Influence of lard-based diacylglycerol on rheological and physicochemical properties of thermally induced gels of porcine myofibrillar protein at different NaCl concentrations

https://doi.org/10.1016/j.foodres.2019.108723Get rights and content

Highlights

  • Gels were prepared with MP and lard diacylglycerol at different NaCl concentrations.

  • Addition of DG and increase in NaCl concentration improved rheological properties.

  • DG promoted a more compact and uniform gel network filled with fat globules.

  • Addition of DG and increase in NaCl concentration enhanced MP gel forming ability.

Abstract

This study investigated the role of lard-based diacylglycerol (DG) on the rheological and physicochemical properties of pork myofibrillar protein (MP) gels at different NaCl concentrations. Incorporated unpurified diacylglycerol (UDG) or purified diacylglycerol (PDG) and increased NaCl concentration significantly enhanced gel compression force, water-holding capacity, storage modulus values (G′), and loss modulus values (G″) (P < 0.05). Scanning electron microscopy results showed that the incorporation of DG promoted a more compact and uniform three-dimensional gel network filled with protein-coated fat globules. The increase in NaCl concentration and addition of UDG or PDG significantly increased the immobilized water contents and decreased the free water contents (P < 0.05) and also caused the increases in the amount of α-helix and β-sheet concomitant with decreases in β-turn and random coil contents. Overall, the increase in NaCl concentration and the incorporation of DG exhibit an enhancing role on the rheological and physicochemical of porcine MP.

Introduction

Myofibrillar protein (MP) is an important functional protein in muscle which is responsible for the thermally induced gelation properties of meat products. MP plays a critical role in producing the three-dimensional viscoelastic gel network via interactions among the proteins by hydrogen bonds, disulfide bonds, and hydrophobic interactions during thermal processing. For comminuted meat products, MP can provide a cohesive force to stabilize fat droplets and water in the three-dimensional network structure and bring meat particles together during thermal processing (Guo et al., 2019). The formed protein gels are notably important for their ability to bind water, stabilize fat, and form cohesive membranes on the surface of fat globules in emulsion systems (Wu, Xiong, Chen, Tang, & Zhou, 2009). The gel formation of MP is influenced by many factors, and one of the most important factors is salt concentration. Myosin accounts for nearly 43% of MP in muscle tissue, and the molecule is highly sensitive to the ionic strength (Yates, Greaser, & Huxley, 1983). At low ionic strength (<0.3 M), myosin molecules aggregate and produce a filament, whereas at high ionic strength (>0.3 M), myosin molecules disperse individually and occur as monomers (Sano, Noguchi, Matsumoto, & Tsuchiya, 1990). Moreover, high salt concentration is the pre-condition for the solubilization which is followed by the gelation of MP. Hegg (1982), revealing that the proper concentration of salt plays a positive role in protein-protein interactions and enhanced quality of the heat-induced gels. NaCl as the most important brine ingredient plays an important role in providing desirable flavour and texture characteristics in comminuted meat products (Gordon & Barbut, 1992). Addition of NaCl to meat products not only influences MP gel behaviour and gel formation but also influences flavour release (Feng, Cao et al., 2018). The perceived saltiness was owing to the Na+ and Cl-, and the flavor intensity depended on salt concentration in meat products (Feng, Cao et al., 2018). However, overconsumption of salt has been shown to lead to degenerative diseases such as hypertension, stroke, cerebrovascular, and cardiovascular diseases (He et al., 2011, Kloss et al., 2015).

Fat is an important ingredient in meat products. Addition of animal fat to meat products can provide good textural characteristics, attractive flavour, and high sensory acceptability (Yoo, Kook, Park, Shim, & Chin, 2007). However, excessive animal fat intake could bring about lifestyle-related diseases, particularly obesity, hypertension, and coronary heart disease (Yoo et al., 2007). Triacylglycerol (TG) is a major component, while diacylglycerol (DG) is a minor component in various plant oils and animal fats. Cheong, Zhang, Xu, and Xu (2009) showed that lard is comprised of 97.90% TG and 2.10% DG. In our previous study, the acylglycerol profile of lard, unpurified DG (UDG), and purified DG (PDG) was analysed by HPLC (Diao, Guan, Kong, & Zhao, 2017). The results revealed that lard was composed of 100% TG; UDG was comprised of 14.50% monoacylglycerol (MG), 61.76% DG, and 23.74% TG; and PDG was comprised of 8.85% MG, 82.03% DG, and 9.12% TG. Compared with TG, DG can reduce postprandial serum TG elevation and suppress fat accumulation in the body (Yasukawa & Katsuragi, 2004). The differences in metabolic characteristics between DG and TG result from differences in metabolic pathways in the small intestinal cells (Yasukawa & Katsuragi, 2004). Moreover, numerous studies in animal and human clinical trials have revealed that DG exerted no adverse effects on health (Morita & Soni, 2009). In addition, compared with TG, DG exerts unique physicochemical properties related to hydrogen bonding due to the presence of a hydrophilic group (hydroxyl group) in DG (Zhao, Sun, Qin, Liu, & Kong, 2018). DG can also improve product texture due to its higher melting point (Yasukawa & Katsuragi, 2004). Meanwhile, the taste, flavour, and texture of DG are similar to those of TG (Nishide, Shimizu, Tiffany, & Ogawa, 2004). Therefore, total or partial substitution of porcine fatback with lard-based DG in meat products may represent an alternative method to improve functional and nutritional properties. Miklos et al. (2014) reported that partial substitution of pork back fat with lard-based DG in fermented sausages could improve the quality of meat products.

Our previous experiments studied the preparation of DG from lard by lipase-catalysed glycerolysis (Diao et al., 2017). Furthermore, we evaluated the influence of ultrasonic pretreatment on DG preparation by enzymatic glycerolysis of lard (Zhao, Sun, Liu, & Kong, 2018). The results revealed that the lard samples, after 4 h of glycerolysis reactions with ultrasonic pretreatment, had similar DG contents to 11 h of glycerolysis reactions without ultrasonic pretreatment, which showed that ultrasonic pretreatment promotes DG preparation from lard. The UDG was prepared by a ultrasonic pretreatment method. The UDG was purified by molecular distillation and labelled as PDG. The melting point of UDP was between 29 °C and 42 °C, and melting point of PDG was between 30 °C and 44 °C (Diao, Guan, Kong, Han, & Zhao, 2016). In addition, lard, UDG, and PDG had very similar fatty acid compositions with C14:0 (myristoleic, 1.10–1.20%), C16:1 (palmitoleic, 1.79–1.92%), C16:0 (palmitic, 23.97–24.60%), C18:2 (linoleic, 7.75–8.03%), C18:1 (oleic, 47.64–48.09%), and C18:0 (stearic, 12.95–13.05%) (Diao et al., 2017). However, there were limited reports about the influence of DG on gelation and processing properties of MP. The aim of this work was to investigate rheological and physicochemical properties of porcine MP as influenced by the addition of lard-based DG at different NaCl concentrations, and the changes in gel microstructure, water distribution, and protein secondary structure were also analysed.

Section snippets

Materials

Fresh pork backfat and fresh pork loin muscle were purchased from the Beidahuang Meat Corporation (Harbin, Heilongjiang, China). Piperazine-1, 4 bisethanesulfonic acid (PIPES) was obtained from the Shanghai Yuanye Chemical Technology Co. Ltd. (Shanghai, China). All reagents in this work were of analytical grades.

Preparation of lard-based diacylglycerol

Lard was obtained by heating fatback at 120 °C with constant agitation. Lard-based diacylglycerol (DG) was synthesized according to the procedure of Zhao et al. (2018). Before the

Gel strength and water-holding capacity

Gel strength was examined by gel compression force testing. As shown in Fig. 1A. as the concentration of NaCl increased from 0 M to 0.6 M, the compression forces significantly increased (P < 0.05) and the maximum gel strength appeared at 0.6 M. The compression forces increased from 0.06, 0.11, 0.14 and 0.16 N (0 M NaCl) to 0.38, 0.45, 0.59 and 0.72 N (0.6 M NaCl) for control (MP alone), lard-, UDG- and PDG-composite gels (P < 0.05), respectively. These results likely occurred because protein

Conclusion

Incorporation of lard-based DG and change of NaCl concentrations had a profound effect on rheological and physicochemical gelation properties of porcine MP. The incorporation of UDG or PDG, the gel compression force and gel WHC were enhanced and the rheological properties (G′ and G″) and gel microstructure were improved with the increase in NaCl concentration. LF-NMR analysis revealed that the incorporation of UDG or PDG and the increase in NaCl concentration increased the entrapped water

Declaration of interest statement

The authors declare no competing financial interest.

Acknowledgements

This study was funded by the National Natural Science Foundation of China (31771990) and National Key Research and Development Program during the 13th Five-year Plan in China (2016YFD0401504).

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