Effect Of Hydrolyzed Sunflower Lecithin On The Heat-Induced Coagulation Of Recombined Evaporated Milk Emulsions

KASINOS, M.; GOÑI M.L.; SABATINO, P.; VAN DER MEEREN, P.

Papendal

Congreso; 8th NIZO Dairy Conference. Functional Enzymes for Dairy Applications; 2013

NIZO

Heat treatment is by far the most important unit operation during milk processing, which ensures the manufacture of a commercially sterile product characterized by a prolonged shelf-life at ambient temperature. Throughout numerous studies during the past decades, β-lactoglobulin, the major protein in the total whey protein fraction in milk, has been described as a heat-labile globular protein, due to major changes which take place upon heating at elevated temperatures, including unfolding and loss of its secondary and tertiary structure. These changes initiate the emergence of an unwanted phenomenon known as heat coagulation as a consequence of whey protein denaturation, followed by its interaction with casein micelles. In an attempt to decrease the degree of intermolecular interactions, the effect of lecithin, a natural source of phospholipids, was examined. To that end, a recombined evaporated milk model system was used containing 16.5 % skimmed milk powder (w/w) and 6.5 % (w/w) sunflower oil with and without native (non-hydrolyzed) sunflower lecithin inclusion prior to homogenization. The obtained results proved that lecithin supplemented recombined evaporated milk emulsions demonstrated a less pronounced increase in viscosity as well as particle size, upon severe heat treatment. Additionally, with the intention to investigate whether hydrolyzed lecithin can prevent aggregation phenomena to a bigger extent as compared to native sunflower lecithin, enzymatic hydrolysis of the latter was performed using Lecitase® Ultra, a protein-engineered phospholipase A1 which cleaves the SN-1 alkyl chain, giving rise to lysophospholipid and fatty acid production. The enzymatic hydrolysis was conducted at multiple time intervals, fluctuating from 10 to 60 minutes. Phosphorus Nuclear Magnetic Resonance (31P-NMR) was utilized to assess whether alterations in phospholipid composition took place upon prolonging the process of enzymatic hydrolysis. The experimental data supported that the overall phospholipid content was drastically decreased by increasing the hydrolysis degree; whereas the phospholipid concentration before the enzymatic hydrolysis was 0.43 mmol per g lecithin, 60 minutes of hydrolysis resulted in a product consisting of only 0.08 mmol per g lecithin. On the other hand, the lysophospholipid content reached a maximum after 20 minutes of hydrolysis and gradually declined as the reaction was extended. Viscosity and particle size distribution measurements of recombined evaporated milk emulsions, revealed that the hydrolysis degree constitutes a very determinant parameter on the heat stability, after heating for 35 minutes at 121°C: while the addition of sunflower lecithin, hydrolyzed for 20 minutes, demonstrated the most pronounced heat stabilizing effect, the aggregation tendency was gradually increased by further increase of the hydrolysis time. Subsequently, the mechanism of interaction between hydrolyzed sunflower lecithin and whey protein was further investigated by residual protein content determination, as well as by oscillatory rheology measurements, on aqueous whey protein solutions. Whereas the former technique confirmed that the extent of hydrolysis has a significant impact on the protein content, the gelation curves obtained by the rheology studies, demonstrated that hydrolyzed lecithin decreased the heat-induced whey protein interactions to a much greater extent as compared to native sunflower lecithin. Generally, this work showed that sunflower lecithin largely improves the heat stability of whey protein containing solutions and emulsions due to its high phospholipid content. As the phospholipid molecular structure plays a decisive role, it follows that the heat stability of whey protein containing products may be optimized by appropriate processing, such as enzymatic hydrolysis of native lecithins.