Drying kinetics, nutritional and functional characterisation of flours produced from edible insects from East Africa

Abstract

The growing global awareness of edible insects as a nutritious food that can be used to contribute in the fight against protein energy malnutrition has met many barriers. These barriers include sustainable processing techniques especially in rural areas where most wild edible insects are collected and the disgust factor which shrinks the consumer market as most consumers are turned-off eating meals having visible whole edible insects. This disgust factor could be circumvented by producing dry insect-based food ingredients that can be included in familiar food as it masked the appearance of the whole edible insect. It is therefore important to understand the drying behaviour, nutritional and functional properties of these edible insect flour to successfully guide their suitability as food ingredients into familiar foods without greatly altering the food’s sensory properties. This study evaluated the effect of sustainable solar dying methods (sun drying, solar cabinet drying, blanching before sun drying or solar cabinet drying) on the drying behaviour of African field cricket; the effect of these sustainable drying methods compared to conventional drying methods (freeze drying and oven drying at 40°C) on the nutritional properties of edible long horn grasshoppers, African field crickets and saturniid caterpillar; and the effect of the sustainable and conventional drying methods on the functional properties of flours produced from defatted edible grasshopper, cricket and caterpillar compared to commercially available soy and whey protein ingredients as a novel protein-rich food ingredients in familiar foods. The decrease in moisture content of African field cricket over time was best described by the empirical quartic polynomial model (fourth order polynomial) for all four sustainable drying methods with a high coefficient of determination (R2 = 0.979 – 0.996), least sum of squared errors (SSE = 0.0046 – 0.0141) and the least reduced chi square (ꭓ2 = 0.0007 – 0.0046). Solar cabinet drying was a faster drying method compared to direct sun drying and blanching before drying greatly reduced the drying time for each drying method. The blanched cricket dried the fastest in the solar cabinet (10 hours), followed by the fresh cricket in the solar cabinet (12 hours), and the fresh cricket placed directly in sunlight recorded the longest drying time (32 hours). This was evident by the calculated moisture diffusivity (Deff) with the highest value (2.0  10-10 m2/s) recorded when crickets were blanched and solar cabinet dried and the least (5.2  10-11 m2/s) when crickets were only sun dried. Blanching may have increased the permeability of the cricket matrix to water and the higher peak drying temperature inside the solar cabinet (approximately 58°C) than the surrounding (45°C) may have enabled the water molecules to quickly attain activation energy and start moving from the cricket core to its surface, followed by evaporation. These results suggest blanching before solar cabinet drying could be a preferential pre-processing method for edible crickets in rural settings where energy and expensive drying equipment are lacking. There were no changes in proximate composition, available lysine, and protein digestibility of whole edible grasshopper, cricket and saturniid caterpillar that were dried using the six different drying methods. Protein Digestibility Corrected Amino Acid Score (PDCAAS) was lower in boiled dried cricket and caterpillar than the other dried forms. Boiled and dried insects had lowest proportions of linoleate and α-linolenate. Despite these losses in boiled dried insects, the essential amino acid indices (EAAI) (0.8 - 0.97) and lipid quality indices (Atherogenicity Index (AI): 0.47 – 0.83 and Thrombogenicity Index (TI): 0.36 – 1.93) of differently dried insects were within desirable limits of greater than 0.7 (EAAI) and less than 2 (AI and TI) for human nutrition. The differently dried edible insects were ground and defatted to produce edible insect protein-rich flours. The nitrogen solubility indices (NSI) for all boiled solar dried grasshopper, cricket, and caterpillar protein concentrates decreased by 35.2, 63.8 and 32.4% respectively while the water solubility indices (WSI) decreased by 37.1, 35.5 and 22.9% respectively compared to the other dried forms of grasshopper, cricket and caterpillars. The water absorption capacity (WAC) (1.7 – 2.97%), oil absorption capacity (OAC) (1.2 – 1.51%), foam stability (FS) (67 – 77.9%), foam capacity (FC) (32 – 49%), emulsion stability (ES) (65 – 78%), emulsion capacity (EC) (81 – 97%) of the insect protein concentrates were not affected by drying methods but varied with insect species. Dried insect protein concentrates had lower foaming and emulsion capacities compared to commercial soy (FC: 55.9%, EC: 94.3%) and whey (FC: 52.9 – 55.6% EC: 93.7 – 96.3%) protein ingredients but exhibited higher foam stability (43.2 – 52.8%) and good emulsion stability (83.4 – 93.0%). The presence of chitin in the defatted flour could have masked the effect the drying method have on the protein functional properties. However, the functional properties of the defatted insect flours suggest that they are not suitable for liquid food products because of its low NSI and WSI but could be suitable for semi solid to solid food products. Therefore, sun drying and solar cabinet drying on their own or with prior boiling could be cost-effective and affordable alternatives to freeze drying and oven drying for preserving edible insects and producing dry edible insect ingredients. These dried insect ingredients could be good candidates for solid and semi-solid food products and the manufacture of texturised proteins.