Physicochemical and Microbiological Stability of Semi-processed Edible Crickets (Acheta domesticus) and Black Soldier Fly Larvae (Hermetia illucens) during Storage

Abstract

Edible insects are a dietary source of proteins for a significant part of rural population in Africa and Asia. Just like any other type of food, edible insects during storage are prone to microbial, chemical and physical changes influenced by both extrinsic and intrinsic factors. In this study the effect of storage time, storage environment and type of packaging were examined. Chemical, microbial and physical changes were all monitored during storage through commonly employed analyses including; peroxide value (PV) , p-anisidine value (P-AV), saponification value (SV), iodine value (IV) and fatty acid analyses for lipid oxidation and total viable count (TVC), yeasts and moulds, Enterobacteriaceae, E.coli and Salmonella counts for microbial stability. Moisture adsorption isotherms were also determined gravimetrically to help identify critical moisture contents of storage that can help optimize and predict the shelf-life of the semi-processed insects. Black soldier fly larvae (BSFL) and adult house cricket collected from specific rearing sites were boiled, solar dried, ground into powder and packaged in polypropylene (PP), plastic (PL) and polyethylene (PE) packages. These were then stored in either ambient or refrigerated conditions for a period of 6 months where by quality assessments were done after every 45 days. Temperature and relative humidity in the trial sites were monitored using a data logger. The PV, P-AV and SV increased (P<0.05) overtime while IV decreased (P<0.05) overtime in all the samples regardless of packaging treatment and type of storage environment. The changes were however significantly higher (P<0.05) in samples stored at ambient conditions than the refrigerated samples. The PV, P-AV and SV of both samples in all the sampling stages were in the order of PP>PE>PL while IV was in the order of PL>PE>PP. The polyunsaturated (PUFA) and monounsaturated (MUFA) fatty acids decreased overtime while the saturated (SFA) fatty acids increased. This was in the order of PP>PE>PL for both insects in both storage environments. All the microorganisms detected at the baseline level also showed a general significant increase (p<0.05) during storage. Yeasts and moulds were not detected at the baseline level in both insect species but were present in all the other subsequent sampling periods. Salmonella was not detected in the adult house cricket but was present in BSFL. Three types of fungi; Aspergillus spp., Alternarie spp. and Penicillium spp. were isolated in all the packages during storage in both insect species. Microbial counts among the different packages was in the order PP>PE>PL. The BSFL sample showed a sigmoidal form of degradation where by most drastic changes were seen at 45-90 days and 135-180 days while in cricket most drastic changes were seen in the period 0-45 days. The moisture adsorption isotherms were of type II according to Brunauer classification and the sorption capacities decreased with increasing temperatures. Cricket had a higher sorption capacity whereas BSFL was more sensitive to temperature changes. Transition from bound to free water in both insect sample begins at approximately 5 g/100 g. Shelf life of up to one year can be achieved if the cricket and BSFL powders are dried to approximately 6.5 g/100 g and 6 g/100 g respectively and packaged in 80μm bags. The results suggest that when the semi-processed insect powders are stored in packages with low water vapour and gas permeability and at lower temperatures, degradation during storage is reduced and a longer shelf life can be achieved. Measures should be put in place to avoid contamination during processing, packaging and storage so as to achieve a longer shelf life.