CHEMISTRY AND BIOCHEMISTRY OF CHEESE AND FERMENTED MILKS 429listed in Table 10.12 (Tamime and Robinson, 1985); yoghurt in its various
forms, is probably the most important type but consumption varies widely
(Table 1.6).
The production of fermented milks no longer depends on acid production
by the indigenous microflora. Instead, the milk is inoculated with a carefully
selected culture of LAB and for some products with LAB plus lactose-
fermenting yeasts (Table 10.12). The principal function of LAB is to produce
acid at an appropriate rate via the pathways summarized in Figure 10.12.
The yoghurt fermentation is essentially homofermentative but the character-
istic flavour of cultured buttermilk is due mainly to diacetyl which is
produced from citrate by Lactococccus lactis ssp. lactis biovar diacetylactis,
which is included in the culture for this product (Figure 10.31).
Kefir and Koumiss contain about 1 and 6% ethanol, respectively, which
is produced by lactose-fermenting yeasts, usually Kluyveromyces marxianus.
The ethanol modifies the flavour of the products and the CO, produced in
the fermentation affects both their flavour and texture. Koumiss, which is
produced traditionally from mares’ milk, mainly in Russia and surrounding
areas of Asia, is not in fact coagulated.
The technology of fermented milks will not be discussed in detail and the
interested reader is referred to Tamime and Robinson (1985), Tamime and
Marshall (1997) and Marshall and Tamime (1997). A flow diagram of the
manufacturing protocol of yoghurt is presented in Figure 10.32. Depending
on the product, the milk used may be full-fat, partially skimmed or fully
skimmed. If it contains fat, the milk is homogenized at 10-20 MPa to
prevent creaming during fermentation. For yoghurt, the milk is usually
supplemented with skim-milk powder to improve gel characteristics. Acid
milk gels are quite stable if left undisturbed but if stirred or shaken, they
synerese, expressing whey, which is undesirable. The tendency to synerese is
reduced by heating the milk at, for example, 90°C x 10min or
120°C x 2min. Heating causes denaturation of whey proteins, especially
P-lactoglobulin, and their interaction with the casein micelles via K--casein.
The whey protein-coated micelles form a finer (smaller whey pockets) gel
than that formed from unheated or HTST pasteurized milk, with less
tendency to synerese.
In some countries, it is common practice to add sucrose to the milk for
yoghurt, to reduce the acid taste. It is also very common practice to add
fruit pulp, fruit essence or other flavouring, e.g. chocolate, to yoghurt, either
to the milk (set yoghurt) or to the yoghurt after fermentation (stirred
yoghurt).
In the manufacture of Labneh and other Middle Eastern fermented milks,
the fermented product is concentrated by removing part of the serum
(whey). This was done traditionally by stirring the yoghurt and transferring
it to muslin bags to partially drain. Concentration can now be achieved by
ultrafiltration, before, but preferably after, fermentation.