The research group led by Professor Fei Peng from Minnan Normal University published an article titled "Study on the high viscosity and gel precursor properties of glycine acylated pectin and its regulatory role on the freeze the stability of sea bass surimi" in the journal Food Chemistry (IF=9.8). The article mainly explores the physicochemical properties of glycine acylated pectin (Gly Pe) prepared by ultra-low temperature enzymatic method (-5 ° C) and its protective mechanism in the freeze-thaw stability of sea bass mince. The study focused on enzymatic acylation modification and systematically characterized the structural changes of Gly Pe, such as enhanced molecular chain flexibility and disrupted crystalline structure. The excellent performance of Gly Pe, which can achieve a viscosity 1000 times higher than that of original pectin (Na Pe) under acidic and low shear conditions, and its regulatory effect on fish mince texture during freeze-thaw cycles were evaluated. The core innovation lies in Gly Pe's sacrifice of some dynamic rigidity to enhance static load-bearing capacity, achieving selective structural reorganization and providing a theoretical basis for the development of new natural cryoprotectants.
1、 Research background
Due to the limitation of its molecular structure, natural pectin has insufficient functional characteristics. For example, high methoxyl pectin needs high acid and high sugar to gel, but its solution viscosity is relatively low, which limits its application in high viscosity food systems. Chemical modification (such as amidation) can improve these defects, but often under harsh conditions. Therefore, mild enzymatic modification has become a trend. In addition, frozen storage will seriously damage the protein network structure of surimi products, resulting in softening of gel strength and texture. Traditional cryoprotectants such as sorbitol and phosphate often require high concentrations to be added and affect the taste. This study is based on the dual characteristics of enhancing intermolecular forces and improving molecular chain flexibility of acylated pectin. It is hypothesized that it can achieve unique protective effects different from traditional protectants through adaptive structural adjustment under freeze-thaw stress.
2、 Research Methods
Preparation and characterization of glycine amidated pectin
The study used an improved ultra-low temperature (-5 ° C) enzymatic method to prepare Gly Pe, using papain as a catalyst, and carried out amidation reaction under the activation of L-cysteine hydrochloride and EDTA, successfully obtaining Gly Pe with a grafting rate of 12.77%. The structures of natural pectin and acylated pectin were comprehensively characterized using various techniques such as HNMR, 13CNMR, FTIR, XPS, and XRD.
Determination of dynamic viscosity of natural pectin and glycinamide pectin
Will undergo behavior dependent on different conditions (pH, solution concentration Ca2+、 The natural pectin and amino acid amidated pectin solution treated with temperature were placed in a 60 ° C water bath and stirred magnetically for 20 minutes until dissolved. Add 3 mol/L NaOH or HCl as needed to control the pH value. Calcium is introduced by adding CaCl2, and pH is adjusted after adding calcium to ensure its stability throughout the process. After cooling to room temperature, useRH-20 rheometer (Shanghai Baosheng Industrial Development Co., Ltd., Shanghai, China)The instrument measures the dynamic viscosity of samples at different rotational speeds, using a rotating spindle to measure viscosity under dynamic conditions.
Evaluation of fish mince samples
The rheological properties of fish paste were measured using a rheometer with frequency and temperature scans to obtain the storage modulus (G ') and loss modulus (G' '). In addition, the texture characteristics (elasticity, cohesiveness, chewiness, stickiness), water retention (WHC) and heat loss rate of surimi gel were also measured.
3、 Results and Discussion (Part)
The results determined by the Baosheng RH20 rheometer showed that both Na Se and Gly Se solutions exhibited shear thinning fluid behavior, but Gly Se exhibited significantly increased viscosity under all test conditions. Under specific optimal conditions (pH 3, 6% concentration, low shear rate), the viscosity even reached 1000 times that of Na Se. This enhancement is attributed to the introduction of amide groups, which promote a more stable and compact molecular network structure by enhancing hydrogen bonding and hydrophobic forces, and enable Gly Pe to maintain moderate viscosity without relying on Ca2+, expanding its application range.
Figure 1. The viscosity behavior of Na Se and Gly Se under different conditions. A and B: PH dependent behavior; C and D: Solution concentration dependent behavior; E and F: Ca2+concentration dependent behavior; G and H: Temperature dependent behavior.
In the application of fish mince, Gly-Pe-S exhibits a unique "selective structural recombination" protective mechanism. Although the storage modulus (G ') measured by the rheometer slightly decreases with the increase of freeze-thaw cycles (indicating a decrease in dynamic stiffness), this change is a manifestation of the network's "rigid flexible balance adjustment". In the macroscopic texture test (large deformation static strength), after 4 freeze-thaw cycles, the chewiness and adhesive retention rates of Gly-Pe-S reached 340% and 385%, respectively, far exceeding the control group and Na Pe-S. This excellent load-bearing capacity is due to the ordered spatial heterostructure formed by Gly Pe during freeze-thaw processes (the synergistic effect of dense load-bearing areas and loose buffering areas). However, this optimization is accompanied by a significant decrease in moisture retention capacity (WHC) (from 95.22% to 80.61%), which is explained as a trade-off mechanism of "sacrifice gain": sacrificing water storage capacity in exchange for enhanced mechanical stability and stress transfer performance.
4、 Conclusion
This study confirmed that glycine amidated pectin prepared by ultra low temperature enzymatic method has better viscosity increasing and gel properties, and the viscosity can be increased by a thousand times under specific conditions. More importantly, it exhibits a good protective effect on the freeze-thaw stability of fish mince, which is attributed to the mechanism of "selective structural reorganization". By optimizing the rigid flexible balance of the network structure, it sacrifices some dynamic stiffness and moisture retention capacity, but gains excellent static bearing capacity for large deformations (i.e. chewiness and adhesiveness). Although the decrease in water retention is one of its application limitations, this study opens up new avenues for developing novel natural food ingredients that can provide freeze protection through adaptive structural adjustment using acylated polysaccharides