Design of an experimental viscoelastic food model system for studying Zygosaccharomyces bailii spoilage in acidic sauces

Abstract

Within the field of predictive microbiology, the number of studies that quantify the effect of food structure on microbial behavior is very limited. This is mainly due to impracticalities related to the use of a nonliquid growth medium. In this study, an experimental food model system for studying yeast spoilage in acid sauces was developed by selecting a suitable thickening/gelling agent. In a first step, a variety of thickening/gelling agents was screened, with respect to the main physicochemical (pH, water activity, and acetic acid and sugar concentrations) and rheological (weak gel viscoelastic behavior and presence of a yield stress) characteristics of acid sauces. Second, the rheological behavior of the selected thickening/gelling agent, Carbopol 980, was extensively studied within the following range of conditions: pH 4.0 to 5.0, acetic acid concentration of 0 to 1.0% (vol/vol), glycerol concentration of 0 to 15% (wt/vol), and Carbopol concentration of 1.0 to 1.5% (wt/vol). Finally, the applicability of the model system was illustrated by performing growth experiments in microtiter plates for Zygosaccharomyces bailii at 0, 0.5, 1.0, and 1.5% (wt/vol) Carbopol, 5% (wt/vol) glycerol, 0% (vol/vol) acetic acid, and pH 5.0. A shift from planktonic growth to growth in colonies was observed when the Carbopol concentration increased from 0.5 to 1.0%. The applicability of the model system was illustrated by estimating mu(max) at 0.5% Carbopol from absorbance detection times.

FIG. 1.

Illustration of viscoelastic behavior of a commercial ketchup (a) and determination of yield stress value using the tangent crossover method for a commercial tomato ketchup, mayonnaise, and salad dressing (b).

FIG. 2.

Illustration of viscoelastic behavior of various samples: growth medium based on agar, CMC, and locust bean gum (LBG) (a); growth medium containing xanthan gum (XG) (b); and growth medium containing Carbopol 980 (C980) in the presence and absence of acetic acid (AA) (c). With the exception of ketchup, all media contained the required amounts of SAB medium, glucose, and fructose.

FIG. 3.

Picture of Z. bailii colonies in a microtiter plate well in a growth medium at 1.0% Carbopol and 5% glycerol and pH 5.0. Colonies on the surface and submerged colonies are visible.

FIG. 4.

Illustration of growth curves obtained in four representative wells at 1.0% Carbopol 980 (a) and 1.5% Carbopol 980 (b).

FIG. 5.

Series of OD growth curves at 0.5% Carbopol used for determination of absorbance detection times (a) and determination of μ_max from linear regression between inoculum size and absorbance detection time in liquid medium (○) and at 0.5% Carbopol (+) (b).

FIG. 6.

Growth curves of Z. bailii in liquid medium (○) and at 0.5% Carbopol (+), with primary fits of the model of Baranyi and Roberts (5).