Comparison of different drying methods on Chinese ginger (Zingiber officinale Roscoe): Changes in volatiles, chemical profile, antioxidant properties, and microstructure
Introduction
Ginger, one of the most ancient spices in the world, has been widely used as a spice or a common condiment for a variety of compound food and beverages (Larsen, Ibrahim, Khaw, & Saw, 1999). It is also an important medicine for treating cold, stomach upset, diarrhea, and nausea. Phytochemical studies show that ginger has antioxidant and anti-inflammatory activities, and some of them exhibit potential cancer preventive activity (Shukla and Singh, 2007, Stoilova et al., 2007, Thomson et al., 2002). The characteristic components of ginger include essential oil and oleoresin, which are responsible for its fragrant and pungent behavior, respectively. Essential oil mainly consists of monoterpenoid and sesquiterpene hydrocarbons, whereas oleoresin is composed of non-volatile phenolics known as gingerols, shogaols and zingerone (Huang, Wang, Chu, & Qin, 2012). The gingerols are identified as the major active components in fresh ginger. Shogaol series of compounds do not intrinsically exist in fresh sample, as they are derived from the corresponding gingerols during thermal processing or long-term storage. Generally, the degradation of gingerol to shogaol takes place either because of the acidic environment or as a result of the increase in temperature (Kubra & Rao, 2012). Studies also have proven that shogaols are more pungent, and exhibit higher antioxidant activity than gingerols (Guo, Wu, Du, Zhang, & Yang, 2014).
Fresh gingers usually contain 85–95% of water and are susceptible to microbial spoilage and chemical deterioration (Mishra, Gauta, & Sharma, 2004). Dehydration of ginger is the most practiced processing procedure to inhibit microbial growth and delay deteriorative biochemical reactions. It is also a fundamental processing method to obtain new products. Dried ginger can be utilized for manufacturing ginger spices, medicine and cosmetics as well as food with ginger flavor such as soft drinks and candies. However, drying process may cause thermal damage and severe changes in physical, chemical and organoleptic properties of aromatic plant. Therefore, the selection of drying method is very important. According to Mujumdar and Law (2010), drying technologies have attracted significant research and development efforts because of the growing demand for better product quality and lower operating cost, as well as lessened environmental impact.
The most conventional drying method is hot air drying (AD), however, its high temperature and long drying cycle usually result in the degradation of important flavor, color and nutritional compounds. Freeze drying (FD) can yield high quality products, but it also leads to high energy consumption, high capital cost and long drying time. Microwave drying (MD) and infrared radiation (IR) have their own place in drying technology, due to the same transfer direction of temperature and moisture, they can offer many advantages such as great energy efficiency and high heat transfer rate. IR has more advantages in uniform heating and high quality of final products (Sellami et al., 2013). Although MW heating can readily deliver energy to generate heat within foods, one of its major drawbacks is the inherent non-uniformity of the electromagnetic field (Zhang, Tang, Mujumdar, & Wang, 2006). Since its local temperature can easily rise to a level that causes scorching, microwave drying usually has been combined with other techniques including convective hot-air, vacuum, and intermittent power application to achieve more uniform, high quality and effective drying (Gunasekaran, 1999, Kaensup et al., 2002, Soysal et al., 2009).
Microwave-convection drying includes two kinds of form, one is microwave and air convection drying conducted in stages, the other being these two dryings carried out simultaneously. In our study, the second form was adopted. In microwave combined convection drying, microwave energy removes the inner moisture of material to the surface and convective air helps removing the surface moisture out of drying chamber, which not only increases the energy efficiency but also reduces the surface temperature of material. Intermittent application of microwave energy has proven itself a good method to avoid uneven heating, improve product quality and increase energy utilization by allowing redistribution of temperature and moisture profiles within the product during off times (Gunasekaran, 1999).
Many authors have studied the variations of volatiles, non-volatiles or antioxidant capacity of ginger induced by drying process. Huang et al. (2012) studied the effects of oven drying, microwaving drying, and silica gel drying methods on the volatile components of ginger and found that microwave and silica gel can be used in drying of ginger to maintain the taste and appearance of fresh ginger. Bartley and Jacobs (2000) reported that the major effects of drying process on ginger are the reduction in gingerol content, increase in terpene hydrocarbons and conversion of some monoterpene alcohols to their corresponding acetates. Gümüşay, Borazan, Ercal, and Demirkol (2015) studied thermal dryings and freeze drying (FD) for ginger in terms of total phenolic content (TPC), ascorbic acid (AA) and antioxidant capacity. He found freeze dried gingers have better antioxidant properties than samples treated by thermal dryings. Yet so far there is no systematic investigation regarding the effects of drying methods on energy consumption, volatile and non-volatile components, antioxidant capacity, and microstructure of ginger at the same time.
The objective of this work was to explore the possibility of using intermittent microwave combined convection drying (IM&CD) for processing of ginger product. Therefore, an investigation was build on the comparison of different drying methods, namely, AD, IR, FD, MD and IM&CD on the energy consumption and quality conservation of ginger.
Section snippets
Reagent and chemicals
Acetonitrile and methanol (HPLC grade) were purchased from Honeywell (Morris, NJ, USA). Authentic standards of 6-, 8-, 10-gingerol and 6-shogaol were purchased from Chromadex Inc. (Irvine, CA, USA). Analytical grade chemicals: Folin–Ciocalteu reagent; gallic acid; ascorbic acid; 2,2-diphenyl-1-picrylhydrazyl (DPPH); 2,2′-azinobis (3-ethylbenzo thiazoline-6-sulfonic acid) diammonium salt (ABTS); 2,4,6-tripyridyl-s-triazine (TPTZ) were procured from Sigma–Aldrich (St. Louis, MO, USA).
Comparison of drying time, energy consumption and extraction yield of different dried gingers
The drying time, energy consumption and extraction yield were different in the selected drying techniques. As shown in Table 1, FD had the longest drying time and highest energy consumption, with drying time of 44.5 ± 2.0 h and energy consumption of 33.7 ± 0.53 kW h/g H2O. Air-dried samples went through the second longer drying time of 12.0 ± 0.5 h, but its energy consumption was relatively low, 3.30 ± 0.08 kW h/g H2O. The second large energy consumption was IR process with the drying time of 6.0 ± 0.7 h. MD and
Conclusions
Based on the results of present investigation, we conclude that drying methods and conditions have profound effect on the quality and energy consumption of the dehydrated product. Compared with AD and MD process, FD, IR and IM&CD had higher retention of chemical profiles, antioxidant activity and cellular structures, which was attributed to their less intense heating. However, FD and IR had relatively higher energy consumption and drying time, especially freeze drying. Therefore, IM&CD is a
Acknowledgments
Special thanks were given to Prof. Wang Zhengfu for his technical support of the intermittent microwave & convective drying equipment and helpful recommendations for the experiment. We also thank the financial support of Province Natural Science Fund of Guangdong (2014A030310208) and Province Science and Technology Plan Projects of Guangdong (2013B020203001) for this research.
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