The influence of inhibit avoid water defect responses by heat pretreatment on hot air drying rate of spinach
Introduction
Spinach (Spinacia oleracea L.) contains many nutrients, such as vitamin C, vitamin E, and folic acid (Lisiewska et al., 2009), but has a limited shelf life during storage. To extend the storage time of substantial nutrients, researchers recently studied a drying model and recorded changes in the quality of spinach during drying (King et al., 2001, Dadali et al., 2008). Particularly, rapid drying of spinach, which inhibits the degradation of nutrient compounds during drying, has been examined (Ozkan et al., 2007, Karaaslan and Tunçer, 2008).
Heat pretreatment to inhibit various undesirable enzymatic reactions may be required to improve the final quality of the product (Nilnakara et al., 2009). In addition, heat pretreatment naturally causes some changes to drying characteristics. Watanabe et al. (2014) reported that heat pretreatment inhibited surface area shrinkage, caused by a physiological response, of cabbage leaves during hot air drying. Water deficits, one of the physiological responses of plants to drying, are characterized by decreases in water content and total plant water potential, resulting in wilting (Kramer, 1983). Watanabe et al. (2014) examined that heated cabbage did not exhibit this physiological response to avoid water deficits because the physiological activity of the sample was got down considerably with heating. Therefore, shrinkage of the surface area of cabbage leaves after heat pretreatment during drying was inhibited (Watanabe et al., 2014). The drying rate of cabbage leaves during drying increased as the area for water transpiration increased (i.e., the surface area). In order to confirm the applicability of the phenomenon to the dried spinach production process, the correlation between the physiological activity of spinach and the decrease in surface area during hot air drying must be clarified quantitatively. However, the correlation between the physiological activity of spinach and the decrease in surface area during hot air drying has not been previously reported.
Cell membrane stability (CMS) and the ratio of variable fluorescence to maximum fluorescence (Fv/Fm) are two indices used to evaluate physiological activity. The CMS is used to evaluate biological reactions of the cell membranes of agricultural products and has been used in many studies as an indicator of the physiological activity of fresh agriculture products (Premachandra and Shimada, 1987, Xiong and Zhu, 2002, Farooq and Azam, 2006, Dupont et al., 2011, Horváth et al., 2012, Lin et al., 2012, Balogh et al., 2013). Electrical impedance spectroscopy (EIS) is one method used to evaluate the CMS (Macdonald, 1992) and has been used as an index of physiological activity (Mwesigwa et al., 2000, Mizukami et al., 2007) or physical properties (Wu et al., 2008, Wu et al., 2009, Ando et al., 2014) for many agricultural products. The Fv/Fm is an index of the maximum quantum yield of PSII photochemistry (Baker, 2008). Measurements of the Fv/Fm have been used in many studies to evaluate physiological activity, particularly a life activity, of fresh agricultural products during the growing period because these tests are rapid and nondestructive (Walker et al., 1990, Krause and Weis, 1991, Willits and Peet, 2001, Rizza et al., 2001, Kocheva et al., 2004). However, these measurements of physiological activity have not been applied in the processing of agricultural products to evaluate drying characteristics of samples. Moreover, the CMS and the Fv/Fm are indices of physiological activity of fresh samples that relate to each other. However, differences in the characteristics of these indices during the same processing of agriculture products have also not been examined.
The objective of this study was to clarify quantitatively the correlation between the physiological activity of spinach and the decrease in surface area during hot air drying, which was useful for constructing of a new drying technique for increasing drying rate by changes in physiological activity of spinach by heat pretreatment. We measured changes in the CMS and the Fv/Fm of spinach during heat pretreatment and determined the relationships between the physiological activity and changes in moisture content and surface area of the spinach during hot air drying.
Section snippets
Materials
The spinach (Spinacia oleracea L.) blades used in the heat pretreatment and drying experiments were purchased from a local market and stored in a refrigerator at 4 °C in the dark prior to the experiment. In this condition, we confirmed the physiological activity of fresh samples were retained. The purchased spinach was used within two days. The average mass of the spinach blades samples was 1.26 ± 0.09 (g). The initial moisture content of the spinach was 10.52 ± 0.41 (d.b. decimal; number of
Moisture content
The changes in the MR of fresh and preheated spinach samples during hot air drying are shown in Fig. 1. The MR decreased as the drying time increased, which resulted in a gentle, downward curve. Fig. 2 shows the relationship between the DRM and MR of fresh and pretreated samples during hot air drying. The DRM decreased with decreasing MR. The DRM of preheated samples in the early stage of drying was 2.51- to 4.42-fold larger than that of samples that were not pretreated. As the drying time
Conclusions
After heat pretreatment, a blade of the spinach was dried by hot air drying. The physiological activities (CMS and Fv/Fm) and the physical properties (moisture content and surface area) of the spinach during heat pretreatment or drying were evaluated. Using obtained data, we discussed the influence of the decrease in physiological activity with heat pretreatment on the hot air drying rate of spinach. The conclusions are summarized as follows:
- 1.
The decrease in the ratio of MR (DRM) of preheated
Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 26850156 [Grant-in-Aid for Young Scientists (B)].
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