Uses of Ethanolic Coconut Husk Extract fot Surimi Gel Strengthening and Enhancement of Oxidative Stability of Emulsion Surimi Gel

Abstract

Effects of coconut husk ethanolic extracts on gelling properties of surimi from sardine (Sardinella albella) were investigated. Extracts prepared using 60% ethanol (E60) and 80% ethanol (E80) with total phenolic content of 463.74 and 453.93 mg tannin/g, respectively, were incorporated into surimi gel. Gels added with E60 or E80 had the increases in breaking force as the levels increased and the highest breaking force was observed when added with 0.125% E60 and 0.075% E80 (P<0.05). Both E60 and E80 had no detrimental effect on sensory attributes of surimi gel. However, slight decrease in whiteness was found in gel added with the coconut husk extracts. Disappearance of major myosin heavy chain took place when incorporated with both E60 and E80, regardless of levels used. Lower autolysis of surimi gel was found in the presence of both extracts. Addition of E60 and E80 could increase the cross-linking of proteins during heating as indicated by the higher G'. Surimi gel added with coconut extracts had highly interconnected network and their microstructure was finer and denser network than that of the control. Heat-induced aggregation of natural actomyosin (NAM) extracted from sardine muscle was studied in the presence of E60, named as ECHE, at various levels (0-0.03%, based on protein content). During heating from 20 to 90 °C, NAM solution showed an increase in turbidity, surface hydrophobicity and disulfide bond contents. Aggregation was more pronounced as ECHE concentration increased (P<0.05), while protein solubility and total sulfhydryl group were decreased (P<0.05). Furthermore, the Ca2+-ATPase activity of NAM was noticeably decreased in the presence of ECHE during heating up to 40 °C. Zeta potential analysis revealed that NAM added with ECHE became less negatively charged when the concentrations of ECHE increased (P<0.05). Cross-linking of protein strands was enhanced in the presence of ECHE during heating as evidenced by the higher G', particle size as well as microstructure. When ECHE at various levels (0-0.25%, based on protein content) was added into low setting surimi prepared from sardine (containing EGTA), breaking force of gels increased as the levels of ECHE increased up to 0.15% (P<0.05). The decrease in whiteness was found in gel added with ECHE. MHC band intensity was decreased, regardless of ECHE concentrations. Addition of ECHE could therefore increase the cross-linking of proteins during setting (40 °C, 30 min), especially with increasing concentrations, as indicated by the increased G'. Gel added with 0.15% ECHE had highly interconnected network with finer and denser structure than the control gel (without ECHE). Impact of ECHE at various levels (0-0.10%, based on protein content) on gel properties of sardine surimi under different gelling conditions including pressurization at 300 MPa, 30 min (HP); and pressurization, followed by heating (90 °C, 20 min) (HP/H) were investigated. At the same level of ECHE, HP/H gel had the higher breaking force (P<0.05) than HP counterpart. The increases in breaking force and water holding capacity were observed as the levels of ECHE were increased up to 0.075% for all gels, regardless of gelling processes used (P<0.05). Lower autolysis of surimi gel was also found in HP/H gel in the presence of ECHE. With addition of ECHE at a concentration of 0.075%, HP/H gel had a network with higher connectivity than HP gel and traditional two-step heated gel. ECHE was characterized and condensed tannin was found as an abundant compound in ECHE. The major free phenolics in ECHE were tannic acid and catechins. Antioxidative activities of ECHE at different levels (50-200 mg/L) tested by all in vitro assays increased as its concentration increased (P<0.05). DPPH radical scavenging activity and metal chelating activity were decreased up to 50% when heated at temperature higher than 90 °C for longer than 60 min (P<0.05). Impact of ECHE (200 and 400 mg/L) on lipid oxidation of shrimp oil-in-water emulsion was monitored throughout 12 days of storage at 30 °C. Lipid oxidation of emulsion added with ECHE was retarded as evidenced by the lower conjugated diene (CD), thiobarbituric acid- reactive substances (TBARS) value and p-anisidine value (AnV) than those of the control. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were more retained in the sample added with ECHE (200 mg/L) at the end of storage (P<0.05). When ECHE at various levels (0-0.25%) was incorporated into sardine surimi gel containing pre-emulsified seabass oil, the addition of seabass oil pre-emulsified with soy protein isolate in the presence of ECHE at levels of 0.20-0.25%, which had the average major mean diameter (d43) of 17.18-33.01 um, yielded surimi gel with the highest breaking force. The resulting gels also had the increases in hardness and cohesiveness (P<0.05). Decrease in whiteness was found in surimi gel added with ECHE, especially with increasing ECHE levels (P<0.05). During storage at 4 °C for 10 days, lipid oxidation of pre-emulsified gel as determined by peroxide value (PV) and TBARS was lowered as the levels of ECHE increased (P<0.05). Nevertheless, addition of ECHE did not affect total viable count and psychrophilic bacterial count in surimi gels. Therefore, ECHE could be a potential antioxidative protein cross-linker for enhancement of gel formation and oxidative stability for surimi or other products.