Milk-and-Dairy-Products-in-Human-Nutrition-FAO

Chapter 3 – Milk and dairy product composition 77
Yoghurt
Yoghurt (0891): A fermented milk food.
Yoghurt, concentrated or unconcentrated (0892): Includes additives such as sugar,
flavouring materials, fruit or cocoa.
Yoghurt is produced by lowering the pH of milk proteins to their isoelectric
points (about pH 4.6) by the fermentation of lactose to lactic acid using starter
bacteria. Yoghurts can be differentiated according to the fat content of the milk used
to produce the yoghurt (non-fat, low-fat or whole milk), the milk source (e.g. cow,
buffalo, goat or sheep milks; for example, traditional Greek yoghurt is produced
with full fat sheep milk) and processing (e.g. UHT-treated yoghurt, fruit-flavoured
yoghurt, yoghurt drinks, smoothies and whipped or aerated yoghurt).
The milk used for yoghurt production varies, including milk concentrated by
evaporation or filtration, by supplementing milk with milk powders or by reconstituting
milk powders directly to the desired concentration (Tamime and Robinson
1999, cited in Williams, 2002). The milk is homogenized and heat-treated, with
typical heat treatments being 85 °C for 30 minutes or 95 °C for 5 minutes. The milk
is then cooled to 42 °C, inoculated with cultures and incubated at 42 °C for about
4.5 h, until the pH decreases (Williams, 2002). The heating step leads to denaturation
of whey proteins. These proteins, together with the caseins, precipitate at low
pH, leading to the properties associated with yoghurt. According to the CODEX
standard for fermented milks (FAO and WHO, 2010f), yoghurt, alternate culture
yoghurt and acidophilus milk should contain a minimum of 2.7 percent milk protein
m/m and less than 15 percent milk fat m/m. The composition of generic yoghurt is
given in Table 3.7.
Dahi (dadhi)
According to some estimates, about 7 percent of all milk produced in India is used
to prepare the traditional fermented milk product dahi (curd, which is equivalent
to yoghurt), intended for direct consumption (Sarkar, 2008). This is significant,
considering that India is now the largest milk producing country in the world.
Although dahi is an age-old indigenous fermented milk product, it has managed to
retain its popularity and remain part of the Indian diet despite changing lifestyles
and food habits (Khurana and Kanawjia, 2007). In Bangladesh, about 4 percent of
milk is made into dahi (Nahar et al., 2007). Dahi is further converted into shrikhand
(or chakka, a sweetened concentrated curd) and lassi (stirred curd).
Dahi is reported to be very nutritious, and possess various therapeutic properties
(Nahar et al., 2007; Sarkar, 2008). In one study where authors produced dahi from cow,
buffalo and goat milks on a lab scale (Nahar et al., 2007), protein content was reported
to be 3.8, 4.3 and 3.3 g/100 g for cow-, buffalo- and goat-milk dahi, respectively. Fat
content was reported to be 4.0, 7.8 and 3.7 g/100 g of dahi produced from cow, buffalo
and goat milks. Values for ash were 0.8, 1.0 and 0.8 g/100 g, respectively. These
values are broadly consistent with the protein, fat and ash contents of the original
milks (Table 3.1). Lactose content of dahi is significantly lower than that of the parent
milk (Boghra and Mathur, 2000). During the production of dahi, folate increases by
165–331 percent and riboflavin and niacin by 160–201 percent; however, when dahi is
converted to chakka some of these vitamins are lost (Atreja and Deodhar, 1987, cited
in Sarkar, 2008). The protein quality of dahi is reported to be higher than that of milk.