Effects of Temperature, Water Activity, and Syrup Film Composition on the Growth of Wallemia sebi : Development and Assessment of a Model Predicting Growth Lags in Syrup Agar and Crystalline Sugar

Abstract

We investigated the effects of temperature, water activity (a w ), and syrup film composition on the CFU growth of Wallemia sebi in crystalline sugar. At a high a w (0.82) at both high (20°C) and low (10°C) temperatures, the CFU growth of W. sebi in both white and extrawhite sugar could be described using a modified Gompertz model. At a low a w (0.76), however, the modified Gompertz model could not be fitted to the CFU data obtained with the two sugars due to long CFU growth lags and low maximum specific CFU growth rates of W. sebi at 20°C and due to the fact that growth did not occur at 10°C. At an a w of 0.82, regardless of the temperature, the carrying capacity (i.e., the cell concentration at t ‫؍‬ ؕ) of extrawhite sugar was lower than that of white sugar. Together with the fact that the syrup film of extrawhite sugar contained less amino-nitrogen relative to other macronutrients than the syrup film of white sugar, these results suggest that CFU growth of W. sebi in extrawhite sugar may be nitrogen limited. We developed a secondary growth model which is able to predict colony growth lags of W. sebi on syrup agar as a function of temperature and a w . The ability of this model to predict CFU growth lags of W. sebi in crystalline sugar was assessed.Crystalline sugar is covered with a very thin, nutrient-containing syrup film that originates from the mother syrup from which the sugar is crystallized (4, 12). Under the right environmental conditions, this syrup film can result in unwanted growth of xerophilic molds in crystalline sugar during storage (12). If contaminated crystalline sugar is used without prior treatment, it may increase the risk of downstream spoilage of sucrose-rich foods. Studies of the effects of different environmental conditions relevant for sugar storage, including temperature, water activity (a w ), and syrup film composition, on the development of xerophilic molds in crystalline sugar are therefore important.Wallemia sebi is a xerophilic mold that is able to grow at an exceptionally wide range of a w values in many types of solutes, and it exhibits rapid and vigorous sporulation (14). W. sebi is ubiquitous and is found in most foods with low and intermediate moisture contents, including sucrose-rich foods such as marzipan and cakes (14). W. sebi has no known sexual state (14) and therefore produces only conidia (asexual spores). During the life cycle of W. sebi, conidia germinate to form hyphae, and fertile hyphae rapidly produce conidia by complete fragmentation (9,14).Secondary growth models describe a change in a certain aspect of microbial growth as a function of environmental conditions (19). Many secondary growth models used for molds describe the maximum radial growth rate as a function of pH, temperature, or a w (3,5,6,13,17,18). The radial growth rate, however, focuses on growth after it has occurred. For the food manufacturer it is often more valuable to know when growth of food spoilage microorganisms occurs or (perhaps even more useful) when it does not. For this purpose, se...

Model of Fungal Development in Stored Barley Ecosystems as a Prognostic Auxiliary Tool for Postharvest Preservation Systems

“…The estimated values of lag phase duration prolonged with a decrease in water activity in grain and with a decrease in storage temperature. This trend is consistent with the results of other researchers (Aldars-García et al 2017;Galati et al 2011;Vindeløv and Arneborg 2002;Wawrzyniak et al 2013;Zimmermann et al 2011). The comparison of the estimated τ lag duration with the maximum number of days, for which the OTA concentration in barley ecosystems was lower than the legislative limits of 3 and 5 mg kg −1 for cereal products for direct human consumption and unprocessed cereals, respectively (EC 2002(EC , 2004(EC , 2005(EC , 2006(EC , 2012, showed that in all cases, the estimated τ lag was shorter than the time, after which the admissible limits of OTA was exceeded (Table 3) (Wawrzyniak et al 2018a).…”

“…Moreover, the measurement of colony diameter on petri dishes does not represent the true tridimensional fungal growth (Garcia et al 2009). The main limitation of the abovementioned method is the fact that it cannot be applied in food analysis, as it focuses on visible growth, whilst for food manufacturers, the knowledge of the activity of food spoilage microorganisms at the earliest stage of its development is more valuable (Garcia et al 2009;Vindeløv and Arneborg 2002). Saxena et al (2001) examined the relationships of ergosterol levels, total viable mould count and ochratoxin A production of Aspergillus ochraceus and Penicillium verrucosum and showed that the viable mould count, expressed in colony forming units (CFU) per gram of grain (cfu g −1 ), makes it possible to detect mould growth and thus the risk of OTA formation sooner than ergosterol used as an indicator of fungal biomass.…”

Use of Stochastic Models To Assess the Effect of Environmental Factors on Microbial Growth

“…Deterministic mathematical models (e.g., Gompertz and logistic equations) are often proposed as the true underlying "perfect" mechanism and fitted to the data via nonlinear fitting techniques (8,13,16,19,52,59). Deviations from the deterministic predictions are accounted for as observational uncer- tainty.…”