Application of in vitro protein solubility for selection of microalgae biomass as protein ingredient in animal and aquafeed

Abstract

Microalgae when grown under certain conditions can be rich sources of protein and can complement conventional protein sources like fishmeal and soymeal, in the aquaculture and animal feed industry. In this study, evaluation of four marine microalgae strains (Picochlorum sp., Nannochloris sp., Nannochloropsis sp., and Chlorella sp.) revealed that in vitro protein solubility and digestibility may serve as key indicators in determining the suitability of microalgae as a protein ingredient in feed. The greenhouse areal biomass productivities, protein and lipid concentrations of these strains ranged between 9–17 g m−2 day−1, 30–38% and 22–24%, respectively. Preliminary in vitro assays using undisrupted biomass of Picochlorum sp. revealed that its protein solubility was 47% and 67% less and digestibility was 28% and 22% less compared with fishmeal and de-oiled soy flour (DOSF), respectively. However, disruption of Picochlorum sp. biomass resulted in 2.5- and 1.5-fold increase in protein solubility and digestibility, respectively, as compared with undisrupted biomass. Further in vitro studies indicated that the soluble protein fractions differed significantly among the four experimental microalgae. The highest in vitro protein solubility (%) and soluble protein fractions (g kg−1 biomass) recorded in the four strains were the following: Picochlorum sp. (53%; 176 g kg−1), Nannochloris sp. (57%; 217 g kg−1), Nannochloropsis sp. (71%; 214 g kg−1), and Chlorella sp. (53%; 197 g kg−1). In addition, extracts from all these four strains were tested for the presence of trypsin inhibitors and found that all these strains have significantly lower trypsin inhibiting activity (TIA) compared with DOSF. The methodology presented in this study combines growth, biochemical composition, protein solubility, in vitro protein digestibility, and TIA and thus provides a reliable strategy in selection of microalgae as protein feed ingredient.