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Introduction
Refrigerated storage of raw milk is used to limit the growth of microorganisms in milk prior to processing. It has been known for some time that the quality and yield of cheese produced from bulk cooled milk may be adversely affected by this procedure (Weatherup et al., 1988; Weatherup and Mullan, 1993). The reduced yield and poor quality may be due to physico-chemical changes in the state of several milk components e.g. dissociation of micellar casein, mainly Κ-casein into a soluble phase, occurs during the first 48 h of storage at 4° and 7° C. This results in losses of fat and curd fines, weaker curd, more moist curd and a slightly lower yield. Partial reversal of dissociation occurs after further storage. The reduced yield and quality can also be due to the activity of proteases and lipases produced by psychrotrophic bacteria.
Despite the work that has been done over many years milk is still being stored for extended periods (1-3+ days on some farms) and cheesemakers are again (2019) reporting problems with the yield and quality of cheese produced using this milk.
Following several queries related to milk quality and cheese manufacture I am providing a report written by Wilf Weatherup and me some years ago that may be helpful.
A simple calculator has been provided using the total viable count of milk prior to pasteurisation and a regression equation to predict the grade value of Cheddar cheese.
Case study
This section contains the results of an actual investigation of an apparent reduction in the yield of Cheddar cheese made in October of year X compared with the same month a year previously.
Five vats of cheese were manufactured in the factory and the milk and cheese were subject to chemical analysis. The samples were analysed at a reputable laboratory.
The following data (Table 1) for milk, cheese and yield (adjusted per 100 kg of milk) were obtained.
This section of the Dairy Science and Food Technology website provides an outline of how the commercial 'cheese yield problem' outlined earlier might be approached and how yield and cheese component retention can be analysed and presented. This is achieved using a unique on-line calculator that is shown in the cheese yield spreadsheet.
Data from 5 vats of commercial Cheddar production are summarised in the cheese yield spreadsheet. The data set are different to the problem given previously. The cheese and milk data were obtained from one day's production. The casein:fat ratio of milk in each vat has been calculated and for information casein as a percentage of protein has been presented.
As more pressure is applied to reducing production costs, attention is increasingly being given to maximising the yield of high moisture cheeses including Cottage cheese. While yield is important, cheese quality must also be considered whenever attempts are being made to improve or optimise yield.
Considerable academic and commercial research has been devoted to optimising the yield of un-dressed or un-creamed Cottage cheese. There is now a significant volume of academic research freely available or available at low cost (On Line databases may require payment by credit card to access some journal articles); this is easy to access. Commercial information of varying quality, some of it surpassing what is available in the research literature, is also available but this is difficult to access.
