Whey Protein Isolate and Lemon Essential Oil Based Edible Coatings for Macadamia Nuts

Abstract

Nut products are susceptible to oxidative rancidity because of their high unsaturated lipid composition. The rancidity may lead to undesirable appearance, odour and flavour. The growth of microorganism is another safety hazard of nut products, and moisture may enhance the growth of microbes. Thus, moisture and oxygen are two key factors that influence the quality of nut products. Since macadamia nuts have the highest content of unsaturated fatty acids among the regular edible nuts, it is imperative to introduce effective ways to protect the nuts from moisture and oxidation rancidity. Edible coatings are extensively studied recently because they are not only valid in prolonging food product shelf-life but also having added bioactive properties to the food product. This gives food products extra benefits, which meet the consumer demands. Protein-based edible coatings are claimed to have excellent oxygen barrier property in literature, which could be applied to food products to prevent lipid oxidation. Glycerol is a general plasticiser applied to edible film/ coating to enhance the physical properties. Lemon essential oil (LEO) is reported to be an effective antioxidant and antimicrobial agent. Therefore, this research applied whey protein isolate (WPI) as coating matrix, glycerol as the plasticiser and LEO as the bioactive agent to generate a bioactive edible coating. The coating procedure and formula were optimised, and results showed dipping once in the coating solution with 10% WPI (w/v) generated the best attributes under current experimental conditions. The antioxidant and antimicrobial capacity of the WPI/ GLY / LEO coating solution was verified through 2,2-Diphenyl-1-picrylhydrazyl (DPPH); 2,2-Azino-biz-3- ethylbenzothiazoline-6-sulfonic acid (ABTS); ferric ion reducing antioxidant power (FRAP) assays and minimal inhibitory concentration(MIC); minimal bactericidal concentration assays (MBC), respectively. The results suggested that the coating solution containing 1-2% LEO (w/v) could restrain the growth of E. coli, while 2% LEO (w/v) could kill E. coli; the MIC and MBC of LEO in the coating solution on S. aureus were 4-8% (w/v) and 8% (w/v), respectively. A sensory evaluation was conducted to obtain the sensory profiling and to test the overall acceptability of the coated macadamia nut products. It was discovered that coated nuts had higher scores in glossiness, flavour and hardness than the uncoated ones. However, the coated nuts with 1% LEO (w/v) were acceptable by consumers and had no significant difference with uncoated nuts in overall acceptability. A 28-day storage trial was performed to study the extent of oxidative rancidity and moisture change under different conditions. The oxidation behaviour and moisture variation were investigated by measuring peroxide value, free fatty acid value, moisture content and water activity. The results showed that the coatings were effective in mitigating oxidation and moisture increase and the efficacy increased as LEO concentration increased. The results of GC analysis correspond to this. In summary, the present study gave a potential solution in the preserving of whole macadamia nuts to the industry.