sanitizers carried into the milk as a result of inadequate equipment
cleaning regimes at the farm or dairy. Inhibition of the starter culture
would result not only in economic losses but could potentially allow
pathogens to grow.
In commercial practice it is usual to supplement the solids content of
the milk to enhance the final texture of the product. The SNF (solids not
fat) content is increased to between 11 and 15%, compared with a level
of around 8.5% in fresh milk. The simplest way of achieving this is by
addition of skim- or whole-milk powder depending on whether a conven-
tional or low-fat product is required. The properties of the product may
also be improved and stabilized by the addition of small amounts of
natural or modified gums which bind water and thicken the product.
If left to stand, the milk fat would separate out to form a cream layer.
To prevent this, the milk is homogenized by passing it through a small
orifice under pressure, typically 100–200 kg cm^2 at 50–60 1 C, to reduce
the size of the fat globules to below 2mm. This improves the product’s
stability, increases the milk’s viscosity, and also makes it appear whiter
as the number of light-reflecting centres is increased.
Before addition of the starter culture, the milk is heated at 80–90 1 C
for about 30 min. Being well in excess of the normal pasteurization
Figure 9.4 Yoghurt production
324 Fermented and Microbial Foods