# Oxidation in Low Moisture Foods as a Function of Surface Lipids and Fat Content
## 1 Introduction  
Lipid oxidation has been studied extensively in various food matrices, such as bulk oils, emulsions, and meats [1, 2, 3, 4, 5]. However, the fundamental mechanisms of lipid oxidation in low-moisture foods (e.g., crackers, breakfast cereals, cakes, cookies, and baked goods) are much less understood, which is somewhat surprising since the shelf-life of many of these products is limited by oxidative reactivity [6]. Gaining a better understanding of the fundamental mechanism of lipid oxidation in these low moisture foods could help in taking a systematic approach to developing antioxidant strategies for this food category. If antioxidant technologies can be developed to prevent rancidity, then any oil can be utilized in these products. Lipid oxidation, which decreases product quality and safety by forming off-aromas and degrading nutrients and potentially forming toxic compounds, starts taking place as soon as biological tissues are processed into foods; this oxidation continues during handling and storage [7]. Crackers and cookies are expected to have long shelf lives, but they will eventually spoil due to oxidative degradation [8]. The mechanisms of the rancidity do not follow the same route for bulk oils and fats as they do for baked goods due to the composition and physical structure of the baked goods [8]. Chemically leavened low moisture foods typically contain flour, fat, water, salt, and baking soda and/or baking powder. Solid fat is generally needed to help in the incorporation of air and to limit the formation of gluten to make the products tender and/or flaky. The leavening agent is another factor affecting cracker quality [9]. Commonly used leavening agents include sodium bicarbonate, ammonium bicarbonate, and monocalcium phosphate [9], and these additives tend to make the crackers alkaline. Recent studies have helped researchers gain a better understanding of lipid oxidation in chemically leavened model crackers. Different from most other food systems, the lag phases of lipid hydroperoxide formation are much shorter than the lag phase of hydroperoxide breakdown products such as hexanal [10]. This suggests that pro-oxidants that decompose hydroperoxides are less active in chemically leavened crackers. A majorpro-oxidant that decomposes hydroperoxides is transition metals; it was found that the addition of iron did not increase oxidation rates, and the presence of metal chelators did not decrease oxidation, suggesting that iron is not a major pro-oxidant in the chemically leavened crackers [10]. Another factor that impacts lipid oxidation in low moisture foods such as crackers is water activity. [PERSON] and [PERSON] (1972) studied the oxidation behavior of potato chips, representing moisture- and oxygen-sensitive foods, and showed that when a product was dried too much and water activity became too low, lipid oxidation reactions would increase [11]. In a chemically leavened system, [PERSON] et al. (2020) found that lipid oxidation decreased with increasing water activity from 0.05 to 0.7 [12]. This was different from potato chips, which had the lowest oxidation rate at intermediate relative humidity (40% RH = water activity, aw = 0.4), with oxidation increasing with increasing relative humidity [11]. It is not clear why lipid oxidation reactions in cracker systems and potato chips respond differently to water activity, but one potential reason could be the physical location of the lipids. In the crackers, the lipids are incorporated into the dough, where they form a heterogeneous matrix with starch and protein. Conversely, in potato chips, the lipids adhere to the exterior of the product during cooking and also penetrate the potato cells upon removal of water during thermal processing. A method used to study lipid oxidation as a factor of lipid location is to isolate surface or rapidly extractible lipids.