Effect of immersion freezing with edible solution on freezing efficiency and physical properties of obscure pufferfish (Takifugu Obscurus) fillets

Highlights

• AF, IF and LNF were shown freezing curves and rates.

• Microstructure, texture, water holding capacity and cell integrity were compared.

• Edible IF(-30 °C) rate was 11.5-fold quicker than AF(-18 °C).

• Physical properties and PCA showed edible IF(-30 °C) had similar aspects as LNF.

Abstract

The impact of immersion freezing (IF) with edible solution (ethanol, NaCl, dextran, active protein, chrysin and naringenin) on physical properties of obscure pufferfish (Takifugu Obscurus) was compared with air freezing (AF) and liquid nitrogen freezing (LNF). Freezing process, microstructure, texture characteristics, water holding capacity, and cell integrity were investigated. Results showed that IF(-30 °C) rate was 11.5/4.7-fold quicker than AF(-18/-50 °C) rate. As for microstructure, ice crystals in IF(-30 °C) fish were much smaller than that in AF. Referring to texture characteristics and water holding capacity, shear force and hardness of IF(-30 °C) fish were 12.9% and 16.7% higher, and drip and cooking loss were 44.1% and 8% lower, than that of AF(-50 °C) fish after 24 weeks storage. Besides, IF led to higher integrity of muscle fibers than AF. For all physical indexes, IF fish had similar quality as LNF fish. Principal component analysis (PCA) further proved these results. In conclusion, edible IF(-30 °C) could retard physical properties deterioration of frozen fish in a better way than AF and have potential to replace LNF.

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.