Dasong Liu

Biochemical and physical changes of grass carp (Ctenopharyngodon idella) fillets stored at −3 and 0 °C

The objective of this study was to investigate the effect of superchilling at -3 °C compared with ice storage at 0 °C on the biochemical and physical properties of grass carp fillets. Fillets stored at -3 °C showed significant changes in whiteness, drip loss and textural hardness, while changes in pH, total volatile basic nitrogen and TCA-soluble peptides were slowed down. Partial denaturation of myosin as demonstrated by differential scanning calorimetry differed between fillets stored at -3 and 0 °C in that the transition peak showed a left shoulder at -3 °C and sharpened at 0 °C. Detachments between muscle cells and formation of cracks within cells were accelerated during storage at -3 °C, and from 10 days on, clear spaces between and within cells were observed with the concurrent appearance of white spots on the surface of fillets, suggesting the formation of both extra- and intracellular large ice crystals.

Collagen and Gelatin

Collagen and gelatin have been widely used in the food, pharmaceutical, and cosmetic industries due to their excellent biocompatibility, easy biodegradability, and weak antigenicity. Fish collagen and gelatin are of renewed interest, owing to the safety and religious concerns of their mammalian counterparts. The structure of collagen has been studied using various modern technologies, and interpretation of the raw data should be done with caution. The structure of collagen may vary with sources and seasons, which may affect its applications and optimal extraction conditions. Numerous studies have investigated the bioactivities and biological effects of collagen, gelatin, and their hydrolysis peptides, using both in vitro and in vivo assay models. In addition to their established nutritional value as a protein source, collagen and collagen-derived products may exert various potential biological activities on cells in the extracellular matrix through the corresponding food-derived peptides after ingestion, and this might justify their applications in dietary supplements and pharmaceutical preparations. Moreover, an increasing number of novel applications have been found for collagen and gelatin. Therefore, this review covers the current understanding of the structure, bioactivities, and biological effects of collagen, gelatin, and gelatin hydrolysates as well as their most recent applications.

Stability of whey protein hydrolysate powders: effects of relative humidity and temperature.

Whey protein hydrolysate (WPH) is now considered as an important and special dairy protein ingredient for its nutritional and functional properties. The objectives of the present study were to investigate the effect of environmental relative humidity (RH) and storage temperature on the physicochemical stability of three WPH powders with hydrolysis degrees (DH) of 5.2%, 8.8% and 14.9%, respectively. The water sorption isotherms of the three WPH powders fitted the Guggenheim-Andersson-DeBoer model well. An increase in water content leaded to a decrease in glass transition temperature (Tg), following a linear Tg vs log water content relationship. Moreover, an increase in DH caused the decrease in Tg at the same water content. Changes in microstructure and colour occurred significantly when the WPH powders were stored at high environmental RH or temperature, especially for those with high DH.

Effects of alkaline pretreatments and acid extraction conditions on the acid-soluble collagen from grass carp (Ctenopharyngodon idella) skin.

This study investigated the effects of alkaline pretreatments and acid extraction conditions on the production of acid-soluble collagen (ASC) from grass carp skin. For alkaline pretreatment, 0.05 and 0.1M NaOH removed non-collagenous proteins without significant loss of ASC at 4, 10, 15 and 20 °C; while 0.2 and 0.5M NaOH caused significant loss of ASC, and 0.5M NaOH caused structural modification of ASC at 15 and 20 °C. For acid extraction at 4, 10, 15 and 20 °C, ASC was partly extracted by 0.1 and 0.2M acetic acid, while 0.5 and 1.0M acetic acid resulted in almost complete extraction. The processing conditions involving 0.05-0.1M NaOH for pretreatment, 0.5M acetic acid for extraction and 4-20 °C for both pretreatment and extraction, produced ASC with the structural integrity being well maintained and hence were recommended to prepare ASC from grass carp skin in practical application.

Maillard-reaction-induced modification and aggregation of proteins and hardening of texture in protein bar model systems.

UNLABELLED The hardening of high-protein bars causes problems in their acceptability to consumers. The objective of this study was to determine the progress of the Maillard reaction in model systems of high-protein nutritional bars containing reducing sugars, and to illustrate the influences of the Maillard reaction on the modification and aggregation of proteins and the hardening of bar matrices during storage. The progress of the Maillard reaction, glycation, and aggregation of proteins, and textural changes in bar matrices were investigated during storage at 25, 35, and 45 °C. The initial development of the Maillard reaction caused little changes in hardness; however, further storage resulted in dramatic modification of protein with formation of high-molecular-weight polymers, resulting in the hardening in texture. The replacement of reducing sugars with nonreducing ingredients such as sugar alcohols in the formula minimized the changes in texture. PRACTICAL APPLICATION The hardening of high-protein bars causes problems in their acceptability to consumers. Maillard reaction is one of the mechanisms contributing to the hardening of bar matrix, particularly for the late stage of storage. The replacement of reducing sugars with nonreducing ingredients such as sugar alcohols in the formula will minimize the changes in texture.

Moisture-Induced Aggregation of Alpha-Lactalbumin: Effects of Temperature, Cations, and pH

UNLABELLED Alpha-lactalbumin is an important dairy protein ingredient, and has been widely used in high-protein foods such as infant formula and nutritional bars for its nutritional and functional properties. The purpose of this study was to investigate the moisture-induced aggregation of alpha-lactalbumin in premixed protein dough model systems, and to illustrate the effects of temperature, cations, and pH on the progress of protein aggregation. Our results suggested that storage temperature was a critical factor for protein aggregation in model systems, and the formation of protein aggregates became faster with increases in storage temperature. Calcium significantly improved the thermal stability of alpha-lactalbumin and slowed down the formation of protein aggregates. The increases in pH accelerated the aggregation of alpha-lactalbumin. Our results also suggested that the formation of intermolecular disulfide bonds together with noncovalent interactions are the main mechanisms resulting in the moisture-induced aggregation of alpha-lactalbumin in model systems. PRACTICAL APPLICATION Alpha-lactalbumin is an important dairy protein ingredient, and has been widely used in high-protein foods such as infant formula and nutritional bars for its nutritional and functional properties. Our results suggested low storage temperature, the presence of calcium and low pH condition can make high-protein food products containing alpha-lactalbumin more stable.

Effects of enzymatic dephosphorylation on infant in vitro gastrointestinal digestibility of milk protein concentrate

This study investigated the effects of dephosphorylation extent on infant in vitro gastric clotting property and gastrointestinal digestibility of milk protein concentrate. Dephosphorylation was affected by phosphatase type and incubation pH. A series of milk protein concentrate with 0-69% dephosphorylation were obtained by incubation with calf intestinal alkaline phosphatase at pH 6.5 for 0-420 min. Both β- and αs1-caseins in the modified milk protein concentrate showed multiply dephosphorylated isoforms with different numbers of phosphate groups depending on the extent of dephosphorylation. With increased dephosphorylation of milk protein concentrate, the gastric clotting extent decreased and the gastrointestinal digestibility increased under infant in vitro conditions. These results suggested the potential of developing a dephosphorylated milk protein concentrate, with improved gastric clotting property and gastrointestinal digestibility, to simulate the multiply phosphorylated patterns of human casein and hence to further the humanization of infant formula on a molecular level.

Effects of skim milk pre-acidification and retentate pH-restoration on spray-drying performance, physico-chemical and functional properties of milk protein concentrates

This study investigates the effects of pre-acidification (pH 6.7-5.4) of skim milk, followed by pH-restoration of the retentates, on spray-drying performance, physico-chemical properties and functionality of the resulting milk protein concentrate (MPC). Powder recovery decreased with decreasing pH of pre-acidification but improved with pH-restoration. Colloidal calcium was gradually solubilized with decreasing pH of pre-acidification but was slightly recovered by pH-restoration. Dissociation of micellar caseins increased with decreasing pH of pre-acidification of skim milk and was further increased by pH-restoration. Casein micelles maintained their overall structures at pre-acidification pH of 6.7-6.0, and partially disintegrated into loosely entangled aggregates at pH 5.7-5.4; while after pH-restoration, micelles generally maintained their overall structures at pre-acidification pH of 6.0, and completely disintegrated at pH 5.7-5.4. Solubility and emulsifying properties of MPC improved with decreasing pH of pre-acidification and with pH-restoration. Heat stability of MPC declined with decreasing pH of pre-acidification but improved with pH-restoration.