Many of the antifungal agents reviewed in this chapter need to be used at extreme concentrations or levels in a food to be effective when used alone. However, a variety of factors can prevent growth of fungi. While fungi tend to be more tolerant to adverse environmental conditions than bacteria, combining inhibitory factors such as temperature, water activity, or pH with antifungal agents can result in considerable improvement of the microbial stability of foods. Suitable combinations of growth-limiting factors at subinhibitory levels can be devised so that certain microorganisms can no longer proliferate.
Sorbic acid at 1000ppm and pH 7.0 will not inhibit mold growth. However, if the pH is lowered to 5.0, growth of most molds will be inhibited (Liewen and Marth 1985). Antioxidants such as BHA and BHT have been shown to potentiate the action of sorbic acid (Scott 1989). In general, antifungal food additives become more effective as environmental conditions move away from the optimum for a particular organism.
The level of a single growth-limiting factor that will inhibit a microorganism is usually determined under conditions in which all other factors are optimum. In preserving foods more that one factor is usually relied upon to control microbial growth. Addition of a substance, which in itself does not give full inhibition, can effectively preserve products in the presence of other subinhibitory factors. The effect of superimposing limiting factors is known as the "hurdles concept" (Leistner 1999).
Little information is currently available on combining subinhibitory factors to preserve food. It is very time consuming and expensive to design preservative systems using the hurdles concept by random design. Predicative modeling can be used to test the consequences of a number of factors changing at the same time. With proper design and interpretation, preservative systems can be designed rapidly and efficiently (Whiting and Buchanan 1994). However, product challenge studies should be conducted to verify the effectiveness of a combination of subinhibitory factors (Labuza and Taoukis 1992).
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