The influence of diet on the larva of two edible insects (Tenebrio molitor and Hermetia illucens)

Abstract

Entomophagy, that is the act of eating insects, is being touted as a promising source of alternative foods, especially with the impending challenge of feeding the growing worldwide population. Insects are highly nutritious, containing physiologically significant levels of essential nutrients such as proteins, dietary fibre, and fat. Insect fat has high levels of essential fatty acids such as linoleic and α-linolenic acids. Of particular interest is the fact that the type, levels and ratio of these essential fatty acids can be modified by altering the diet of the insects. In this study the larvae fat from two edible insects (mealworms (T. molitor) and black soldier fly (H. illucens)) were studied to ascertain their fatty acid characteristics (classes, composition, and positional distribution after digestion) and physicochemical properties (colour, thermal stability, melting and crystallisation profile), as well as how the type of feed given to these insects influences the composition and ratios of essential fatty acids). Fat content on a dry matter basis were 28.84% and 40.51% respectively for T. molitor and H. illucens larvae. The larvae fats differed in colour, with colour codes of #3D210F and #2B1C12 for T. molitor and H. illucens respectively. Fatty acid (FA) composition showed that T. molitor larvae fat had approximately 25% saturated fatty acid (SFA), 75% unsaturated fatty acid (UFA), 39% monounsaturated fatty acids (MUFA) and 36% polyunsaturated fatty acids (PUFA). The most prevalent FA detected were oleic acid, linoleic acid and palmitic acid. The omega 6 to omega 3 ratio (ω6:ω3) and SFA:UFA ratios were ~24:1 and 0.3:1 respectively. H. illucens on the other hand was composed of approximately 56%, 44%, 28% and 16% SFA, UFA, MUFA and PUFA respectively. The most abundant FA were lauric, palmitic, myristic acid, oleic and linoleic acid. The ω6:ω3 and SFA:UFA were present at ratios of 3.3:1 and 1.3:1 respectively. An analysis of the lipid classes showed that both insect larvae fat contained triglyceride (TG), free fatty acids, diglycerides, monoglycerides and cholesterol. Positional distribution of the FA on the TG molecule after pancreatic lipase action showed that the FA are distributed differently on the outside (sn-1, 3) or inside (sn-2) positions for both fat samples. For T. molitor, palmitic, myristic and palmitoleic acids showed higher preference of being distributed on the sn-1, 3positions. Other FA such as stearic, oleic and linoleic acids are present in high percentage on either the sn-1, 3 and/or sn-2 positions. A higher ω6:ω3 were present on the sn-2 ( ~26:1) than on the sn-1, 3 (~20:1) positions. The MUFA showed preference of distribution on the sn-2 position (46%), while the SFA and PUFA are about 5% more distributed on the sn-1, 3 than the sn-2 position. In H. illucens, a higher percentage of distribution of SFA was detected on the sn-2 position (50.7%) than on the sn-1, 3 positions (36.7%). The MUFA were found predominantly distributed on the sn-2 position (~35.2%) than sn-1, 3 (~9.8%). The thermal study showed that melting of T. molitor and H. illucens larvae fat occurred over a temperature range of -19.05°C to 5.99°C (enthalpy, 40.13 J/g) and 8.22°C to 39.65°C (enthalpy, 25.35 J/g) respectively. The crystallisation occurred between -12.36°C to -14.20°C (enthalpy, -0.61 J/g) and 6.73°C to -17.64°C (enthalpy, -15.71 J/g) for T. molitor and H. illucens larvae fat respectively. Moreover, onset temperature for thermal decomposition were 303.47°C and 149.61°C for T. molitor and H. illucens larvae fat respectively, reflecting differences in chain length and composition of FA in the larvae fats. Moreover, a feeding trial was performed on both insect larvae. The larvae were raised on seven or eight feeds in which the basal diet had been supplemented with seed meals (flaxseed, chiaseed, hempseed and rapeseed) to various levels. Some of the diets increased the PUFA compositions for both larvae. The ω6:ω3 ratio in T. molitor raised on the basal diet was 50:1 while that of the larvae raised on the supplement diets ranged from 2.5:1 to 29.3:1. However the ratio of the SFA:UFA was similar to the larvae raised on the basal diet. The FA composition of H. illucens larvae fat raised on supplemented diet showed a decrease in SFA, and an increase in MUFA and PUFA. The ω6:ω3 ratio of the larvae raised on basal diet was 9.9:1 while that of the larvae raised on the supplement diets ranged from 0.5:1 to 7.1:1. The ratio of the SFA:UFA (ranging from ~ 1.5:1 to 4:1) also reduced for larvae raised on the supplement diets, compared to the larvae raised on basal diet (SFA:UFA ratio of ~ 6.6:1). In summary, larvae fat from mealworms (T. molitor) and black soldier fly (H. illucens) differ in the composition and positional distribution of fatty acids, as well as in the physicochemical characteristics (colour, melting and crystallisation properties, and thermal decomposition profile). Moreover, the different diets had an effect on the type and composition (especially PUFA levels and w6:w3 ratios) of fatty acids expressed in larvae biomass. These findings suggest that larvae fat of these two insects have huge potential for exploration by in the food and feed industry.