Growth dynamics of Bacillus cereus in pasteurised milk was examined in storage tests performed at 5, 7, 9, 11, and 13°C. The contents of B. cereus in pasteurised milk ranged from the absence in 4 ml of milk to 2.3 cfu/ml. The initial total plate counts varied from 1.1 × 10 4 to 3.0 × 10 4 cfu/ml (n = 15). Growth curves of Bacillus cereus showed that strains naturally present in pasteurised milk grew well at 5 and 7°C. A square root model was used for the growth rate analysis in relation to the storage temperature of milk (√µ = 0.026 (T-T min ); R 2 = 0.93). Lag-time of B. cereus was described by a modified Arrhenius-type equation (λ = -45.667 + 1035.3/T; R 2 = 0.94). The comparison of the time prediction calculated for B. cereus to reach the density of 10 4 cfu/ml in pasteurised milk and that for the total plate counts to reach levels of 5.0 × 10 4 cfu/ml proved that these plate counts were reached in all tests earlier than the level of 10 4 cfu/ml could be reached by B. cereus. It is thus concluded that pasteurised milk is spoiled by the growth of saprophytic psychrotrophic bacteria before B. cereus can produce hazardous levels of its enterotoxin. Keywords: pasteurised milk; B. cereus; shelf-life 196Vol. 21, Czech J. Food Sci.ubiquitous in raw milk, survive the pasteurisation process, and produce different enterotoxins which may cause food poisoning of the diarrhea or emetic types (G����� et al. 1993; N�������� & T����� 1993). Moreover, many of the strains can grow at 4-6°C. Recently, a new psychrotolerant species, B. weihenstephanensis, was detected by L������ et al. (1998) and M��� et al. (1999). Heat resistance of B. cereus spores varies with different strains. The following D 95 -values of different B. cereus strains were found: 8.2-8.4 min by L������ et al. (1999), 4.4-4.6 min by G������� et al. (1999), 0.9-11 min by F�������� et al. (2001) at pH = 7, and 0.80 to 1.68 min at pH = 6.5 adjusted by organic acids by L��������� and M����� (2001). D-values of B. cereus spores published by M��� (1996) were at 121°C in the range of 2-4 s.Spores of B. cereus are very hydrophobic (D���� et al. 1984) and can adhere to the surfaces of stainless steel. The a�ached spores may subsequently germinate, multiply, take part in forming biofilm and then resporulate (A�������� et al. 1995; T� G����� et al. 1997; A��������� et al. 2000; P��� et al. 2001).B. cereus outbreaks were associated mostly with the ingestion of catered meals (T��� 1995; M����� et al. 1999). Only several outbreaks were mentioned in which milk or milk products could have been the cause of food poisoning (B����� et al. 1994; N����-���� et al. 1997). The poisoning, however, seems to be underestimated in the literature and statistics because of its short duration and because it might have been assumed, by mistake, as having been caused by S. aureus or C. perfringens enterotoxins (G����� et al. 1995). Symptoms such as nausea, diarrhea, and vomiting appeared within 7 h after consumption in 70% of the persons involved.Relationships between the sources ...
L������ D., V���� Ľ., G����� F. (2003): Effect of lactic acid on the growth dynamics of Candida maltosa YP1. Czech J. Food Sci., 21: 43-49.The growth dynamics of the oxidative imperfect yeast strain Candida maltosa YP1 isolated from the surface of fruit yoghurt was studied in relation to the lactic acid concentration ranging from 0 to 1.6% (w/v). The maximal specific growth rate of 0.36 h -1 and minimal lag-phase duration of 2.9 h were found in the glucose solution without lactic acid at 25°C. The decrease of the natural logarithm of both the specific growth rate (ln µ) and the lag-phase prolongation (ln λ) in the dependence on the increase of lactic acid concentration (0-1.59%) was significantly linear (ln µ = -1.1458 -0.6056 c; R = 0.9577). Based on these equations, the prediction of the time necessary for C. maltosa YP1 to reach 1 × 10 6 CFU/ml in the dependance on lactic acid concentration and, the initial density of the yeast culture was calculated. For example, C. maltosa YP1 was able to reach the level of 1 × 10 6 CFU/ml in a model glucose solution at the initial concentration N 0 = 1 CFU/ml, 0.9% lactic acid and 25°C within 2 d. The growth predictions presented indicate a considerable resistance of C. maltosa YP1 to lactic acid in the concentration of up to 1.3% (w/v).
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