Microwave-vacuum heating parameters for processing savory crisp bighead carp (Hypophthalmichthys nobilis) slices

https://doi.org/10.1016/j.jfoodeng.2006.03.008Get rights and content

Abstract

The effects of moisture content of the pre-dehydrated fish slices, microwave (MW) power output, and vacuum degree on the quality of the savory crisp bighead carp slices were studied. The single factor tests indicated that, the optimal moisture content, microwave power output, and vacuum degree were 19.8 ± 2% (50 °C hot air drying for 3 h), 686 ± 3.5 W and 0.09 MPa, respectively. The results showed that proper moisture content of the pre-dehydrated fish slices and microwave power could increase the expansion ratio and crispness of the fish slices, and then improve the sensory quality of the finished products. The higher vacuum degree could enhance the puffing effect, improve the crispness of the fish slices, increase the energy efficiency of microwave heating, and obviously decrease the burnt spots formation on the surface of the finished products. The processing parameters of the savory crisp bighead carp slices were optimized by a L15(51 × 32) orthogonal design based on the single factor tests. The optimum processing parameters were finally determined as that the moisture content of the pre-dehydrated fish slices controlled at 15.3 ± 1% (50 °C hot air drying for 3.5 h), microwave power output selected at 686 ± 3.5 W, microwave heating for 12 s at vacuum degree of 0.095 MPa, and then heating for another 10 s after balanced for 1 min.

Introduction

Freshwater fishery develops very rapidly in China. In 2003, the total yield of the freshwater fishery was 17.96 billion Kg, which accounted for 40% of the total aquaculture yield (National Bureau of Statistics of China, 2004). Moreover, the gross product of the low value freshwater fishes such as silver carp, bighead carp etc. exceed 60% of the total freshwater fishery yield (Duan, Zhang, & Tang, 2004). Because of under-developed freshwater fish processing industry and relatively poor technology, most of the silver carp and bighead carp were sold alive or at frozen status (Yan, 2002), and few of successfully processed products were available in Chinese markets (He, Li, & Fang, 1999). Obviously, the fish processing industry has become the bottle-neck of the freshwater fishery development. More innovative processing techniques are the key to the industry. Fish slices are important products of the fish snack foods, but at present, the main processing technology are still roasting and frying in China. Although many people attempted to utilize microwave in freshwater fish slices processing (Chi et al., 1999, Wu and Li, 2002, Yang et al., 1995), there existed some problems: (1) the color of the microwave processed products were worse than the far-infrared roasted products; (2) the heating was imbalanced, the products tends to form burnt spots; (3) the drying effect of the microwave heating was excessively, the texture and taste of the finished products are not good enough.

The edge effect of the microwave electric field was partly caused by the accidented surface of the pre-dehydrated fish slice, which will induce part of the surface temperature rise too fast, and then burnt spots are formed (Xu et al., 2004, Yang et al., 1995). In order to overcome the temperature imbalance of the microwave heating and improve the taste of the finished products, the effects of microwave-vacuum heating on the qualities of bighead carp slices were studied. The microwave-vacuum heating technology not only have the advantages of microwave heating (heating rapidly, high efficiency, good controllable and sanitation) (Maskan, 2000; Zheng et al., 2004), but the lower boiling point of water caused by the vacuum environment improve the energy efficiency and decrease the formation of burnt spots in the surface of the final products. Furthermore, this new processing technology can also increase the expansion ratio and improve the texture of the finished products. In the present study, we take the low value fish of bighead carp as material, a prospective high protein snack food (savory crisp bighead carp slices) was prepared by optimization of microwave-vacuum heating parameters. The final products not only have the aroma of the roasting fish slices, but also achieve good crispness.

Section snippets

Materials

Fresh bighead carps (about 1.5 kg) were purchased at the local supermarket. Before storage, the head, viscus, scales and fims were removed, and then the carcasses were cut into two pieces along the fish chine. The fish pieces were then stored in a refrigerator at −46 °C until further processing. A single batch of bighead carps, with the initial moisture content (wet basis) of 81.5 ± 0.7%, was used in the experiment. Millet, salt, sugar and seasoners such as sauce, vinegar, yellow wine etc. were

Effect of moisture content on the qualities of the fish slices

Microwave heating is realized by polar molecules vibrating in the microwave field with high speed. In food, water molecule is apt to polarize and form two poles of anode and cathode, so qualities of food produced by microwave heating are greatly affected by their moisture content (Lin & Anantheswaran, 1988).

At constant vacuum degree of 0.06 MPa and microwave power output of 686 ± 3.5 W, after heating for 10 s, 15 s, 20 s and 25 s, the expansion ratio of the pre-dehydrated fish slices was shown in Fig. 2

Conclusions

In the present work, pre-dehydrated fish slices containing relative high moisture content (⩾19.8%) had bigger expansion ratio than that of containing low moisture content (⩽15.3 ± 1%), which resulted in a lower crispness of the finished products and forming air bubbles in the surface of the fish slices. But too low moisture content (<13.5%) would result in ‘rigidness’ of the finished products. A relative high microwave power level could increase expansion ratio and crispness of the fish slices,

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