Starter Cultures and Fermented Milk Products

Lesson 25. FLAVOUR IN CULTURED BUTTER MILK, ADVANCES IN CBM PRODUCTION

Module 14. Cultured butter milk

Lesson 25

FLAVOUR IN CULTURED BUTTER MILK, ADVANCES IN CBM PRODUCTION

25.1 Flavor in Cultured Butter Milk

The true cultured butter milk is popular for its flavour. The product has a fine delicate flavour of diacetyl with acidic taste. The flavour is delicate and is affected by several cultural and environmental factors. Hence, efforts are concentrated in CBM industry to produce optimum concentration of flavour and than maintain it still the product is sold.

The principal flavouring component in cultured butter milk is diacetyl produced by the aroma bacteria. This is produced by fermentation of citric acid (about0.2% present in fresh milk). To prevent deterioration of fine flavor citric acid may be added (0.1-0.2%) to milk before pasteurization to provide sufficient substrate and maintain diacetyl levels between 2-4 ppm. A proper balance of different flavour compounds namely diacetyl, acetoin, acetate and CO₂will give a characteristic flavor to the product.

Lactococcus l actis subsp diacetylactis and Leuconostoc are the major diacetyl producing microorganisms in cultured butter milk. Neither L. lactis subsp diacetylactis nor Leuconostoc can use citrate as an energy source. However they produce different end products from citrate. L. lactis subsp diacetylactis,however grows better in milk than does the leuonostoc and as its name suggests,accumulates high concentration of diacetyl (up to 5 ppm). It can therefore be used to produce both acid and aroma. This organism unfortunately produces acetaldehyde, an undesirable flavor in butter milk (described as a “green” or yoghurt like off flavour). On the other hand, Leuconostoc don’t have this disadvantage and they help to scavenge acetaldehyde.

25.2 Modes of Diacetyl Production

Diacety is produced by fermentation of citric acid by selected lactic acid bacteria in milk. These are two theories which explain the pathways for formation of diacetyl in milk.

One theory explains that diacety is formed by oxidative decarboxylation of α-acetolactate which is excreted in to milk by bacterial cells (Figure 25. 1)

Fig. 25.1 Pathways for flavour production in cultured butter milk

1. According to Van Beynum Pette (1938); Deman (1956); Seitz et al. (1963) the diacetyl is formed by oxidative decarboxylation of α-acetolactate which is excreted into milk by bacterial cells.

2. According to Speckman & Collins(1968) and Cogan (1984) diacetyl is formed in the bacterial cells by reaction of acetyl CoA and activated acetaldehyde. This process occurs totally intracellularly and not via α-acetolactate.

In L.lactis subsp diacetylactis maximum accumulation generally coincides with the total disappearance of citrate after which the diacetyl and acetoin level decreases. The presence of citrate represses the synthesis of acetoin and 2, 3butydene glycol reductases) which explain why diacetyl and action accumulates during growth of this organism.

Fig. 25.2 Relation among diacetyl, citric acid pH in cultured butter milk

Leuconostocutilize citrate rapidly once growth has begun producing acetate and traces of2,3- butydene glycol but no diacetyl or acetoin unless citrate is present at low pH. This is thought to be due to inhibition of acetolactate synthatase activity by several intermediates of heterofermentation. The inhibition by some of these intermediates is relieved at low pH allowing the production of acetoin.This however, does not explain why diacetyl does not accumulate during growth of leuconostoc.

25.3 Factors Affecting Diacetyl Content in Cultured Butter Milk

Flavour in cultured butter milk is due to presence of diacetyl and acetoin. These compounds are affected by several factors as listed below:

1. Type of culture and culture balance : cultures inherent physiological properties decides the metabolism. More important is the balance among different strains and especially the proportion of acid and aroma producers in culture decides the fate of diacetyl.

2. Citric acid concentration in milk: Citrate is the substrate as well end product inhibitor of the diacetyl and hence it is most important. Surplus citrate in the product after fermentation helps stabilize diacetyl content.

3. Acidity and pH : The metabolism of citrate involving various enzyme activities depends mainly on pH of the medium. Further, it is seen that diacetyl in cultures like Leuconostocs is produced externally from alpha-acetolactate in presence of lactic acid. It is found that pH 4.6 is optimum for stabilizing the concentration of diacetyl.

_4. Aeration and oxygen content in milk: The diacety and acetoin can easily reduce to 2,3 butynediol, which is flavourless. As all these compounds interchange based on oxidation-reduction potential, aeration and oxygen are important.

5. Type of packaging material :It decided permeability for oxygen. Fro examples, when bottles are used as packaging material, there can be decrease in diacetyl; while when polyethylenepouches are used, because of permeability to oxygen, the diacetyl may remain stable.

6. Concentration of Manganese and magnesium in milk: These ions act as co-factors for keyenzyme activities in metabolic pathway and hence affect the concentration of end product.

7. Age of milk and other contaminants : If the milk is stored and other contaminants which can produce diacetyl reductase are present, it may reduce flavour.

8. Temperature of incubation: The growth of cultures and the activities of enzymes in flavour production pathway depend on temperature.

9. Agitation and cooling : It can affect the growth of culture as well as oxygen content in product and hence the diacetyl concentration. It is observed that fast cooling and aeration for 1hour has favourable effect. Too slow cooling in incubation vessel has adverse effect. If it is not cooled immediately, especially in large tanks,the diacetyl may quickly decrease.

25.4 Advances in Cultured Butter Milk Technology

Over the period, several changes and modifications have come in manufacture of CBM.Some examples of modifications suggested based on research are given below.

1. Double stage homogenization of milk for 1.7% fat butter milk adds to richness of flavor.

2. Controlled fermentation process involving the chilling of buttermilk at pH 5.2 to arrest the growth of added L. diacetylactis followed by acidification of the product to pH 4.5 with added lactic acid has been reported gives fine flavor through increased diacetyl, but without developing acetaldehydeovertones.

3. Various types of flavoured butter milks can be produced with incorporation of natural fruits or essences.

4. To improve texture and viscosity of the product, membrane filteration technology can be used. Concentration of milk by UF can be done to increases protein and decrease lactose content.

Vacuum de-aeration before cooling for removal of excess carbon dioxide, helps get better flavor in CBM.