Agriculture Reference
In-Depth Information
17.3.5 Yogurt
The word 'yogurt' is derived from the Turkish word 'jugurt' and refers to
fermented milks with acidic fl avours. Yogurt is the best known of a range of
fermented milk products made by the acidifi cation of milk with lactic acid
bacteria, but without the use of chymosin which would demand classifi cation as
cheese. Yogurt is usually made from standardised milk with fat content of 0-3.5%
and NFMS content of 11-13%, although the fat content may be as high as 10%.
The increase in milk solids may be achieved by the addition of skimmed milk
powder or concentration of a portion of the milk by ultrafi ltration.
Milk for yogurt fermentation is homogenised (not fat-free milk) to disperse
milkfat globules and then heat treated to 90-95°C for around fi ve minutes. This gels
the whey proteins and induces β -lactoglobulin- κ -casein interactions, which improve
the texture of the product and reduce syneresis. Heat treatment also promotes
chemical changes which improve the milk solids as a fermentation substrate.
Following heat treatment the milk is cooled to 41-43°C and inoculated with yogurt
starter comprising a specifi c blend of the symbiotic micro-organisms Lactobacillus
delbrueckii subsp. Bulgaricus and Streptococcus salivarius subsp. Thermophilus.
In around three hours, the milk pH falls to 4.2-4.5 and a coagulum forms. Stirred
yogurt fermented in tanks must be cooled to 15-20°C to arrest fermentation and
prevent further acid production, following which fruit, sugar and additives may be
added prior to packing. Packed product is chilled to 0-4°C. Set yogurt is made by
pot fi lling milk with added starter (often over a layer of fruit purée), fermenting in
incubators, then blast chilling. The absence of agitation leaves the coagulum intact.
Consumers see the separation of whey (syneresis) from yogurt during shelf life
as a defect. Additives may be used to stabilise against syneresis including pectins,
guar gum, corn starch and gelatine which also contribute to texture and viscosity.
Fruited yogurts are popular and are fl avoured with fruit purées and natural fruit
fl avours. Sugar is often added to sweeten yogurt and fruit acids such as citric and
malic acid may be added to sharpen the sensation of fruit fl avours and reduce the
perception of lactic acid. In the production of 'Swiss-style' yogurt, fruit and other
ingredients/additives are mixed into set yogurt. Some yogurts are made using
probiotic cultures (e.g. organisms of the genus Bifi dobacterium ).
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17.3.6 Ice cream
Ice cream is a milk-based dessert rooted in seventeenth century Italy and not the
journeys of Marco Polo (Quinzio 2009). The basic components of ice cream are fat
(milkfat as in dairy ice cream or vegetable fat), NFMS (from whole milk, skimmed
milk, skimmed milk powder (SMP) and other materials depending on recipe),
sweeteners (sucrose or glucose solids), emulsifi ers, stabilisers, fl avourings and
colourings. A premium ice cream contains, for example, 15% fat, 10% NFMS, 17%
sugar and 0.3% emulsifi er-stabiliser blend (Andreasen and Nielsen 1998). Ice
creams of lower quality of design have reduced solids content, with vegetable fat
replacing milkfat and are highly aerated to increase overrun. Table 17.7 compares
the ingredients in premium and economy products.
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