Chemical and Biological Properties of Silk Protein and Application of Silk Protein for Functional Food Product Development

Abstract

Silk proteins have a number of advantageous substances including proteins and pigments. The proteins, sericin and fibroin, are widely studied for medical applications due to their good physiochemical properties and biological activities such as anti-oxidation, anti-inflammatory, biocompatibility, acceleration of cell proliferation, and activation of collagen synthesis. Recently, many studies have focused on the use of silk proteins for the production of drugs or cosmetics; however there has been little information on the development of food products from silk-cocoon proteins. Therefore, the objective of this study was to study an appropriate silk protein extraction method to obtain the chemical and biological properties of silk protein for development into functional food products. Four strains of silk cocoons were selected on the basis of the availability of strains in Northeastern Thailand, namely Leaungsaraburi, Nangsew, Nangtui and Eri. The extraction method of silk protein involved distilled water at 100°C for 2, 4, 6 and 8 hours and also with enzyme (Alcalase). The resulting extracts were evaluated for protein content, amino acids, total phenolic content (TPC), phenolic compounds, total flavonoid content (TFC), flavonoid compounds, DPPH radical-scavenging activity, ABTS radical scavenging capacity assay, FRAP assay, anti-glycation, α-amylase and α-glucosidase inhibitory activities. Furthermore, the molecular mass distribution was assessed by SDS-PAGE. The results showed that longer extraction times provided higher contents of all parameters using extraction times of 2-6 hours; however, these values started to decline after 6 hours of extraction. Interestingly, using enzyme extraction gave the highest values of all parameters for all cultivars studied. The SDS-PAGE results showed that silk protein molecules broke down after 4 hours of extraction. The molecular weights of protein were in range of 10-15, 45-65 and over 75 kDa whereas using the enzyme, the molecular weights of the proteins were between 25-35 and 35-45 kDa. In a toxicity assessment, the samples tested did not show toxicity to the cells studied in Cancer cells of the breast (MCF-7), lung (HCT116) and in normal cells that are not cancerous cells (Vero). However, the major concern of the product is its bitterness taste. Subjectively, we had tested the extract with someone from the company (Siam Natural Product, Co. Ltd). It was quite obvious that the water extract provided a better taste with less bitterness than the enzyme extract. More importantly, the operating cost to produce the enzyme extract is rather more expensive for industry. Therefore, water extraction for 6 hrs was selected for further development of the possible silk food product. A silk drink product was developed using a mixture-design program with factors consisting of 90-100% silk protein extract (X1) and 0-5% honey (X2). The most acceptable formula consisted of 92.4% of silk protein extract and 7.6% of honey.

In conclusion, this research has demonstrated that the silk cocoon has proved to possess proteins with high bioactivities, extractable using water and enzyme. This potential of silk protein extracts can be developed into valuable functional foods and nutraceuticals.

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