Influence of frozen storage temperature on the microstructures and physicochemical properties of pre-frozen perch (Micropterus salmoides)
Introduction
Freezing is widely used for fish and fish products preservation industry. During the storage and market distribution, the temperature of frozen product maintains below −10 °C (Bøgh-Sørensen, 2006). It's widely believed that there is a strong correlation between the stability of frozen food products and storage temperature. The shelf-life of frozen fish greatly extended as storage temperature decreased from −18 to −30 °C (Bengtsson, Liljemark, Olsson, & Nilsson, 1972, pp. 301–311). Furthermore, no quality difference was found for Atlantic salmon and Atlantic cod stored at a temperature in the range of −40 and −70 °C (Burgaard & Jørgensen, 2011; Mørkøre & Lilleholt, 2007). The most stable state of food is under its glass transition temperature (Rahman, 2006). As for the material with high content of water and protein, fish tissue for instance, the glass transition temperature of tuna (Nedenskov Jensen, Jøgensen, & Nielsen, 2003), cod (Nesvadba, 1993) and salmon (Tolstorebrov, Eikevik, & Bantle, 2014) are −72, −77 and −84 °C, respectively. However, fish muscle could be highly apt to rupture at a temperature lower than −85 °C, thus −85 °C is generally considered as the limiting temperature for industrial freezing of fish (Tolstorebrov, Eikevik, & Bantle, 2016).
The quality of frozen product could be affected by several processing parameters, such as the freezing method (Cui, 2011), freezing rate (Kaale, Eikevik, Rustad, et al., 2013), pre-freezing condition (Liao, 2014), storage temperature (Burgaard & Jørgensen, 2011) and storage duration (Kaale & Eikevik, 2013a). These factors are greatly related to the water state in food at low temperature. Generally, all water in frozen food is categorized into freezable and unfreezable water, and the essential difference is whether the water could be frozen at −40 °C (Reid & Fennema, 2007). Ice crystals are formed from freezable water during freezing and storage period, and the information of ice crystals, such as the amount and the size of ice crystals, in frozen food can be used as a vital parameter to reflect the influence of processing methods on the quality changes of frozen food (Kaale & Eikevik, 2014). For example, small and uniform intracellular ice crystals are developed in frozen food with fast freezing rate than slow rate (Kaale, Eikevik, Bardal, Kjorsvik, & Nordtvedt, 2013); the size of ice crystals grow fast when the storage temperature raises and the duration extends (Kaale & Eikevik, 2013b); while pre-freezing procedure could effectively increase the efficiency of freezing, thus suppress the destructive effect caused by ice crystals (Liao, 2014).
Perch (Micropterus salmoides), a popular economic freshwater species in China, is famous for its rich nutrition, great taste and suitable price. The total cultivation output of perch is about 0.35 million ton in mainland China in 2015 (Yearbook, 2016). For now, perch is mainly sold in fresh and alive in the market, which would easily lead to quality deterioration and short shelf-life. Split frozen products, like frozen fish block, is a common type of high-end aquatic products in the market. However, the frozen storage condition on the quality of perch block is still limited. Therefore, the objective of this work was to investigate the frozen storage temperature on the ice crystal microstructure and physicochemical properties of pre-frozen perch block after stored for 1 month.
Section snippets
Materials and freezing process
Fresh and alive perch, approximate 1 kg, was purchased from a local market (Wuhan, China). Cubic samples (1.5 × 1.5 × 1.5 cm3) were obtained from the dorsal muscle of perch. All samples were subjected to pre-frozen by immersing into liquid nitrogen (Baowu Co., Ltd, China) for 1 h first, and then classified into 4 batches randomly. Each batch contained at least ten samples, and frozen stored at a temperature of −18 °C (stored at −18 °C freezer, BCD-215DC, Qindao Haier Co., Ltd, China), −40 °C
Microstructure
Fig. 1 depicts the histology of frozen perch muscle fiber stored at different temperature, in comparison with the pre-frozen and fresh tissues. Perch tissues were subjected to pre-frozen at liquid nitrogen for 1 h before stored at cryogenic freezer. Being well known for its nontoxic, small weight loss and quick heat exchange, liquid nitrogen is wildly applied in food and biomedicine (Zhang & Hua, 2000). However, the obvious cracks on the surfaces of perch muscle (pre-frozen and thawed tissues,
Conclusions
Frozen storage temperature affected the formation of ice crystals and final quality of perch tissues. With the increase of storage temperature from −80 to −18 °C, the quality of pre-frozen perch was gradually deteriorated. The microstructures of perch samples visually displayed the gradual increase of cell space and average ice crystal area, meaning the breakage of muscle tissues. Meanwhile, the results indicated the higher loss of firmness texture, moisture and protein oxidation when
Acknowledgement
The authors would like to thank the financial support from China Agriculture Research System (CARS-46), Freshwater products Processing Innovation Team of Hubei Innovation Center of Agriculture Science and Technology (2016-620-000-001-034) and Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Technology and Business University (BTBU).
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