M T Frohlich Wyder and H P Bachmann

As we already know, Swiss cheese has a particular dominating flavour due to propionic acid fermentation which is described mainly as sweet and nutty [125]. However, a flavour defect may occur that becomes evident only with progressing ripening. The most common flavour defects in Swiss cheese are produced by:

• butyric acid fermentation;

• excessive lipolysis;

• excessive proteolysis.

Butyric acid fermentation is totally undesirable, since lactate fermentation by Clostridium tyrobutyricum into butyric acid, acetic acid, carbon dioxide and hydrogen causes the cheese loaf to blow [91]. Furthermore, even small amounts of butyric acid cause off-flavours. Therefore, in Switzerland, Emmental cheeses have to be manufactured with milk from cows that have not been fed silage. Feeding cows with silage of low microbiological quality is the primary route of contamination of the milk with spores of Cl. tyrobutyricum. As few as 50 spores per litre of cheese milk are sufficient to cause a butyric acid fermentation.

Spores can also be eliminated either by physical treatment, i.e. bactofugation or microfiltration prior to processing, or by the use of additives such as nitrate, lysozyme or nisin in order to restrict germination. However, these additives are not permitted in Switzerland for the production of Emmental cheeses. A particularly serious defect results from the presence of Clostridium sporogenes. This species leads to a non-specific and very intense proteolysis, leading to putrid spots in the cheese loaf (Fig. 1).

Lipolysis in Emmental cheese is catalysed by bacterial lipases and the indigenous lipoprotein lipase in milk which is, however, thermolabile and

Fig. 1 Intense proteolysis by Clostridium sporogenes with putrid spots (source: Agroscope Liebefeld-Posieux; CH-3003 Berne).

therefore its activity is reduced by cooking at temperatures over 50 0C. Lactic acid bacteria have only limited lipolytic activity, with Streptococcus thermo-philus having the highest. Propionibacteria, in contrast, have lipolytic activity, 10-100 times more than lactic acid bacteria and which is highly strain-dependent. Lipolysis in Swiss-type cheeses is consequently mainly caused by propionibacteria and is generally recognised as necessary to produce typical Swiss cheese flavour. The amount of free fatty acids present varies from 2 to 7gkg_1. Nevertheless, higher contents give flavour defects such as rancidity (caused mainly by butyric and caproic acids) [90]. The release of free fatty acids starts in the warm room simultaneously with the growth of propionibacteria.

Other bacterial, but undesirable, lipases may originate from the raw milk flora. These lipases become especially evident if the raw milk has been stored under unfavourable conditions before processing (too long and at too high temperatures) and these enzymes are usually heat stable.

Excessive proteolysis gives an overripe and sharp taste and a shorter body. This defect becomes particularly evident when a large amount of casein is decomposed into low-molecular compounds and amino acids. The latter are further metabolised to strong flavour compounds, e.g. sulphurous compounds. This is certainly a desirable process in other cheese types, but in Swiss-type cheeses the specific propionic acid flavour should dominate.

Excessive aspartase activity has also a great impact on flavour development [121]. Propionibacteria with strong aspartase activity need the amino acid aspartate for this pathway. The more aspartate is available, the stronger is their metabolism. A strong aspartase activity leads also to a stronger propionic acid fermentation and, as a result, to the defect of late fermentation. Consequently, more propionic and acetic acids are liberated, and, if present at excessive concentrations, may also lead to an overripe and sharp taste.

Frequently, the course of proteolysis [90] in a cheese loaf varies from one zone to the other, a phenomenon that is due to temperature changes in the cheese loaf during lactic acid fermentation. Since the outer zone cools faster, there often develops a bacterial flora which is proteolytically more active than the microorganisms in the centre of the loaf. This usually leads to cheese defects such as short and firm body, sharp taste, or the development of whitish colour under the rind.

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