Strategies to increase the stability of intermediate moisture foods towards Zygosaccharomyces rouxii: the effect of temperature, ethanol, pH and water activity, with or without the influence of organic acids.

Intermediate moisture foods (IMF) are in general microbiologically stable products. However, due to health concerns consumer demands are increasingly forcing producers to lower the fat, sugar and preservatives content, which impede the stability of the IMF products. One of the strategies to counteract these problems is the storage of IMF products at lower temperatures. Thorough knowledge on growth/no growth boundaries of Zygosaccharomyces rouxii in IMF products, also at different storage temperatures is an important tool for ensuring microbiologically stability. In this study, growth/no growth models for Z. rouxii, developed by Vermeulen et al. (2012) were further extended by incorporating the factor temperature. Three different data sets were build: (i) without organic acids, (ii) with acetic acid (10,000 ppm on product basis) and (iii) with sorbic acid (1500 ppm on product basis). For each of these data sets three different growth/no growth models were developed after 30, 60 and 90 days. The results show that the influence of temperature is only significant in the lower temperature range (8-15 °C). Also, the effect of pH is negligible (pH 5.0-6.2) unless organic acids are present. More specific, acetic acid had only an additive effect to ethanol and aw at low pH, whereas sorbic acid had also an additive effect at the higher pH values. For incubation periods longer than 30 days the growth/no growth boundary remained stable but enlarged gradually between day 60 and 90, except for the lower temperature range (<12 °C) where the boundary shifts to more stringent environmental conditions.

Incidence, diversity and characteristics of spores of psychrotolerant spore formers in various REPFEDS produced in Belgium.

The major objectives of this study were to determine the incidence of psychrotolerant spore formers from REPFEDS marketed in Belgium, and their diversity and characteristics. Spore formers in general were found as spores on 38.3% of the food samples and in 85% food products types evaluated. 76% of the food samples containing spore formers had spores before enrichment. A total of 86 spore formers were isolated from the samples. 28 of 86 bacterial spore formers (32.6%) were capable of vegetative growth at 7 °C. 96% (27/28) of these psychrotolerant spore formers were determined to belong to Bacillus or related genera. According to a (GTG)5-PCR analysis, 24 of these 28 isolates were genetically distinct from each other. 10.7% (3/28) of the bacilli were determined to belong to the Bacillus cereus group, namely B. cereus (chicken curry and Edam cheese) and Bacillus mycoides (Emmental cheese). Almost half of the bacilli (12/27) were putatively identified as Bacillus pumilus, which occurs ubiquitously in nature and has been associated with outbreaks of foodborne disease. Only one psychrotolerant clostridium, Clostridium tyrobutyricum, was isolated in the study. The results of this study show the highly diverse ecology and spoilage potential of psychrotolerant spore formers in REPFEDs marketed in Belgium.

The combined effect of pasteurization intensity, water activity, pH and incubation temperature on the survival and outgrowth of spores of Bacillus cereus and Bacillus pumilus in artificial media and food products.

The objective of the study was to evaluate the combined effects of pasteurization intensity (no heat treatment and 10 min at 70, 80 and 90 °C), water activity (aw) (0.960-0.990), pH (5.5-7.0) and storage temperature (7 and 10 °C) on the survival and outgrowth of psychrotolerant spores of Bacillus cereus FF119b and Bacillus pumilus FF128a. The experiments were performed in both artificial media and a validation was performed on real food products (cream, béchamel sauce and mixed vegetable soup). It was determined that in general, heat treatments of 10 min at 70 °C or 80 °C activated the spores of both B. cereus FF119b and B. pumilus FF128a, resulting in faster outgrowth compared to native (non-heat treated) spores. A pasteurization treatment of 10 min at 90 °C generally resulted in the longest lag periods before outgrowth of both isolates. Some of the spores were inactivated by this heat treatment, with more inactivation being observed the lower the pH value of the heating medium. Despite this, it was also observed that under some conditions the remaining (surviving) spores were actually activated as their outgrowth took place after a shorter period of time compared to native non-heated spores. While the response of B. cereus FF119b to the pasteurization intensity in cream and béchamel sauce was similar to the trends observed in the artificial media at 10 °C, in difference, outgrowth was only observed at 7 °C in both products when the spores had been heated for 10 min at 80 °C. Moreover, no inactivation was observed in cream or béchamel sauce when the spores were heated for 10 min at 90 °C in these two products. This was attributed to the protective effect of fat in the cream and the ingredients in the béchamel sauce. The study provides some insight into the potential microbial (stability and safety) consequences of the current trend towards milder heat treatments which is being pursued in the food industry.

Validated empirical models describing the combined effect of water activity and pH on the heat resistance of spores of a psychrotolerant Bacillus cereus strain in broth and béchamel sauce.

The major objective of this study was to evaluate and model the combined effect of the water activity (a(w)) and pH of the heating menstrum on the heat resistance of spores of a psychrotolerant Bacillus cereus strain isolated from béchamel sauce. Two models, a quadratic polynomial equation and a reparameterized function, were assessed for their ability to describe the combined influence of a(w) and pH on the D(85°C)-values of the B. cereus isolate in tryptone soy broth. The performance of the models was validated by challenging the models with data independently collected in broth and béchamel sauce. Both models were found to adequately describe the validation data obtained in broth. However, it was determined that in béchamel sauce the predictions of the polynomial function not only showed bias (bias factor = 1.156) but were also fail-dangerous, as they deviated from the validation data by 17.2%. The reparameterized function was determined to be a good predictor of the D(85°C)-values in béchamel sauce as it showed no bias (bias factor = 1.033) and its predictions differed by only 7.9% from the validation data. The reparameterized function can be used to provide estimates of the minimum processing conditions required to achieve desired levels of spore inactivation within the a(w) and pH ranges studied and to determine the potential changes in heat resistance of B. cereus spores when a(w) and pH are changed, for example, during product reformulation. As validation of heat resistance models is rarely performed, let alone in actual food products, the models evaluated and validated in this study (in particular the reparameterized function) are of immediate relevance to the food industry.

Incidence, diversity and toxin gene characteristics of Bacillus cereus group strains isolated from food products marketed in Belgium.

The major objectives of this study were to determine the incidence, diversity and characteristics of Bacillus cereus group spp. isolated from food products marketed in Belgium. The food products investigated in this study included cooked pasta, lasagna, béchamel sauce, bolognaise sauce, fresh minced beef, fresh-cut vegetables and raw basmati rice. B. cereus group spp. were detected in 56.3% (324 of 575) of the samples giving rise to 380 strains. The highest incidence (100%) occurred in the raw basmati rice. Although only 10 (2.6%) of the 380 isolates were determined to be psychrotolerant (able to grow at ≤7°C), 25 (6.2%), 189 (49.7%) and 334 (87.9%) isolates were able to grow at mild temperature abuse conditions of 8°C, 9°C and 10°C, respectively. The large diversity of the isolates obtained (overall and between isolates obtained from the same product type) was highlighted by the results of the (GTG)(5) PCR fingerprinting of 80 selected isolates. Sixty-one of these 80 isolates belonged to 15 distinct clusters (≥85% Pearson correlation) whereas the remaining 19 were each clustered separately. Further diversity was also found in the distribution of toxin genes as 16 different profiles were observed in the 80 selected isolates. Whilst none of 80 selected strains harboured the ces gene required for the production of the emetic toxin cereulide, 42 strains (52.5%) carried all seven genes required for the production of the diarrhoeal enterotoxins: haemolytic BL, non-haemolytic enterotoxin and cytotoxin K. The results of this study highlight not only the omnipresence but also the highly diverse ecology of B. cereus spp. within and across several food product types available on the retail market in Belgium. They should also provide the impetus for more studies to enable detailed risk assessment studies to be performed.

Influence of NaCl reduction and replacement on the growth of Lactobacillus sakei in broth, cooked ham and white sauce.

The growth inhibiting effects of NaCl and selected simple salt replacers (CaCl(2), MgCl(2), KCl and MgSO(4)) on the growth of Lactobacillus sakei were studied in de Man Rogosa Sharpe broth at 7 degrees C over a water phase concentration of 0 to 6.4%. The divalent chloride salts (CaCl(2) in particular) generally had the largest antimicrobial activities at equivalent water phase concentrations, molalities or water activity (a(w)) values. MgSO(4) had not only the least antimicrobial activity but also the smallest a(w) depressing capacity. The results also showed that the antimicrobial effects of CaCl(2) were not fully accounted for by its a(w) depressing effects. Challenge tests performed on cooked ham and white sauce showed that reduction of NaCl levels by 28 and 33%, respectively, had no influence on the microbial stability of these products to L. sakei. Ultimately the study concluded that the microbiological consequences of the full or partial replacement of NaCl on the growth of L. sakei largely depend on the initial level of NaCl, the level of replacement and the nature of the salt replacer used. Altered stability to L. sakei is most likely given a high initial NaCl level, combined with a large level of partial replacement with either CaCl(2) (increased stability) or MgSO(4) (reduced stability) as the replacer.