Evaluation of temperature and storage time on oxidation of peanut butter

Peanut is a high-protein, nutrient-rich legume that is popular all around the world. Peanut butter is a kind of food paste made from ground roasted peanuts. It is found to be one of the most susceptible food products to oxidation due to its high content of PUFAs. Peroxide value is the only mentioned parameter for oxidation of peanut butter according to ISIRI 5690. Although oxidation pathways are mostly known, there are a lot of ambiguities about oxidation in real situations after production and during storage so there is a lack of studies in this field.   The present study investigated the effect of packaging and storage conditions on the oxidation of peanut butter in polypropylene single serving cup (PP) and polyethylene terepthalate (PET) jar due to widespread applications of these plastic materials in food industries. Peanut butter stabilized with mono and di-glycerides was filled in PP single serving cup (size: 30 g), and PET jar (250 g), then heat sealing was applied. Determination of chemical composition (moisture, fat, protein, sugar, ash and salt), fatty acids profile and microbial quality (total count, coliform, E. coli, coagulase positive Staphylococci, Salmonella, mold and yeast) was performed for produced samples. The study was conducted for 160 days at storage temperature of 4, 25 and 40°C on peanut butter and storage stability at specific time intervals (0, 30, 60, 90, 120, 160 days) was evaluated for changes in p-Anisidine value, peroxide value, acidity and sensory analysis. All analyses were carried out in triplicate and significant differences were determined using Duncan's multiple range test (p<0.05).   Peanut butter had a fat content of 54.55±0.85% which contained 80.92% unsaturated fatty acids (57.57% MUFAs and 23.35% PUFAs). PUFAs are susceptible to oxidation and are easily incorporated into the chain reaction mechanisms of lipid degradation. There was no sign of microbial contamination in produced peanut butter. Time and temperature had a significant interaction on peroxide value, p-Anisisdine value, acidity and sensory analyses in both of packaging materials. Different trends were observed in peroxide value data of peanut butter packaged in different plastics. Peroxide values were higher in PP compared with PET due to higher oxygen permeability through PP. PP cups stored at 40 ̊C had the highest amounts of peroxide value (52.55± 2.27 mEq/kg) and p-Anisisdine value (4.10± 0.22). Oxidation of peanut butter in PET jar occurred at lower rate. One notable point is that peroxide value at 40°C (4.61±1.04 mEq/kg) was less than 4°C (15.51±1.09 mEq/kg) and 25°C (14.19±0.78 mEq/kg) after 160 days. These values never reached the maximum standard limit (12 mEq/kg) at 40°C. Temperature of 4°C might not provide enough energy to convert hydroperoxides into other oxidation products so peroxides accumulated in peanut butter. Though significant differences were observed among data of acidity in both of packaging materials but differences were not large enough to be of value in a practical sense. As it was expected peanut butter did not provide its initial fresh taste especially at higher temperature but no sign of off-flavor and rancidity was detected by panelists. According to the obtained results,using polypropylene single serving cup for peanut butter led to decrease its shelf life due to its higher permeability to oxygen. In this situation samples should be kept under cold storage. To overcome this problem coated PP or other plastic materials with higher oxygen barrier properties are suggested. Peroxide value was the only indication of oxidation which was mentioned in ISIRI 5690, while it seems that determination of such a parameter in this product is not a proper index of quality.