The Effect of Nonthermal Pretreatment on the Drying Kinetics and Quality of Black Garlic
1. Introduction
Allium sativum
L.). The process is usually performed for an extended period of time (up to several weeks) at increased temperature (60–90 °C) and high relative humidity (50–95%) [8,
Black garlic is produced during the aging of fresh garlic (L.). The process is usually performed for an extended period of time (up to several weeks) at increased temperature (60–90 °C) and high relative humidity (50–95%) [ 1 ]. Consequently, the material changes its physical and chemical properties, acquiring the typical dark color in the process. The main bioactive compound responsible for the properties of black garlic is S-allyl cysteine (SAC), which is formed as a result of the conversion of unstable alliin present in fresh garlic [ 2 ]. Moreover, the strong taste and smell of garlic changes due to the decreased amount of allicin, which is responsible for its strong off-flavor [ 3 ]. An increasing amount of Maillard reaction products in black garlic also influences the taste and aroma, which results in a typical sweet and sour taste that some describe as plum [ 4 ]. Black garlic can be characterized by many health-promoting properties, including anti-inflammatory [ 5 6 ], antidiabetic [ 7 9 ], anticarcinogenic [ 10 11 ] as well as the ability to reduce blood pressure [ 12 13 ]. These properties make black garlic a very attractive functional food ingredient that can be used in the development of various snacks or other food products, especially in Japan, China, Korea, and also in the USA, where it is gaining recognition [ 14 ]. Moreover, fresh garlic is already recognized and accepted in society and there is already a big market for garlic-derived products [ 1 ]. Hence, black garlic is among the fastest-growing health food [ 15 ]. Therefore, application of various treatments, including drying, can enable obtaining powders that could be used as spices or additives to functional food products.
Cassia alata
[
Convective drying is among the most common drying methods. The process is based on the principle of water evaporation as a result of airflow through the material that absorbs the moisture from the surface. This is followed by the internal diffusion of moisture from the inside of the sample to the outer layer to enable water evaporation. Convective drying has previously been used in studies on garlic [ 16 ], Thai basil [ 17 ], hemp flowers [ 18 ], and kiwiberry [ 19 ]. On the other hand, vacuum and vacuum-microwave drying enable reaching a lower final moisture content in the final product. Moreover, vacuum-microwave drying can significantly accelerate the drying process due to volumetric heating occurring as a result of microwaves application, which is intensified by the pressure diffusion mechanism of the Darcy type, resulting from the pressure gradient between the center of the material and surrounding vacuum. These methods were previously used, among others, in studies on true lavender [ 20 ] and 21 ]. Drying is a very energy-intensive process, which is mainly due to the long processing times. This can be reduced by combining different drying methods, such as combined convective pre-drying to remove easily accessible unbound water followed by vacuum-microwave finishing drying aimed at reducing the drying time and improving the quality of the material. As a result, significant energy savings can be observed, such as in studies on garlic [ 16 ] as well as osmotic dehydration and drying of apples [ 22 ].
Cannabis sativa
[
Another way of reducing process duration and improving the quality of dried products is the application of various pretreatment methods. The most common are thermal methods, including freezing, blanching, or osmotic dehydration. However, nonthermal methods are also gaining recognition due to their effect on the shortening of the drying duration and limiting microbiological contamination [ 23 ]. Among these methods is a pulsed electric field (PEF). The use of a PEF is a pretreatment method based on the application of a very short, high-intensity electric field that leads to a cell membrane disintegration [ 24 ]. Consequently, electroporation occurs and leads to the intensification of internal water diffusion and therefore a significant reduction in drying time as shown in recent studies on parsnips [ 25 ], onions [ 26 ], carrots [ 27 ] and red bell pepper [ 28 ]. Moreover, the application of a PEF as a pretreatment did not affect the nutritional value of apple juice but led to an inactivation of microorganisms extending the product shelf life [ 29 ] as well as improving the microbiological safety of the product. A PEF was also previously used before osmotic dehydration, where it showed a significant reduction in the time of osmotic dehydration of blueberries [ 30 ]. Another type of nonthermal pretreatment is a constant electric field (CEF). In this method, the material is placed between the electrodes, and a generated constant electric field interacts with the material changing its properties. A CEF was previously applied in studies on Camellia [ 31 ] and 32 ]. A magnetic field (MF) has been used to accelerate the germination and growth of various seeds, including sunflower seeds [ 33 ] as well as in the freezing of plant materials due to its positive effect on the formation of small ice crystals and enhanced freezing rate [ 34 35 ]. Moreover, this method was used during convective drying in order to change its properties and accelerate water removal during drying [ 36 ].
Application of various nonthermal pretreatment methods such as a pulsed electric field, a constant electric field, and a magnetic field before drying has previously been discussed in the literature, but never in terms of black garlic drying. Therefore, this study aims to determine the influence of process parameters and nonthermal pretreatments on the drying kinetics and quality of the final product, including changes in the antioxidant and antidiabetic potential of black garlic after treatment.
