The nature of the flavour of cheese, the reason for variety differences, and the control and measurement of cheese flavour have for long been some of the most scientifically interesting and commercially important problems in the dairy industry.
Flavour is a complex of odour and taste. In cheese, the latter is due to fat and protein break-down products, in addition to the lactic acid and salt always present. Free fatty acids, especially the lower ones, and amino acids are probably largely responsible for the characteristic tastes of different varieties of cheese.
During the last few years, attention has been focused on the amino acids formed or liberated during the ripening process and considerable advance has been made in their estimation.658
The odour of cheese, as distinct from taste, was formerly considered to be largely due to volatile fatty acids, and methods for their estimation have been described.659-64
2. Chemical Nature of Cheese Flavour
(a) General. There has been hitherto a wide gulf between the practical cheesemaker, grader and factor on the one hand and the scientific worker on the other, as far as the assessment of flavour is concerned. The grader uses such terms as over-acid, sweet, mature, tainted, etc. whereas the scientist endeavours to measure such qualities in terms of pH value, lactic acid content, protein degradation, free fatty acids, esters, ketones, aldehydes and other substances known or suspected to be responsible for these qualities.
With modern analytical techniques, it is becoming possible to detect and measure these substances at levels of 1 ppm or less and so flavour (aroma and taste) can now be broken down into the constituents of sensory assessment and an attempt made to correlate these with specific chemical compounds.
Modern Chromatographie techniques have added considerably to our knowledge of the nature of cheese flavour. While the importance of free fatty acids has been confirmed, it is now recognized that quite different sub-stances have a key role in cheese flavour.
(b) Hard cheese. Data for free fatty acids in Cheddar cheese have been published.665-667 Whereas the major volatile fatty acids in Cheddar are acetic, butyric and caproic, in Swiss cheese there is predominance of propionic and small amounts of acetic and butyric, together with very small amounts of branched chain lower fatty acids.668 Clostridia, producing butyric acid from lactic acid, may induce rancidity in Cheddar cheese.669 it appears that the balance between free fatty acids and sulphydryl compounds is one of the most important aspects of flavour in hard cheese.67o Analytical techniques have been developed.67i, 672 See also Milk taints, p. 68.
(c) Blue-veined cheese. Fatty acids, especially the lower (C2, C4) and inter-mediate (Ce, Ce, C10) members, undoubtedly play an important role in the flavour of blue-veined cheese. In cheese made from sheep milk (e.g. Roque-fort), butyric acid is relatively low and the CO to Qo acids are relatively high.
Differences in fatty acid make-up can be correlated with flavour.673 About 80 % of the fatty acids in cheese exist as salts, i.e. combined with sodium, calcium, etc. Sampling of cheese for assessment of chemical degradation is always difficult, and especially so for blue-veined cheese. A complete wedge from outside to the centre should always be used (cf. p. 31).
(d) Flavour substances other than fatty acids. More recently, attention has turned to carbonyl compounds and volatile sulphur compounds.674-9 Walker 680 has claimed to be able to produce the flavour of a 3 month old Cheddar cheese by adding a mixture of methyl ketones, fatty acids and a source of H2S to curd, and Kristoffersen68i has more definitely linked flavours with-SH groups. Incubation of the milk (5 hr at 37°C) enhanced and heating the milk (62-68°C) delayed the appearance of active -SH groups and Cheddar flavour.682 Cheddar cheese flavour and -SH groups are not correlated with bacterial counts. Lawrence683 has evolved a micro-method for determining H2S in cheese, but its role in flavour is not clear. See surveys of the
SUbject.684,685
The natural objective of flavour control work in cheese, both for the scientist and the grader, would be to obtain a typical analysis or "profile"
of flavour constituents, both qualitatively and quantitatively, for each variety of cheese. Kroger and Patton,686 working with 15 varieties, obtained 30 peaks corresponding to alcohols, ketones, aldehydes, esters, sulphides and other volatile compounds. Each variety had its own profile but ethanol and acetone were found in all, and acetaldehyde, butanone, pentan-2-one, iso-propanol and dimethyl sulphide (cf. p. 71) occurred frequently. The con-centrations change on ripening. Thus in Cheddar ethanol declines and
138 J. G. DAVIS
butanone increases; at later stages n-propanol and butan-2-ol are prominent.
Kroger and Patton suggest that the ethanol content could be used instru-mentally as a criterion of maturity.
3, Micro-organisms and Cheese Flavour
The role of the starter and the microflora of the milk in flavour develop-ment has always been highly controversial.684 Originally starter was con-sidered to affect flavour but later experimental work discredited this theory.
Franklin and Sharpe687 found that the more severe the heat treatment the fewer were the bacteria and the lower the flavour score, and the greater the number of species present at the time of examination the higher was the score.
The flavour score was also influenced by the starter used and the pH value, but not by the fat, salt and moisture contents of the cheese.
4. Routine Assessment of Cheese Flavour
Although the grader tests the cheese at 14 or 28 days or at such other time as may be appropriate for any variety of cheese, the grading so given is un-satisfactory for certain purposes. It is purely a subjective assessment devoid of quantitative measurements, the grader may use terms incomprehensible to the scientist or the consumer, he cannot forecast accurately the quality of the cheese when fully ripe, and the practical grading and laboratory test results may be quite unrelatable.
With the increasing interest in quality and flavour in cheese, and the increasing influence of the laboratory in the food industry, an objective method for the assessment of flavour or at least of the degree of ripening, is obviously required. No laboratory tests can adequately assess flavour in the subjective sense, because no two persons always agree entirely on the flavour quality of a food. However, flavour and degree of ripeness in a cheese broadly go together, and by making laboratory tests for those constituents which change in concentration during ripening and affect flavour, it should be possible to make some progress in flavour assessment.
(a) Body and texture. These two properties are related and often confused, but are distinct. Body is the "firmness of feel", i.e. it can be measured without seeing the cheese, whereas texture is the appearance of the cheese, excluding colour.
Weak body, the commonest body defect of cheese, is usually due to reten-tion of moisture due to faulty manufacture or slow starter (p. 143). In farm-house cheese it may be due to mastitis milk, as any abnormal milk tends to give a weak-bodied cheese.
In hard cheese texture is mainly concerned with openness which may be classified as mechanical, slit or (rarely) as "sweet eye". This type of fault
may be due to irregularities in acid development or faulty manufacture. The most useful practical device for minimizing this type of fault is vacuum pressing.688