Effect of immersion freezing with edible solution on freezing efficiency and physical properties of obscure pufferfish (Takifugu Obscurus) fillets
Introduction
The obscure pufferfish (Takifugu obscurus) is widely welcomed for its rich nutrition and special flavor; and annual sales keep increasing. Currently, this fish is mainly sold without processing, which causes a waste of this resource and a burden to the environment if this perishable fish is not consumed and stored properly. Among different processing methods, fish freezing is preferable because it could largely preserve the originality of raw material (Cheng, Sun, Zhu, & Zhang, 2017). Previous studies have shown that quality of frozen food has relationship with the morphology and distribution of ice crystals; and formation and growth of ice crystals are affected by freezing rate (Alizadeh, Chapleau, De-Lamballerie, & Le-Bail, 2007; Kaale & Eikevik, 2015). When freezing rate is fast, distribution of ice crystals is very close to distribution of water in raw material and induces less mechanical damage to food (Cheng et al., 2017).
Referring to freezing methods, liquid nitrogen freezing (LNF) has very high heat transfer coefficient (425 W/m2·K) (Guo & Tao, 2003) and extremely fast freezing rate induced by large temperature difference (food temperature of about 4–25 °C Vs freezing medium temperature of about −196 °C). However, compared to air freezing (AF) which has low heat transfer coefficient (15–17 W/m2·K) (Rodezno et al., 2013), LNF takes significantly higher cost and the consumption is not in cyclic utilization which for one-time use is huge and needs about 0.8–2 kg liquid nitrogen to freeze 1 kg of the good (Guo, Tao, Hua, & Wang, 2002). Therefore, though LNF has remarkable advantage for its freezing efficiency, high cost limits its application in bulk food commodities.
In contrast, immersion freezing (IF) has a balance in both cost and freezing rate, compared to LNF and AF. For the expense, IF solution could be used in cycles and saves the cost. For the freezing process, IF has theoretical feasibility of relatively high heat transfer coefficient (100–140 W/m2·K) due to liquid mobility (Rodezno et al., 2013), which will be a promising food freezing method (Pan et al., 2018; Sun, Sun, Xia, Xu, & Kong, 2018). Pan et al. (2018) found that freezing rate of IF (−50 °C) to vacuum-packaged food was approximately 4.5 times higher than that of AF (−18 °C) on the same initial temperature. In that study, 95% of ethanol was applied and therefore vacuum package was used in avoid of the problem of high content of ethanol with food. In comparison, IF solution used in this study was made up with edible components with much less ethanol (25% ethanol with sodium chloride, sugar, ice-nuclear active protein and some antioxidant) and it allowed direct contact with food by good liquid mobility property even at a low temperature of −30 °C. This may lead to a great increase in freezing rate, which was different from previous studies and needed verification.
Therefore, the process of IF with edible solution and its effect on physical properties of frozen obscure pufferfish (Takifugu Obscurus) were investigated. In this study, IF was compared with AF and LNF in order to clarify whether IF with edible solution could be an effective replacement of LNF to freeze freshwater fish.
Section snippets
Filleting of obscure pufferfish
Fresh live obscure pufferfish (Takifugu Obscurus) (weight: 400 ± 50 g; length: 15 ± 2.5 cm; n = 36) in fresh water source were bought from Tian Peng Aquatic Products Market in Wuxi (China) in February 2018 and immediately killed without struggle of fish by experienced salesperson. These obscure pufferfish, covered with crushed ice, were transported to the lab within 60 min. After moving out of heads, tails and viscera, the obscure pufferfish were cleaned with cold sterile distilled water,
Freezing process of obscure pufferfish fillets
As shown in Fig. 1A, all the freezing curves except for liquid nitrogen freezing (LNF) could be divided into three stages: falling, smoothing, and falling again. Combined with Fig. 1B, it could be seen that the smoothing stage was related to the process going through the freezing point (−1.1 °C) of obscure pufferfish (also referred to maximal crystallization zone), which was higher than that of the sea fish such as cod (Gadus morhua L.) (−2.2 °C) (Bjørkevoll, Reboredo, & Fossen, 2017) and
Conclusions
The results of this study showed that immersion freezing (−18 °C and −30 °C) using edible solution led smaller ice crystals, higher shear force and hardness, less drip and cooking loss, and more integrity of cells and muscle fiber in obscure pufferfish (Takifugu Obscurus) during frozen storage than air freezing (−18 °C and −50 °C). Besides, the texture characteristics, water holding capacity and membrane integrity of frozen obscure pufferfish fillets treated with edible immersion freezing
Declaration of competing interest
None.
Acknowledgements
Thanks for the following funds: NSFC 31701677; Postdoctoral Science Foundation of Jiangsu Province (No.1701098B); Postdoctoral Science Foundation of China (No.2017M621634); China Agricultural Research System (CARS-45); Project from Jiangsu Fisheries Administrator (Y2018-25); China National First-class Discipline Program of Food Science and Technology (JUFSTR20180201); and Yi Tong Postdoctoral Program in Jiangsu.
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