Effect of cassava starch gel, fish gel and mixed gels and thermal treatment on structure development and various quality parameters in microwave vacuum-dried gel slices

Abstract

Three types of gels including tapioca starch gel, fish gel and mixed gels of both (cassava starch: fish = 1:1) were prepared under different thermal conditions. Slices (4 mm thick) derived from these the gels were dehydrated to a final moisture content of 7% in a microwave vacuum dryer. The micro/macro structure and selected quality parameters of the dried gel chips were measured e.g. shape, relative volume, bulk density, texture, color, sensory and microstructure. The measured gel characteristics were analyzed and related to structure development and selected quality attributes. Both fish and mixed gel chips expanded in both diameter and thickness while the starch gel chips shrank in diameter, but expanded in thickness; the former two represented a continuous cross-section composed of a cellular structure and expanded uniformly in thickness while the latter developed an open cross-section and the center expanded more than the edge region, indicative of a disc and pillow shape, respectively. The higher severity of thermal treatment favored greater expansion and reduced hardness and bulk density, and increased crispness in the mixed gel chips; they developed a smooth and fine surface and pore wall but decreased lightness and whiteness due to starch gelatinization in the starch and mixed gel chips. Only the partial dried mixed gel chips are acceptable to the panelists. A homogenous co-gel in the mixed gel contributed to the higher uniform expansion and cellular structure. These findings will help developing the restructured microwave vacuum-dried product by blending the starch with the fish.

Introduction

Microwave vacuum drying (MVD) has been widely considered as a potential dehydration technology to obtain high-quality dried food products (Markowski, Bondaruk, & Błaszczak, 2009). Coupled with rapid dehydration at low temperature in the absence of oxygen, MVD is suited to create an expanded and/or porous structure for some dried products such as apple chips and strawberry halves (Erle & Schubert, 2001; Sham, Scaman, & Durance, 2001), potato (Bondaruk, Markowski, & Błaszczak, 2007; Markowski et al., 2009), blue honeysuckle snack (Liu et al., 2010), durian chips (Bai-Ngew, Therdthai, & Dhamvithee, 2011) etc, which contributes to a desirable, crispy texture for the dried products similar to that created by frying (Sham et al., 2001). Therefore, it is an attractive alternative method to produce oil-free and healthier snack-type food products.

Texture is one of the most important criteria affecting consumer acceptance of snack-type chips and is described in terms of crispness, hardness and crunchiness (Huang, Zhang, Mujumdar, & Lim, 2011; Salvador, Varela, Sanz, & Fiszman, 2009). Hardness is an important parameter used to quantify case hardening in dried fruits and is related to expansion while crispness is conceived as being related to the cellular structure of foods (Hao, Zhang, Mujumdar, & Lim, 2011; Saeleaw & Schleining, 2011). The higher the degree of expansion the thinner the walls of the expanded products; then less force is required to puncture the product (Arimi, Duggan, O'Sullivan, Lyng, & O'Riordan, 2011). Pores play an important role in the crispness and texture puffed snack. A chip should be firm and snap easily when bent, emitting a crunchy sound (Sobukola, Dueik, & Bouchon, in press). Certain degree of sensory hardness is necessary for higher crispness (Salvador et al., 2009). During MVD processing, the low chamber pressure used results in relatively high internal vapor pressure as a result of microwaves penetrating into the interior of the chips and water being vaporized in situ. This in turn results in an outward force, which expands the fruit structure to create voids, and render a crisp texture (Mui, Durance, & Scaman, 2002).

In addition texture/crispness, properties, such as volume, shape as well as color are the other main quality attributes perceived by the consumer. MVD causes the volume and shape changes in the materials upon drying. Three typical shapes in MVD products have been observed e.g., non-uniform pillow-shaped expansion, uniform expansion and shrinkage as well (Durance & Wang, 2002; Erle & Schubert, 2001; Figiel, 2009, 2010; Lin, Durance, & Scaman, 1998; Liu et al., 2010; Markowski et al., 2009; Wang, Zhang, & Mujumdar, 2012a). Shrinkage is often seen in most MVD fruits and vegetables but this depends also on the power level.

The sensory characteristics are the main indicators of acceptance of food products by the consumer. Due to the replacement of the frying process with drying alternatives the variation in flavor of the finished products cannot be measured instrumentally, thus making affective sensory analysis indispensable in this type of research (Bai-Ngew et al., 2011).

These quality attributes of food depend strongly on the special structure which is developed during drying of foods and the volume changes caused by drying, which are closely associated with the characteristics of the material to be dried as well as the drying conditions used (Arimi, Duggan, O'Riordan, O'Sullivan, & Lyng, 2010; Arimi et al., 2011; Durance & Wang, 2002; Lefort, Durance, & Upadhyaya, 2003; Maté, Quartaert, Meerdink, & vant Riet, 1998).

Most of the studies on MVD focus on modeling the drying kinetics and evaluating the quality of the finished products for non-restructured agricultural products containing multi-components, carrot cubes (Lin et al., 1998), shrimp (Lin, Timothy, & Scaman, 1999), apple chips (Erle & Schubert, 2001; Han, Yin, Li, Yang, & Ma, 2010; Sham et al., 2001), banana chips (Mui et al., 2002), potato slices or cubes (Bondaruk et al., 2007; Lefort et al., 2003; Markowski et al., 2009; Song, Zhang, Mujumdar, & Fan, 2009), bighead carp (Hypophthalmichthys nobilis) slices (Zhang, Zhang, Shan, & Fang, 2007), beetroots (Figiel, 2010) and durian chips (Bai-Ngew et al., 2011). The role of individual ingredient, especially structural ingredients such as protein, starch, fiber etc., and thermal treatment condition in the MVD products has not been well elucidated.

Recently some restructured dried crispy chips have been developed using MVD. In the restructured chips materials rich in starch, such potato, bananas or the pure starch or fish muscle, are used to blend with other components such as the blanched and ground plant tissue (apple, lychee, wild cabbage) (Hao et al., 2011; Huang et al., 2011; Liu, Zheng, Jia, Ding, & Gao, 2009, 2010; Qiao, Huang, & Xia, in press; Wang, Zhang, Mujumdar, Wang, & Zhu, 2012; Zhang, Zhang, & Mujumdar, 2011). The advantages of such fabricated products include reproducibility, variable process and cost control, the harmonization and improvement in flavor and decrease hot spots and avoid charring, increase in utilization rate and improvement processing properties of the material, uniformity and lack of defects when compared with heterogenous materials like raw potato, fish, which may instill important variations to MVD products (Sobukola et al., in press; Wang, Zhang, & Mujumdar, 2011). However, the use of starch or fish in these products is most based on practical experience and lack of scientific experimental evidence. In addition, the pasty mass is poured into a plate and then dried; shaping is found to be an unsolved processing problem.

Both cassava starch and fish muscle possess good capacity of forming gel either in the mixed form or alone and are frequently used for producing fried products such as fish crackers and cassava crackers (Kyaw, Cheow, Yu, & Dzulkifly, 2001; Kyaw, Yu, Cheow, & Dzulkifly, 1999; Kyaw, Yu, Cheow, Dzulkifly, & Howell, 2001; Saeleaw & Schleining, 2011; Suknark, Phillips, & Huang, 1999; Tongdang, Meenun, & Chainui, 2008; Wang, Zhang, & Mujumdar, 2012b; Yu, Mitchell, & Abdullah, 1981). The gel from cassava starch and fish muscle presents excellent handling and processing property, where other functional or nutritional ingredients can be easily formulated. Therefore drying gels provides a viable option for MVD products. The ratio of cassava starch to fish and the gelatinization conditions strongly affect gel characteristics; they influence expansion and texture of the fried products. It has been found only starch granules with more amylopectin that are fully gelatinized are able to participate in the formation of a stable expanded structure and the use of other ingredients, especially protein, decrease significantly their expansion and crispness. Our earlier study revealed that MVD leads to uniform expansion of the restructured fish meat heated at conventional thermal conditions and developed a consistent crispy texture in the dehydrated restructured fish products (Wang et al., 2012a). This seemingly indicates that the protein is responsible for the expanded structure in MVD. Little information is available on MVD gel products from the gels produced at different thermal treatment conditions.

Therefore, this research is aimed at 1) characterizing selected quality for the MVD products based on both instrument and sensory method; 2) elucidating the role of two typical types of prepared gel ingredients, viz. fish protein and starch, as well as gelatinization conditions (different thermal treatment conditions) in structure development of the MVD products using microstructure observation for the gel and dried products and gel characteristics analysis.