# Oxidation in Low Moisture Foods as a Function of Surface Lipids and Fat Content
## 3 Results  
### Differences in Lipid Oxidation between Surface and Total Lipids  
In the first experiment, whole crackers were incubated and, periodically, the surface and total lipids were extracted; lipid hydroperoxides were determined to see if the surface lipids oxidize before total lipids as they do in spray-dried powders. In order to extract the surface fat from dried powders, various techniques that use different solvents (hexane, butanol, chloroform, isopropanol, and methanol), different extraction times, and different sample-to-solvent ratios have been studied [13; 14; 15; 16; 17]. The ideal system would constantly extract just enough fat to measure oxidation products, assuming that the short extraction time would primarily extract surface lipids. Extracting with chloroform:methanol (2:1) mixture for 1 min was found to extract enough lipid (1.58 \(\pm\) 0.05% of total cracker; 16 mg \(\pm\) 0.6 mg lipid/g cracker; 12.51 \(\pm\) 0.37% of total lipid in the cracker sample) to conduct hydroperoxide analyses. Headspace analyses were not conducted as volatile compounds would be lost during extraction. The lag phase for lipid hydroperoxide formation for both surface and total lipids was 8 days (Figure 2), indicating that oxidation rates were similar. Total lipids had higher hydroperoxide concentrations after 18 days, but this is not relevant to food quality and shelf life since both surface and interior lipids would be rancid by this time because sensory detection of oxidation products occurs shortly after the lag phase. Lipid hydroperoxide concentrations decreased in the later stages of storage, which is common as lipid hydroperoxide formation becomes slower than hydroperoxide breakdown upon extensive storage [29; 30; 31; 32; 33; 34].  
To verify that surface lipids do not oxidize faster than interior lipids, an experiment where crackers were stored intact (whole) or after they were crushed with a mortar and pestle was also conducted. Crushing the crackers alters the physical structures that protect the interior lipids, and if this were the case, one would expect the crushed crackers to oxidize faster than the whole crackers. When crackers with 8.1% f  
Figure 2: Surface oil vs. total lipid hydroperoxide formation at 55 \({}^{\circ}\)C. Data points are the average of 3 measurements \(\pm\) standard deviations.  
content in commercial crackers) were stored as whole or crushed, the lag phase of lipid hydroperoxide formation was the same at 8 days (Figure 3). Since these crackers were not subjected to solvent extraction, headspace hexanal could be monitored as well. The hexanal lag phase was 24 and 27 days for the whole and crushed crackers, respectively (Figure 4). Since hexanal formation was faster in whole than crushed crackers, this suggests that disrupting the cracker structure did not expose the interior lipids in a way that they would be more susceptible to oxidation.  
Figure 4: Hexanal formation in at 55 \({}^{\circ}\)C in crushed and round crackers. Data points are the average of 3 measurements \(\pm\) standard deviations.  
Figure 3: Lipid hydroperoxide formation at 55 \({}^{\circ}\)C in crushed and whole crackers. Data points are the average of 3 measurements \(\pm\) standard deviations.