Effects of water activity, sugars, and proteins on lipid oxidative stability of low moisture model crackers

Understanding lipid oxidation mechanisms in low moisture foods is necessary to develop antioxidant strategies to increase shelf life and/or to improve nutritional quality by increasing polyunsaturated fatty acid concentrations. In this study, we examined the influence of water activity (a_w), sugars (glucose, maltose, maltodextrin, and cyclodextrin), and proteins (casein and gluten) on the lipid hydroperoxide and hexanal lag phases of model crackers. Oxidative stability of crackers was in an order: a_w 0.7 > a_w 0.4 > a_w 0.2 > a_w 0.05. Higher water activities resulted in bigger differences between hydroperoxide lag phases and hexanal lag phases. Compared to non-reducing cyclodextrin and no added sugar controls, reducing sugars including glucose, maltose, and maltodextrin at the same dextrose equivalence increased both hydroperoxide and hexanal lag phases. At the same dextrose equivalence, oxidative stability was in the order of maltose > maltodextrin > glucose > control (no sugar added). The antioxidant effectiveness of maltose, a low sweetness profile sugar, increased with increasing concentrations from 1.1 to 13.8%. Increasing a_w increased the antioxidant activity of maltose. For example, 1.1% maltose increased both hydroperoxides and hexanal lag phases by 9 days at an a_w of 0.2, but increased hydroperoxide lag phase by 24 days and hexanal lag phase by 15 days at an a_w of 0.7. Gluten was able to inhibit lipid oxidation with activity increasing with increasing a_w while casein showed minimal antioxidant impact. Antioxidant activity of gluten decreased when its sulfhydryl groups were blocked by N-ethylmaleimide suggesting that cysteine was an important antioxidant component of gluten. Adjusting water activity and addition of reducing sugars and gluten could be strategies to increase oxidative stability of low moisture crackers.