefit of fermentation is the
conversion of sugar and other carbohydrate into preservative organic acids eg. converting juice into wine, grains into beer, carbon dioxide to leaver bread and sugars in vegetables. Fig 1. Steps involved in ethanol fermentation Fermentation of food serves five important purposes. 1. Enrichment of the diet through development of a diversity of flavour, aromas and texture in food substrate. 2. Preservation of food through production of lactic acid, alcohol, acetic acid, and alkaline fermentation 3. Biological enrichment of food substrate with protein essential amino acids and vitamins. 4. Elimination of anti-nutritional factors. 5. Decrease in cooking time and fuel requirement. 6.24.5 Fermentation of milk Milk can be fermented by inoculation of fresh milk with appropriate bacteria and placing it at a temperature that favour bacterial growth. At the end of the incubation, milk sugar, lactose is converted to lactic acid which can be experienced by a tart or sour taste. The reason behind this is that, the lowered ph caused by lactic acid preserves the milk by preventing the growth of putrefactive and/or pathogenic bacteria that do not grow in acid conditions.
6.24.6 Preparation of fermented milk products Fermented milk products are prepared from milk of almost all domesticated animals since ages. This traditional fermentation usually occurs as a result of activities of natural microbial flora present in the food or added from the surroundings. Among the bacteria, the most important group carrying out the fermentation is lactic acid bacteria, which are generally regarded as safe for human consumption. Fermented milk products, also called as cultured milk products are dairy foods that have been fermented with lactic acid bacteria. This fermentation process increases the shelflife of the product, while enhancing the taste and improving the digestibility of milk. Most important lactic acid bacterial group includes Lactobacillus, Lactococcus and Leuconostoc. The term cultured milk products are used to designate fermented milks that are prepared by using starter cultures and pre-defined temperature. During fermentation, certain physical and chemical changes occur in milk as a result of growth and fermentative activities of lactic acid bacteria used as a starter cultures. Cultured milk products are produced by making use of special lactic acid bacterial cultures, which fall into two broad categories. 1. Those made with use of lactic acid bacteria that grow well at ambient temperature (25-30 C) are known as mesophilic starter cultures 2. Those made with use of lactic acid bacteria that grow well under warm temperature (38-45 o C) are called as thermophilic starter cultures Depending on the type of cultures used, the cultured dairy products will vary in their flavour, texture and consistency and may be categorized as follows. 1. Products made by use of mesophilic lactic starter cultures may use one of the following starter culture types. O-type these are starter cultures which uses Lactococcus lactis subsp l.lactis and Lactococcus lactic subsp cremoris, these produce only lactic acid are called homofermentatic. D-type these are starter culture which contain in addition to the O-type bacteria, also a flavour producing lactic bacteria known as Streptococcus lactic subsp lactis var diacetylactis as the name implies, produces a flavour compound known as diacetyl cream butter or buttermilk by-product. L-type this type of starter culture contain in addition to the O-type bacteria, also Leuconostoc mesenteroides subsp mesenteroides, as the main compound producing bacteria. It produces diacetyl, acetic acid and other flavour compounds.
LD-type these starter cultures contain a combination of Str.lactis subsp lactis var diacetylactis and Leucconostoc mesenteroides subsp mesenteroides to give a blend of improved flavour of aroma. 2. Products made by use of thermophilic starter culture The main product of this group is yogurt. The typical lactic acid bacteria in yogurt starter culture are Streptococuss salivaricus subsp thermophili and Lactobacilus delbrueckic sub sp bulgaricus. The former is responsible for fermenting lactose to lactic acid, whereas Lb. delbruedcic sub sp bulgaricus is responsible for flavour production mainly in the form of acetaldehyde. Some special curd (Indian Dahi) starter cultures use Lactobacillus acidophilus, which produce a natural antibiotic, acidophilin and bifidobacterium, which are part of natural bacterial flora of the human gut. The primary benefit of using cultured milk is that in people with lactose intolerance, they are defective in the enzyme lactase which breaks down lactose to glucose and galactose for further utilisation. Consumption of cultured milk does not pose any problem, because a large part of the milk sugar has already been digested for them by the lactic acid bacteria. The production of cultured milk products begins with good quality milk and a good starter culture (Fig 2). The milk is first subjected to filtration to remove physical dirt. This is followed by pasteurization of the milk at 80 85 o C for 30 minutes in a boiling pasteurizer of a 50 litre capacity. Once the pasteurization is complete, the milk should be cooled to 20 25 o C either in a cold water basin or trough. Then the milk is inoculated with 2-3 % active starter culture, stirred well for 3-5 minutes to ensure uniform distribution of starter culture and Then milk is finally incubated at ambient temperature overnight (for 16-18 hours). During the production, care should be taken for intact coagulation. This could be achieved by cooling the coagulants before stirring or breaking the curd. This may improve viscosity and consistency of the product.
Fig 2 General steps involved in the preparation of fermented dairy products 6.24.7 Characteristics of ideal starter cultures 1. It should be quick and steady in acid production 2. It should produce product with fine and clear lactic flavour 3. It should not produce any pigments, gas, off-flavour and bitterness in the finished products. 4. It should be associative in nature in product development. 6.24.7.1 Role of starter cultures in fermented milk When starter cultures grow in milk, it affects the constituents of milk and brings fermentative metabolic changes. It produces different intermediary or end products, which gives typical characteristics to fermented milk such as 1. Produce lactic acid 2. Bring about coagulation of protein and form gel. 3. Produce volatile flavour compounds like diacetyl acetaldehyde and several intermediate compounds. 4. Possess other compounds like CO2, alcohol, propionic acid which are essential in products like cheese. 5. Control the growth of pathogens and spoilage organisms.
6. Some dietary cultures like Lb.acidophiles have health benefits and produces antibacterial substances. 7. It helps in texturing and ripening of cheese. 6.24.7 Curd (Indian Dahi) Curd (Indian Dahi) is a mixture of milk and cream which is fermented by a culture of lactic acid producing bacteria Lactobacillus bulgaricus and Streptococcus thermophilus. In addition, other bacteria may also be added to the culture. Sweetners (eg. sugar, honey, and aspartame) flavourings (eg. vanilla, coffee) and other ingredient (eg. fruits, preserves, stabilizers such as gelatine) may also be added. Yogurt contains at least 3.25% milk fat and 8.25% solid non-fat. The consistency, flavour and aroma may vary from one region to another, the basic ingredients and manufacturing processes are consistent Yogurt is strictly defined as a milk product produced by the action of two bacteria - Streptococcus thermophilus and Lactobacillus delbrueckii ssp bulgaricus for improving its therapeutic significance. Although milk of various animals has been used for curd (Indian Dahi) product in various parts of the world, the most preferred is cow s milk. In order to ensure the development of the curd (Indian Dahi) culture, the following criteria for the raw milk must be met. Low bacterial content Free from antibiotics, sanitizing chemicals, colostrums and rancid milk No contamination by bacteriophages The starter culture for most curd (Indian Dahi) production is a symbiotic blend of two bacteria, Streptococcus salivarius sub sp (ST) and Lactobacillus delbruecki sub sp bulgaricus(lb). Both of them grow independently, however the rate of acid production is higher, when used together than either of the two organisms grown individually. Comparatively, ST grows faster and produces both carbon dioxide which stimulates LB growth on the other hand, proteolytic activity of LB produces stimulatory peptides and aminoacids for use by ST. Hence, these microorganisms are ultimately responsible for the formation of typical yogurt flavour and texture. The curd (Indian Dahi) mixture coagulate during fermentation due to drop in ph. The initial drop in ph of yogurt mix to approx 3.0 is due to Streptococci. The further decrease to ph 4.0 may be due to Lactobacilli. The products which contribute to flavour of curd (Indian Dahi) are lactic acid, acetaldehyde, acetic acid, and diacetyl.
Curd (Indian Dahi) is produced through fermentation of milk by lactic acid bacteria. The milk is first heat treated, homogenized and is then cooled to allow the addition of bacteria or starter culture (Fig 3). At appropriate temperature and moisture, the bacteria are able to ferment the Fig 3. Schematic flow chart showing Yogurt manufacturing process lactose into lactic acid. The milk proteins then coagulate and set to form curd (Indian Dahi). A colourless liquid called acetaldehyde is also produced during fermentation contributing to the flavour of the curd (Indian Dahi). Fermenting milks with different microorganisms has also provided an opportunity to develop a wide range of products with different flavours, textures, consistencies, and recently health attributes. These include Live curd (Indian Dahi), which contain bacteria that are harmless, but still present and alive
Probiotic curd (Indian Dahi) which contain live probiotic microorganisms that are suggested to beneficial to health Bioyogurts which are very popular and are made using Bifidobacterium bifidum (bifidobacteria) and/or Lactobacillus acidophilus. Biocurd (Indian Dahi) has a milder, creamier flavour which aids in digestion and promotes good health. 6.24.8 Buttermilk Buttermilk is the liquid left behind after churning butter out of cream. It also refers to a range of fermented milk drinks. This fermented dairy product also known as cultured butter milk is produced from cow s milk which has a characteristic sour taste caused by lactic acid bacteria. It is made using one of two species of bacteria either Lactococcus lactis or Lacto bacillus bulgaricus which creates more tartness. The tartness of buttermilk is due to the acidity in the milk. The acidity is primarily due to lactic acid produced by lactic acid bacteria, while fermenting lactose. As the bacteria produces lactic acid, the ph of the milk decreases and casein, the primary milk protein coagulates causing the curdling of the milk. This process makes buttermilk thicker than plain milk. The cultured buttermilk appears to be more viscous than traditional buttermilk as it contains more lactic acid. Buttermilk prepared in the traditional way is considered beneficial to health as it contains probiotic microbes. The probiotic nature of buttermilk is said to beneficial to the gut and improve immunity when taken regularly. Buttermilk contains vitamins, potassium, calcium and traces of phosphorus. It is a favourite traditional drink during summer, as it is soothing to the stomach and alleviates minor stomach upsets. In India, flavouring ingredients such as asafoetida, coriander leaves, ginger, curry leaves and salt are mixed with buttermilk to enhance its digestion-aiding properties. 6.24.9 Cheese Cheese is a food derived from milk which is produced in a wide range of flavours, textures and forms by coagulation of the milk protein, casein. It comprises proteins and fat from milk, usually the milk of cows, buffalo, goats or sheep. During production, the milk is usually acidified, and adding the enzyme rennet causes coagulation. Hundreds of types of cheese from various countries are known. They differ in their styles, textures and flavours depending on the origin of the milk whether they have been pasteurized the butterfat content, the bacteria and mold, the processing and aging.
In cheese, the milk is curdled by adding acids such as vinegar or lemon juice. Most cheeses are acidified to a lesser degree by bacteria, which in turn causes milk sugars to lactic acid, and then the addition of rennet completes the curdling. Cheese is classified as hard or soft, depending on their moisture content and whether they are ripened by bacteria or molds or unripened. During the preparation of cheese, the whole milk is pasteurized which is then brought to a temperature of 31 o C followed by the addition of lactic acid producing starter culture and required colouring matter, after about 30 minutes to the mildly acidic milk, rennet solution is added, stirred and allowed to set the curd for 30 minutes (Fig 4). Fig 4. Cheese Manufacturing Process The curd is then cut into small cubes which is then heated to 38 o C and held at that temperature for about 45 minutes. This heating squeezes whey from cubes, which is then drained off. In addition, heat increases the rate of acid production which makes curd cubes shrink. The curd is then allowed to mat. The mat is then subjected to the process of cheddaring, where the matted cult is cut into block and pilling them on one another. This is then followed by addition of salt, preserving and ripening to make cheese.
6.24.10 Conclusions This module deals with the fermentation of milk and manufacturing of fermented milk products like yogurt, buttermilk and cheese. This method involves the use of microbes as starter culture to facilitate the process. These microorganisms, mainly of lactic acid bacteria are responsible for the conversion of lactose in milk to lactic acid, which may in turn lower the ph of the product, harmful for the growth of other milk spoiling microorganisms. Other minor products like diacetyl, acetaldehyde produced during fermentation imparts colour, flavour and texture to the fermented milk products. The subsequent module will highlighten on the preparation of fermented fish and meat products