Microencapsulation of fish oil with brewer’s spent grain proteins: Effect of citric acid and emulsion pH

The use of brewer's spent grain (BSG) proteins as wall material for encapsulating fish oil was investigated for the first time. This study also demonstrated the effect of citric acid (CA)-crosslinking and emulsion pH on the encapsulating ability of BSG proteins. Four different BSG microcapsules were prepared, including non-crosslinked and crosslinked BSG microcapsules at pH either above or below the isoelectric point (pI) of BSG proteins, and their performance was compared with OSA-starch. Crosslinked BSG emulsions had larger droplet size than non-crosslinked ones. The largest droplet sizes were found in emulsions where pH < pI due to some degree of protein aggregation. Encapsulation efficiency of BSG microcapsules ranged from 81 to 93%. BSG microcapsules, regardless of crosslinking, showed considerably higher oxidative stability than OSA-starch microcapsules. CA-crosslinking further enhanced their oxidative stability. The highest and the lowest oxidative stabilities among BSG microcapsules belonged to the microcapsules prepared at pH < pI, where some degree of protein aggregation occurred in the emulsions. Crosslinked BSG microcapsule prepared at pH < pI exhibited the highest oxidative stability with peroxide and thiobarbituric acid reactive substances (TBARS) values remaining in the range allowed for fresh oils even after 15 days of storage at 40 °C. It was attributed to its denser shell matrix due to CA-crosslinking and multilayer formation resulting from protein aggregation. Non-crosslinked microcapsules prepared at pH < pI represented the lowest oxidation stability among BSG microcapsules due to larger pores. The results suggest a synergistic effect of CA-crosslinking and protein aggregation, which significantly reinforced shell matrix and effectively reduced oxygen permeability.