Attenuated-total-reflection Fourier-transformed spectroscopy as a rapid tool to reveal the molecular structure of insect powders as ingredients for animal feeds

Unsustainable agriculture contributes to disastrous global effects ‒ insect-based feed shows potential due to their sustainable, nutritional, and waste upcycling properties. Current EU legislation restricts insect-based meals to fish, pork, and poultry feed; but the near-future shows a great potential for wider acceptance in livestock feed. Black soldier fly larvae (BSFL), mealworm (MW), field cricket (FC), and banded cricket (BC) were sourced within three consecutive weeks ‒ each batch was prepared, freeze-dried, then milled, and stored at -20 °C. Chemical composition of whole-insect meals was analysed for ether extract (EE), crude ash, and nitrogenic contents using standard wet chemistry protocols. Monogastric in vitro digestibility was determined through replicating gastric and full intestinal digestion; during this, R-amino acid content was determined through protein hydrolysis kinetics. Additionally, ATR-FTIR was used for molecular analysis, including identification of nutrient-associated functional spectral bands ‒ structural differences were compared through principal component analysis. Insect-based ATR-FTIR analysis demonstrates notable differences in Amide regions, suggesting distinct protein secondary structures, but overall, FC and BC contain the highest crude protein (CP) levels. The lowest CP content was in BSFL; however, BSFL contained the highest ash content‒likely consequence of high calcium. Dry matter (DM) yielded lowest in the crickets (FC‒28.6; BC‒26.9 g/100 g), and highest in MW‒38.5 g/100 g; the sum of CP+EE in MW represented >80% DM, but with higher EE contents‒CP: EE = 2.45. Data shows greater chitin content in crickets than BSFL+MW. Crickets showed greater neutral detergent fibre (NDF) than BSFL+MW; however, acid-detergent fibre (ADF) was similar among all species, suggesting NDF may include amalgams of interlinked nutrients released by acid digestion. This first study shows for the first time evidence that rearing conditions and substrates influences molecular structure. Exponential solubilisation was observed during pepsin+pancreatin digestion for all, but BSFL exhibited the highest degree-of-hydrolysis during the pancreatin phase, surpassing others. Analysis indicates protein hydrolysis differences are linked to trypsin activity susceptibility.