Abstract
Insects are a promising alternative protein source. One of the bottlenecks in applying insects in food is the fast darkening initiated during grinding. Besides enzymatic browning, non-enzymatic factors can cause off-colour formation, which differs between species. This study investigates the impact of iron, phenoloxidase, and polyphenols on off-colour formation in insect larvae. Hermetia illucens showed a blackish colour, whereas Tenebrio molitor turned brown and Alphitobius diaperinus remained the lightest. This off-colour formation appeared correlated with the iron content in the larvae, which was 61 ± 9.71, 54 ± 1.72 and 221 ± 6.07 mg/kg dw for T. molitor, A. diaperinus and H. illucens, respectively. In model systems, the formation of iron-L-3,4-dihydroxyphenylalanine (L-DOPA) bis- and tris-complexes were evidenced by direct injection into ESI-TOF-MS, based on their charges combined with iron isotope patterns. The reversibility of the binding of iron to phenolics, and thereby loss of blackening, was confirmed by EDTA addition. Besides complex formation, oxidation of L-DOPA by redox reactions with iron occurred mainly at low pH, whereas auto-oxidation of L-DOPA mainly occurred at pH 10. Tyrosinase (i.e. phenoloxidase) activity did not change complex formation. The similarity in off-colour formation between the model system and insects indicated an important role for iron-phenolic complexation in blackening.
Highlights
Insects are nowadays investigated as alternative protein source to meet the growing protein demand in the future[1]
The highest enzyme activity was found for T. molitor, A. diaperinus and the lowest activity was found for H. illucens, based on oxygen consumption measurements with L-3,4-dihydroxyphenylalanine (L-DOPA) as substrate[4]
Dark brown or black colour was observed at pH 7, whereas at pH 3 the colour formation was limited for all species, probably because phenoloxidase was inactive at low pH4
Summary
Insects are nowadays investigated as alternative protein source to meet the growing protein demand in the future[1]. The oxidative enzyme activity of the different species cannot explain the differences in off-colour formation of Hermetia illucens compared to that of Tenebrio molitor and Alphitobius diaperinus. Insects contain transferrin proteins to transport iron in serum and ferritin proteins to store it. Ferritin can bind multiple ferrous iron (Fe2+) which is converted into the ferric form and stored as such in the oxoferrihydrite core[9]. Of L-tyrosine[12], besides small amounts of L-tyrosine derivatives like L-dopamine and L-DOPA, often in phosphorylated or glycosylated form[10]. These polyphenols are prone to oxidation by endogenous enzymes or iron, the latter has never been studied in insects. The product DOPAquinone can react further non-enzymatically to form melanins or crosslinks with proteins[14]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
