Insects have been identified as excellent alternative source of proteins due to their high protein content. The acceptance of insects increases when used as ingredient in an invisible manner and hence grinding is necessary. Off-colour formation occurs upon grinding larvae, which can hamper their potential use as ingredient for food and feed. The aim of this thesis was to investigate potential of insect larvae as protein source, the mechanisms responsible for the browning or blackening of larvae during grinding, and its impact on protein functionality. This was investigated for the larvae of three species: Tenebrio molitor, Alphitobius diaperinus and Hermetia illucens.
The specific Kp factor of 4.76±0.09 was determined for the three species to determine protein content based on nitrogen, and 5.60±0.39 after protein extraction and purification. Thus, the general Kp factor of 6.25, used until now, overestimated the protein content in insects. Off-colour formation upon grinding was caused by both enzymatic and non-enzymatic browning. Phenoloxidase was found to be mainly responsible for browning in T. molitor and likely in A. diaperinus, whereas iron-phenolic complexation likely contributed to the black colour in H. illucens. A model system of L-DOPA and iron was used to elucidate the structures of the iron-L- DOPA complexes by mass spectrometry.
Enzymatic browning did not influence the solubility of the proteins of all three species. Upon in-vitro hydrolysis by pepsin and trypsin, soluble proteins from H. illucens were more digestible compared to those of T. molitor and A. diaperinus. Phenoloxidase activity during processing negatively affected in-vitro pepsin hydrolysis. Besides phenoloxidase activity, also endogenous proteases remained active at pH 8 in extracts of insect larvae. Summarizing, endogenous enzyme activities and iron complexation should be taken into account for future application, as well as the specific Kp factor to prevent overestimation of the protein content of insects.