Effect of food ingredient on microwave freeze drying of instant vegetable soup
Introduction
Commercial instant soups are available in the market in most countries. Freeze-dried instant vegetable soups are prepared with various vegetables as main raw materials, followed by seasoning, molding and freeze drying. The dried instant soup should retain the color, flavor and nutrition of the original soup mix. Adding some water to the instant soup, it must recover the original state of the soup. However, high costs restricted the popularity of freeze-dried instant soups.
Freeze drying produces the highest-quality dried foods. But a major problem with conventional freeze drying (FD) is the long drying time needed, which in turn leads to high energy consumption and high capital costs (Hammami and Rene, 1997, Liu et al., 2008). This is partly due to the poor heat transfer rate associated with the conventional electric heating method which transfers heat for drying by conduction. It is well known that microwave radiation generates rapid volumetric heating of a wet material by altering the electromagnetic field to interact primarily with polar water molecules and ions in food materials (Varith, Dijkanarukkul, Achariyaviriya, & Achariyaviriya, 2007). To increase the freeze drying rate, utilization of microwave heating instead of the traditional conduction heating has been proven to give better heat and mass transfer rate (Duan et al., 2008a, Duan et al., 2008b, Duan et al., 2007, Lombraña et al., 2001, Sunderland, 1980, Wang and Shi, 1999, Wang et al., 2009). This technique is called microwave freeze drying (MFD). However, some problems still exist in application of MFD technology. The degree of heating of a food material subjected to microwave processing is strongly influenced by their dielectric properties (dielectric constant and dielectric loss factor) (Venkatesh & Raghavan, 2004). When water was frozen, the dielectric constant and dielectric loss factor decreased sharply (Venkatesh & Raghavan, 2004). Hence when ordinary MFD is applied for dehydration of high moisture content materials, the drying rate cannot be improved distinctly. To enhance the MFD rate, a study of dielectric material assisted MFD has been conducted by some authors (Tao et al., 2005, Wang and Chen, 2003, Wang and Chen, 2005, Wang et al., 2005, Wu et al., 2004). Their theoretical and experimental study indicates that the MFD process can be enhanced significantly by the addition of dielectric cores with high loss factor to the porous or liquid materials to be dried. The dielectric cores function as another heat source by absorbing microwave energy.
The dielectric properties of food materials are affected by the chemical composition such as moisture, sugar, salt, and protein contents (Brinley et al., 2008, Heredia et al., 2007, Ng et al., 2008, Venkatesh and Raghavan, 2004), so the change of chemical composition of materials also has potential to enhance MFD process. However, little study has been done on this aspect. Vegetable soup is a complicated and high water content food. It contains food ingredients such as salt, sugar, amino acid and so on. Hence, matching concentrates of these ingredients to improve dielectric properties during preparation of vegetable soup, there is a potential to improve MFD rate and product quality.
Response surface methodology (RSM) is a statistical procedure frequently used for optimization of complex processes and evaluating interactive effects. It has been successfully used in the optimizing process variables (Changrue et al., 2008, Erbay and Icier, 2009, Ozdemir et al., 2008).
Therefore, the objective of this work was to investigate effect of NaCl content (1–15 g/100 g water), sucrose content (5–25 g/100 g water) and sodium glutamate content (0.5–10 g/100 g water) on the MFD characteristics of instant vegetable soup, and optimize their addition levels when used in combination by RSM. The optimum conditions were also verified by a comparison of MFD instant vegetable soup with and without these ingredients.
Section snippets
Raw materials
Vegetables (cabbage, tomato, carrot, spinach and mushroom) employed in the experiments were purchased locally. They were stored at a temperature of 4 ± 0.5 °C until the drying experiments. The ingredients including salt (NaCl), sugar (sucrose) and sodium glutamate were purchased from local supermarket.
Experimental equipment
Experiments were performed with a microwave freeze dryer, which has been described earlier (Duan et al., 2008a). A schematic diagram of the equipment is presented in Fig. 1. As shown in Fig. 1,
Drying kinetics
Drying curves for instant vegetable soup with different NaCl, sucrose and sodium glutamate content are presented in Fig. 2(a), (b) and (c), respectively. As seen in Fig. 2(a), drying of instant vegetable soup with higher NaCl content resulted in shorter drying time. It was observed that MFD rate increased with the increase of NaCl content when its addition level within 1–10 g/100 g water. However, when NaCl content reached 10 g/100 g water, a further increase in NaCl content did not improve any
Conclusions
The microwave freeze drying time of instant vegetable soup could be reduced by adding NaCl, sucrose and sodium glutamate. The NaCl and sucrose addition had significant effect on drying rate and sensory scores. The addition levels of the three food ingredients when used in combination were optimized using RSM. An optimal zone was reached at NaCl content of 3.2–5.3 g/100 g water, sucrose content of 2–6.8 g/100 g water, and sodium glutamate content of below 4.5 g/100 g water, respectively. Under
Acknowledgements
The authors express their appreciation to the National Natural Science Foundation of China (No.20776062) for the financial support.
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