fish tissue containing at least 2.4% acetic acid and 6% sodium chloride in its water phase. This corresponds to pH values of 4.0±4.2 and awvalues between 0.955 and 0.960. These conditions also assure the inactivation of nematode larvae and are stipulated in official regulations such as Annex 1 to the German
`Fischhygiene-Verordnung'. The marinated tissue is then placed in a brine containing spices. According to the German Code of Practice (`LeitsaÈtze'), the pH of the final product must be 4.8 or below, corresponding to a concentration of acetic acid of at least 1.8% in its water phase. For the production of Scandinavian-type anchovies, the brine contains sugars, acidulants (sometimes also lactic acid bacteria) moderate levels of salts, and occasionally nitrate, and the fish is `fermented' for 1±2 days at ambient temperature prior to ripening at chill temperatures. The pH does not normally drop to 5.5 or below, and the products are stabilized mainly by salt and by added preservatives. Benzoic and sorbic acids are frequently used to inhibit yeasts and moulds (levels of up to 0.2% ± based on the sum of the content of the undissociated form of these acids
± are permitted in Annex III to the EU Directive 95/2 on Food Additives).
7.6 The use of pH control to preserve vegetables, fruits,
pasteurization of the product. A survey of pasteurized sauerkraut in Germany revealed pH values between 3.7 and 4.2, corresponding to about 100 mmoles of lactic and 50 mmoles of acetic acid per kg, respectively (cf. BuckenhuÈskeset al., 1990). To achieve sufficient microbial stability and safety, a final pH of 4.2 should not be exceeded (Holzapfel, 1996). This is also true for Spanish-style fermented green table olives (target pH 3.8±4.2, corresponding to about 100 mmoles of lactic acid/kg) and fermented cucumbers. These products contain less acetic acid than does sauerkraut because fermentation takes place at higher salt concentrations and for longer periods. Under such conditions, homofermentative lactobacilli easily outgrow the heterofermentative leunocostocs (Nout and Rombouts, 2000). Untreated ripe (black) olives are fermented for shorter periods at higher salt concentrations, and lactic acid accumulates only to levels of 0.3±
0.5% titratable acid, corresponding to a pH of around 4.5 (Garrido-FernaÂndezet al., 1995). Fermented vegetable juices are produced by adding homofermentative lactic acid bacteria that lower the pH to 3.8±4.0 (cf.
BuckenhuÈskeset al., 1990).
In contrast, most ripe fleshy fruits formed by multiannual plants (pome, stone, citrus, tropical fruits; berries), as well as tomatoes have pH values below 4.7, and growth potential for bacterial pathogens is low. The pH is mainly determined by the content of citric and malic acids in the fruits, and growth of fungi is possible throughout the whole pH range down to pH 2.2 (on lemons).
However, the pH determines the requirements for heat treatment of fruit products such as canned fruits, jams and fruit juices. In the range of pH 4.0±4.5,
`butyric' anaerobic spore-forming bacteria (Clostridium butyricum, Clostridium pasteurianum), and, at storage temperatures above 25ëC, Bacillus coagulans may still grow. This necessitates a more severe heat treatment (equivalent to an exposure to 100ëC for 1±2 minutes).
Acidulants such as citric acid may be added to fruits with pH values in this range or even above, e.g., tomatoes and pears. At pH values below 4.0, a pasteurization treatment equivalent to an exposure to 80ëC for about ten minutes (or to 92±99ëC for 10±60 seconds) is usually applied. This treatment, however, is not sufficient to inactivate spores of Alicyclobacillus spp. (capable of developing at low pH and temperatures above 25±30ëC), and not always sufficient to inactivate heat-resistant ascospores ofByssochlamys spp.and some other fungal species. These moulds may be inhibited by low aw(as prevailing in many jams) and/or sorbic and benzoic acids. The latter preservatives are, at levels below 0.1%, permitted by EU Directive 95/2 for the preservation of certain low-sugar jams and soft drinks. Acidulants are often added to fruit products for purposes other than microbial stability. Citric acid delays enzymatic browning and modifies the gelling properties of pectins, thus affecting the viscosity of jams and similar products.
`Drop' apples may become contaminated with animal faeces, and unpasteurized apple juice (`cider') was identified as a vector for foodborne infections. Hence, the United States Food and Drug Administration demands a warning on the label unless a reduction ofEscherichia coliO157:H7 by five log The control of pH 119
cycles is ascertained. Non-thermal processes by which this reduction is achieved were designed (Uljas and Ingham, 1999; Comes and Beelman, 2002). These authors found that inactivation rates of salmonellae andEscherichia coliwere higher in juices of low pH, higher storage temperatures (25±35ëC), and in the presence of preservatives such as benzoic acid.
7.6.1 Mayonnaise, salad dressings, ketchups, and delicatessen salads Mayonnaise, salad dressings, ketchups and comparable products are preserved by a combination of low pH (adjusted by acetic and citric acid), low aw(mainly depending on the relative levels of oil, water and salt in the formulation), and ± sometimes ± preservatives such as sorbic and benzoic acids. The target pH value depends on the level of other inhibitory factors such as aw and the presence of preservatives, but is generally about 4.0 (see Table 7.3). In contrast to ketchups, emulsion-type products cannot be pasteurized after mixing. When designing the formulation of the latter products, one must take into account the partition coefficient of the preservative agents between the water and the oil phase.
When sauces are used for the preparation of delicatessen salads, the acid is diluted and the relative proportions of sauces and solids should be adjusted so that the final salad has a pH value of 5.0 or below, and a concentration of undissociated acetic acid in the water phase is at least 0.1% or 16 mM. It should also be noted that considerable time is needed for the acid to penetrate through the solids. During this time there is a risk of growth of acid-sensitive bacteria in the solids (Baumgart, 1996).
Delicatessen salads may be spoiled by yeasts, moulds and/or lactic acid bacteria tolerating acetic acid. Growth of yeast and moulds is inhibited by sorbate. This preservative, however, has little if any effect on lactic acid bacteria, and some yeasts (especially Zygosaccharomyces baillii) are rather resistant to it. Hence, sorbate is mostly used in combination with benzoate. EU Directive 95/2 permits levels up to 0.15% (calculated as the sum of the undissociated forms) of these acids.
Table 7.3 Some critical limits for pH and acidity in mayonnaises, dressings and ketchups
Product Fat content aw pH Acetic acid in aqueous phase
% % mmoles of
undissociated acid/kg Mayonnaise 80 (emulsified) ca. 0.93 4.1 2% 270 mM
Emulsified salad 50 (emulsified) ca. 0.95 4.2 1% 130 mM dressings
Tomato ketchup <1 ca. 0.94 3.8±4.0 0.9 % 130 mM Based on data provided by Baumgart (1996) and Michels and Koning (2000).
7.6.2 Cereal products
The baking process destroys vegetative micro-organisms and mould spores, but not bacterial spores. In some wheat breads prepared without dough acidification, the combination of the hurdles pH (5.4±6.0) and aw(0.94±0.97) may still permit growth of someBacillusstrains. To prevent this, acetate is sometimes added to the flour to give a level of about 1±2 mmoles of undissociated acetic acid in the bread. Sourdoughs have pH values around 4.0, brought about by fermentation of maltose and glucose to lactic and acetic acids by homofermentative and heterofermentative lactic acid bacteria. The titratable acidity of the final dough ranges from 30 to 120 meq/kg, with a molar ratio between lactic and acetic acid of about 5. However, preservation is not the main purpose of sourdough use.
Rather, the acid improves water uptake by the pentosans and inhibits amylases in rye flour. This is essential for breadmaking if the dough contains more than about 20% of rye flour.
Sorbate or propionate may be added to the dough in order to delay mould spoilage of baked goods. However, EU Directive 95/2 restricts the use of these preservatives to baked goods more prone to spoilage by moulds, namely, sliced and some other prepackaged products. The Directive also limits the levels to 0.1±0.3%, depending on the product.