Freezing-then-aging treatment improved the protein digestibility of beef in an in vitro infant digestion model
Introduction
Complementary foods are served to infants between six and 24 months of age to meet their nutrient requirements for active growth and development during this period. Beef is a source of complementary food because it supplies balanced essential amino acids and iron that are generally lacking in infants (Lee et al., 2019).
The digestibility of proteins incorporated in complementary foods, as well as amino acid composition are important parameters. The protein digestibility is lower in infants than adults, owing to immature gastrointestinal tracts. Infants have a higher stomach pH, lower concentration of digestive proteases, and slower gastric emptying rate than adults (Nguyen et al., 2015). The intact proteins that are not digested sufficiently can be fermented in the colon and cause health problems, such as diarrhea and allergic reactions (Lee, Jo, Lee, et al., 2020). Despite the great bioavailability of beef proteins in the human body, infants can have hindered digestion of beef proteins. Therefore, beef that is used in complementary foods should have enhanced proteolysis during digestion.
The degradation of myofibrillar proteins during aging resulted in the improvement of the protein digestibility of beef (Lee, Jo, Lee, et al., 2020). The degradation of myofibrillar proteins in meat during aging is attributed to the activity of endogenous proteases, such as calpains, proteasomes, lysosomal cathepsins, and caspase-3 (Cao et al., 2013, Kaur et al., 2020). The degradation of myofibrillar proteins by endogenous proteases induces the loss of the structural integrity of meat protein, thereby increasing the substrate availability for digestive proteases during digestion (Lee, Jo, Yong, et al., 2020).
Freezing is a conventional method that extends the shelf-life of meat and meat products. The decrease in temperature during the freezing process induces the formation of ice crystals, and increases the solute concentrations in the unfrozen water in muscles, which can alter the physicochemical environments in muscles (Zhang & Ertbjerg, 2019). Therefore, muscle organelles and myofibrillar proteins can be denatured by the formation of ice crystals and high ionic strength in the muscle (Zhang & Ertbjerg, 2018). The cold denaturation of myofibrillar proteins, caused by the reduced free energy barrier for unfolding, is a phenomenon in frozen meat that results in the partial unfolding and structural integrity loss of myofibrillar proteins (Arsiccio et al., 2020). Furthermore, when the biological systems of enzymes, such as mitochondria, erythrocytes, and lysosomes, in muscle cells are exposed to cold shock, the enzymes can be released into the extracellular space (Gaarder et al., 2012, Kang et al., 2014).
The deteriorations of quality such as texture and juiciness in frozen-thawed meat are well known. However, it cannot be a problem in the meat for complementary foods which are generally served as puree because infants have no or low masticatory ability (Lee, Jo, Lee, et al., 2020). Therefore, we hypothesized that if freezing induces the loss of structural integrity of myofibrillar proteins and an increase in protease activity in beef, the protein digestibility of beef can be improved by increasing the degradation of myofibrillar proteins with the combination process of aging and freezing of beef. In addition, the combination process of aging and freezing may be the way to improve the protein digestibility of beef for complementary foods.
Therefore, this study investigated the effect of aging of freeze-thawed beef on the activities of endogenous proteases and their protein structure, and observed how these changes affected protein digestion in an in vitro infant protein digestion model.
Section snippets
Sample preparation
Three semitendinosus muscles from three heifer carcasses at 24 h postmortem were purchased from a local market. Each muscle was divided into seven pieces. One piece was used as the day 1 sample, and six were allocated to two treatments (FA, freezing-then-aging; AO, aging only) and three aging periods (3, 7, and 14 days). The three different muscles were considered as three batches (replications). The beef for the AO treatment was vacuum-packed and aged at 4 °C. The vacuum-packaged beef for FA
Protein oxidation
Chemical modification of protein structure by oxidation leads to a decrease in protein digestibility, as it reduces the accessibility of digestive enzymes (Duque-Estrada et al., 2019). Protein oxidation is considered a major problem in frozen meat because freezing of meat generates oxidative conditions with increased solute concentrations in the unfrozen water (Zhang & Ertbjerg, 2019).
No differences in carbonyl content between FA and AO groups were found at any aging day (Table 1). In addition,
Conclusion
The effect of freeze-aging of beef on protein digestibility in an in vitro infant digestion model was investigated in this study. Although there was no significant change in tertiary structure between FA and AO, the activity of cathepsin B increased with the freezing process before aging. The FA group exhibited a higher 10% TCA-soluble α-amino group content than the AO group, on day 14 of aging. As assessed from the increase in the degradation of myofibrillar protein and the decrease in the
Funding
This work was supported by the National Research Foundation of Korea (NRF); the Korean government (MSIT) [grant number 2019R1F1A059255]; and partly supported by Korea Institute for Animal Products Quality Evaluation.
CRediT authorship contribution statement
Seonmin Lee: Formal analysis, Data curation, Writing - original draft. Kyung Jo: Formal analysis. Hyun Gyung Jeong: Formal analysis. Hae In Yong: Formal analysis. Yun-Sang Choi: Formal analysis. Dongjun Kim: Investigation. Samooel Jung: Conceptualization, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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