Ice Recrystallization Inhibition Activity of Soy Protein Hydrolysates

Identifying and developing ice recrystallization inhibitors from sustainable food proteins such as soy protein isolate (SPI) can lead to practical applications in both pharmaceutical and food industries. The objective of this study was to investigate the ice recrystallization inhibition (IRI) activity of SPI hydrolysates, and this was achieved by using an IRI activity-guided fractionation approach and relating IRI activity to interfacial molecular activity measured by vibrational sum frequency generation (VSFG). In addition, the impact of molecular weight (MW) and enzyme specificity was analyzed using three different proteases (Alcalase, trypsin, and pancreatin) and varying hydrolysis times. Using preparative chromatography, hydrolysates from each enzyme treatment were fractionated into five different MW fractions (F1-F5), which were then characterized by high-performance liquid chromatography (HPLC). All SPI hydrolysates had IRI activity, resulting in a 57-29% ice crystal diameter reduction when compared to native SPI. The F1 fraction (of 4-14 kDa) was most effective among all tested hydrolysates, while the lower MW peptide fractions lacked activity. One sample (SPI-ALC 20-F1) had a 52% reduction of ice crystal size at a lower concentration of 2% compared to the typical 4% used. SFG showed a difference in H-bonding and hydrophobic interactions of the molecules on the water/air interface, which may be linked to IRI activity. This study demonstrates for the first time the ability of SPI hydrolysates to inhibit ice crystal growth and the potential application of SFG to study molecular interaction at the interface that may help illustrate the mechanism of action.

Characterizations and the Mechanism Underlying Cryoprotective Activity of Peptides from Enzymatic Hydrolysates of Pseudosciaena crocea

Antifreeze peptides are a class of small molecule protein hydrolysates that protect frozen products from cold damage under freezing or subcooling conditions. In this study, three different Pseudosciaena crocea (P. crocea) peptides were from pepsin, trypsin, and neutral protease enzymatic hydrolysis. It aimed to elect the P. crocea peptides with better activity through molecular weight, antioxidant activity, and amino acid analysis, as well as to compare the cryoprotective effects with a commercial cryoprotectant. The results showed that the untreated fillets were prone to be oxidized, and the water-holding capacity after freeze-thaw cycle decreased. However, the treatment of the trypsin hydrolysate of P. crocea protein significantly promoted the water-holding capacity level and reduced the loss of Ca²⁺-ATP enzyme activity and the structural integrity damage of myofibrillar protein in surimi. Moreover, compared with 4% sucrose-added fillets, trypsin hydrolysate treatment enhanced the umami of frozen fillets and reduced the unnecessary sweetness. Therefore, the trypsin hydrolysate of P. crocea protein could be used as a natural cryoprotectant for aquatic products. Hence, this study provides technical support for its use as a food additive to improve the quality of aquatic products after thawing and provides a theoretical basis and experimental foundation for the in-depth research and application of antifreeze peptides.

Identification of Novel Antifreeze Peptides from Takifugu obscurus Skin and Molecular Mechanism in Inhibiting Ice Crystal Growth

Foodborne hydrolyzed antifreeze peptides have been widely used in the food industry and the biomedical field. However, the components of hydrolyzed peptides are complex and the molecular mechanism remains unclear. This study focused on identification and mechanism analysis of novel antifreeze peptides from Takifugu obscurus skin by traditional methods and computer-assisted techniques. Results showed that three peptides (EGPRAGGAPG, GDAGPSGPAGPTG, and GEAGPAGPAG) possessed cryoprotection via reducing the freezing point and inhibiting ice crystal growth. Molecular docking confirmed that the cryoprotective property was related to peptide structure, especially α-helix, and hydrogen bond sites. Moreover, the antifreeze peptides were double-faces, which controlled ice crystals while affecting the arrangement of surrounding water molecules, thus exhibiting a strong antifreeze activity. This investigation deepens the comprehension of the mechanism of antifreeze peptides at molecular scale, and the novel efficient antifreeze peptides can be developed in antifreeze materials design and applied in food industry.

Cryoprotective Activity and Action Mechanism of Antifreeze Peptides Obtained from Tilapia Scales on Streptococcus thermophilus during Cold Stress

Cold stress adversely affects cell viability and acidification, and new cryoprotective methods continue to be needed in cold-chain food industry. Given this, we investigated the cryoprotective effects and action mechanism of antifreeze peptides obtained from tilapia scales (TSAPP) on Streptococcus thermophilus during cold stress. Our results showed that the molecular weight of TSAPP ranged from 180 to 2000 Da and its thermal hysteresis activity was 0.29 °C. Growth of S. thermophilus was improved after treatment with TSAPP (1 mg/mL) under cold stress. This growth was notable when compared with the effects of other cryoprotectants. Furthermore, TSAPP improved the metabolic activity of S. thermophilus during cold stress. TSAPP likely offered its cellular protection by maintaining cell membrane fluidity through hydrogen bonding of the phospholipid bilayer. These results indicate that TSAPP has potential as a novel biological peptide material with cryoprotective activity for future use in probiotic or other processed food applications.

Intracellular Expression of Antifreeze Peptides in Food Grade Lactococcus lactis and Evaluation of Their Cryoprotective Activity

Antifreeze peptides can protect living organisms from low temperatures by preventing damage or killing due to ice crystal formation between cells. Therefore, antifreeze peptides can be used as a low temperature protectant for cryopreservation of cells and tissues, and also in food production. In this study, a recombinant SF-P gene was constructed and inserted into pNZ8149 to construct a food grade expression vector, which was then electroporated into Lactococcus lactis NZ3900. The expression of the target protein was induced using Nisin, and the optimal expression condition was determined to be a pH of 6.0, Nisin concentration of 25 ng/mL, temperature of 37 °C, and incubation time of 6 hr. Compared to the strain NZ3900 and the recombinant strain SF-P₁ without addition of Nisin, the recombinant strain SF-P₂ showed the highest cell survival and thermal hysteresis activity, and had a reduction in the changes of activities of extracellular and intracellular lactate dehydrogenase and β-galactosidase after freezing. Moreover, analysis by SEM showed that SF-P₂ cells were more completely and regularly shaped than other strains, displayed no obvious leakage of cell contents, and had an intact boundary between cells after freezing. These results indicate that the recombinant strain SF-P₂ has a protective effect against freezing. This paper presents a food grade expression system for an antifreeze peptide SF-P using L. lactis as a host, and shows that the intracellular expression of antifreeze peptide could protect the cellular integrity and physiological functions of L. lactis.

PRACTICAL APPLICATION

The recombinant Lactococcus lactis with intracellular expression of antifreeze peptides SF-P could reduce the damage of bacteria cells induced by freezing or freeze drying, so, it could be applied in the process of freezing food without separation, such as the manufacture of yoghurt ice cream, frozen dough, and so on.