Effect of different drying methods on the myosin structure, amino acid composition, protein digestibility and volatile profile of squid fillets
Introduction
Dried aquatic products are popular as one of the convenient and delicious food items (Rahman, 2006). The annual world production of dried aquatic products is about 350,000 tons, and increases about 7% annually in China (Zhang & Sigurjon, 2008). Hot-air drying (AD) and freeze drying (FD) are two principal methods to preserve perishable foods. AD is cost effective, but can cause substantial quality losses. FD can help maintain nutrients, colour, flavour and texture with indistinguishable changes from original products, but requires high equipment investment and operating cost (Deng, Luo et al., 2014). Heat pump drying (HPD) is a novel and promising drying technique with comparative advantages, such as independent control of the operation parameters, lower energy consumption and less quality loss, and is suitable for drying heat-sensitive seafood (Deng et al., 2014, Deng et al., 2011, Qian et al., 2011).
The protein quality of meat products depends on its amino acid composition and digestibility (Hahn, Faubion, Ring, Doherty, & Rooney, 1982). Heating causes chemical changes in the amino acid residues, which could modify the structural, digestible and functional properties of proteins depending on the applied thermal treatment and processing conditions (Deng et al., 2014, Dima et al., 2012, Oliveira et al., 2013, Reed and Park, 2011, Santé-Lhoutellier et al., 2008). It was reported that 30 min of cooking decreases the contents of tryptophan, tyrosine and phenylalanine in beef and results in incomplete digestion of small molecular weight (<10 kDa) peptides (Kaur, Maudens, Haisman, Boland, & Singh, 2014). However, no significant differences were found in in vitro digestibility of the lyophilised in cooked beef samples (Oliveira et al., 2013). The cooked, canned, and stored tuna did not present significant deteriorations in protein digestibility or biological value (García-Arias, Navarro, & García-Linares, 2004). Changes in the digestibility of meat proteins are mainly ascribed to the modifications of protein structure (Kazemi, Ngadi, & Gariépy, 2011). Myosin is the major muscle protein that constitutes approximately 55–60% of the myofibrillar fractions in muscle tissues of mammals, birds, and fish (Reed & Park, 2011), but is heat sensitive and denatures in the temperature range of 43–58 °C (Deng et al., 2014). Therefore, evaluating the conformational modification and level of denaturation of myosin during different drying processes is important to better understand the role of myosin in the digestibility and functional property alteration of myofibrillar proteins. Unfortunately, there is lack of information on the impacts of different drying methods on myosin structure, amino acid composition, and protein digestibility in dried squids and other aquatic products.
Amino acids and peptides contribute directly to the flavour of seafood flesh, but could be destroyed by thermal treatment due to the formation of ammonia compounds, which resulted in unpleasant odours (Dima et al., 2012). Although the volatile changes of cooked or boiled seafood have been well investigated (Morita et al., 2002, Ohshima et al., 1991), there is lack of data about how the drying treatments may affect the volatile compounds of squids. About 99 identified compounds were found in squid dried at 125–230 °C (Kawai et al., 1991). The heterocycles containing sulphur and nitrogen atoms in the same ring contributed to the odour of dried squid (Shahidi, 1998). To our best knowledge, no comprehensive study has been conducted to investigate the effect of different drying methods on the volatile components of dried squids.
A common analytical technique used to identify volatiles in the seafood is gas chromatography with mass spectrometry (GC–MS) (Fratini et al., 2012, Ohshima et al., 1991, Xie and Lou, 2012). However, GC–MS is insufficient for separation of volatile components because it may result in co-elutions of different components (Parastar, Radović, Bayona, & Tauler, 2013). Two-dimensional gas chromatography with time-of-flight mass spectrometric detector (GC × GC–TOFMS) is a suitable alternative for qualitative and quantitative analysis of complex matrices, like food and beverage volatiles, due to its higher peak capacity, sensitivity, and selectivity (Parastar et al., 2013). Unfortunately, no detailed information is available on the volatile compounds of dried squids using GC × GC–TOFMS.
Our previous studies have demonstrated the influences of different drying methods on the proximate composition, drying behaviour, antioxidant activity, texture, microstructure, and rehydration kinetics of squid fillets (Deng et al., 2011, Deng et al., 2014, Deng et al., 2014). The objectives of this study were to identify and quantify the major classes of volatile compounds in the squid fillets using simultaneous distillation–extraction (SDE) coupled with GC × GC–TOFMS, and to investigate the effects of different drying methods (AD, FD and HPD) on amino acids composition, protein digestibility, and myosin structure of squid fillets.
Section snippets
Materials
North Pacific squids (Todarodes pacificus) (660 ± 10 g per a whole squid) were captured from the North Pacific ocean in January 2014 by the East China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences and frozen immediately. Frozen samples were first defrosted in a refrigerator at 4 °C overnight, and then thawed with running water. The internal organs, arms, and tentacles were removed by hands to obtain the mantle. The mantle muscle was cut into rectangular sheets with size of
Amino acid composition and protein digestibility
Compositions of amino acids of all samples are reported in Table 1. It has been well known that amino acids are susceptible to the processing conditions depending on the material species, amino acid type, and process method (Boye et al., 2012, Wu and Mao, 2008). After drying treatment (FD, AD and HPD), the total essential amino acids content were significantly higher than those in the raw squids (P < 0.05), except that the contents of leucine, isoleucine and valine in the raw squids were
Conclusion
Freeze-dried squids showed similar amino acid composition to that of raw squids and possessed the highest in vitro digestibility among all the samples. Amino acids composition of AD and HPD squids were alike, but in vitro digestibility of HPD squids was significantly higher than that of AD squids. AD caused more damage to squid myosin structure than HPD, while FD effectively retained the myosin integrity. Drying decreased total number of volatile compounds, but increased the content of total
Acknowledgements
This research was financially supported by the “National Natural Science Foundation (Nos. 31271955, 31301586)”, “Twelfth National Five-Year Plan (No. 2013BAD18B02)”, and “National High Technology Research and Development Program (863 Program) (No. 2012AA092303)”.
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