Influence of Post-Harvest Techniques on Nutritional and Microbial Quality of Selected Edible Insects

Abstract

Edible insects have traditionally been a dietary source in many African and Asian rural populations. Their potential use as food has increased globally due to their high nutritional content. Microbiological quality and safety concerns regarding edible insects however calls for simple processing methods which are capable of decontaminating this food type while maintaining their nutritional profiles. The aim of this study was to evaluate the impact of traditional post-harvest processing techniques: boiling, toasting, solar drying, oven drying, boiling + solar drying, boiling + oven drying, toasting + solar drying, and toasting + oven drying on the nutritional composition and microbiological quality of adult house cricket (Acheta domesticus), adult grasshopper (Ruspolia differens), black soldier fly (Hermetia illucens) pre-pupae and cotton leaf worm (Spodoptera littoralis) 5th instar larvae. All the insect species were collected from the insect rearing and containment unit of the International Centre for Insect Physiology and Ecology (ICIPE) in Kenya, except for R. differens which was harvested from the wild in Uganda. The chemical and microbiological analysis were performed in all the processed edible insect samples, with their raw counterparts as the control. The experiments were replicated three times and the data analysed using Stata SE version 12. Raw edible insects had crude protein and crude fat values ranging from 36.3-52.3% and 17.4-29.6%. Processing significantly increased the crude protein (P < 0.001) by 1.2-22% in the order: toasting > boiling > oven-drying > solar-drying, with the exception of solar-dried A. domesticus (P = 0.144), solar-dried and oven-dried R. differens (P = 0.95; P = 1.00) whose crude protein increments were insignificant. Conversely, processing significantly decreased (P < 0.001) the crude fat by 0.8-51% following the same order. However, solar-dried edible insects had the least crude fat reduction, which was insignificant (H. illucens: P = 0.051; A. domesticus: P = 1.000; R. differens: P = 0.148; S. littoralis P = 1.000). Both H. illucens and S. littoralis had more saturated fatty acids than A. domesticus and R. differens. Processing significantly increased (P < 0.001) the saturated fatty acids 1-1.6 times, while unsaturated fatty acids significantly decreased (P < 0.001) by 0.6-0.9 factors in all the edible insect samples, with toasting and toasting + drying having the highest impact. Toasted + oven-dried insect samples had the least desirable amount of linoleic acid (2.96-25.13%) and polyunsaturated fatty acid/saturated fatty acid (PUFA/SAFA) ratio (0.04-0.49). Hermetia illucens had the most stable omega-6/omega-3 (n-6/n-3) ratio (1-1.01) during processing, compared to all the other edible insects in this study. The iodine value was significantly decreased (P < 0.01) by 0.2-1 factors, while the peroxide and saponification values were significantly increased (P < 0.01) by 1.1-25.4 and 0.9-3.9 factors respectively in all the processed edible insect samples. Solar and oven drying significantly increased (P < 0.01) the cholesterol content of all the edible insect species by 2.3-15.9%, while the other processes significantly decreased (P < 0.001) the cholesterol value. Processes that involved boiling (boiling and boiling + drying) led to a 2.4 to 80.2% decrease in the mineral content of all the edible insect species, with boiled insect samples having the highest decrease. However, processes such as toasting, drying and toasting + drying significantly increased the insect’s mineral content (P < 0.01). Combined processing (boiling/toasting + drying) had the most negative impact on all the vitamins and amino acids evaluated. Raw insect samples had high microbial counts. For instance, the total viable counts (TVC), yeasts and moulds counts (YMC) and Enterobacteriaceae counts ranged from 7-9.1 Log cfu/g, 6.4-8.2 Log cfu/g and 5.3-7.9 Log cfu/g respectively. Moreover, these samples tested positive for Salmonella spp, hence unfit for consumption. Boiling and toasting of all edible insects samples reduced microbial TVC by 4-5 log cycles, Staphylococcus aureus by 4.5-6.4 log cycles and totally eliminated Enterobacteriaceae, faecal coliforms, Lactose positive enteric (Lac+) bacteria, Salmonella spp, and YMC. Oven drying significantly lowered the edible insects TVC (P < 0.001) and YMC (P < 0.001) by 1-2 and 2-4 log cycles respectively, while solar drying had no significant effect (P > 0.05) on these parameters. Combined processing maintained the microbial quality of the boiled/toasted edible insects. These results suggest that the edible insects in this study are highly nutritious but harbour both spoilage and pathogenic microorganisms, which could be detrimental upon ingestion. Actionable hazard control mechanisms like boiling and toasting, which are considered basic and traditional, could be used to decontaminate edible insects and improve their microbial safety.