Materials and methods
Results and discussion
Ultrasound has been demonstrated as an effective pretreatment to enhance the convective drying process of mulberry leaves. For example, the total processing time and energy consumption needed to reduce moisture content in mulberry leaves to 0.10kg water/kg dry matter was decreased by 17.2% and 17.3%, when samples were pretreated by ultrasound at 63.0W/L for 10min (central point). The modeling results provided by the tachykinins model considering external transfer resistance revealed that the increase of AED and ultrasound duration could increase De and hm in the current experimental range. Meanwhile, ultrasound pretreatment had a deeper influence on internal moisture transport than on external moisture transfer during drying.
On the other hand, although there was a slight solid loss in mulberry leaves after ultrasound pretreatment, the quality properties, such as color, antioxidant activity and contents of bioactive compounds of dried mulberry leaves pretreated by ultrasound (63.0W/L, 10min) were comparable to those of control samples. In conclusion, ultrasound pretreatment in a proper way can shorten the drying time of mulberry leaves and save the energy consumed, while the quality of final product can also be satisfactory. Future studies should pay attention to scale-up of ultrasound equipment based on the identified De and hm.
This work was funded by the Independent Innovation of Agricultural Sciences in Jiangsu Province, China coded CX(15)1026.
Rapeseed oil is the third most widely consumed vegetable oil worldwide after palm and soybean, and ranks first in Europe for frying oil, margarine and salad dressings [1,2]. The rapeseed oil production process involves several steps including preparation of seeds, mechanical pressing and solvent extraction of the press cake (Fig. 1) .
The oldest method for oil recovery from seeds is pressing to squeeze the oil out of the solid residue . However, matrices containing more than 30% oil, such as rapeseed, which contains nearly 50%, require more than simple pressing to recover all the oil available in the seed. Conventional processing begins by a conditioning step to increase seed plasticity by warming to 50–60°C. Seeds are then flaked between smooth rollers to increase the surface area and help release oil. The flakes undergo a cooking step lasting 20–60min, which brings the temperature into the range 90–120°C. This step produces significant changes in the cell, especially by destroying oil bodies and causing oil droplets to merge and migrate across cell walls. As a result, the mechanical extraction by pressing that follows the cooking step removes 65–70% of the oil contained in seeds. It permits to continue the cell wall disruption needed to obtain a high rate of oil recovery in press cake [4,5]. The press cake then undergoes solvent extraction. This step is performed in countercurrent extractors using hydrocarbon solvents, with 40–60% n-hexane, sourced from fossil resources and registered under the REACH Regulation as a category 2 reprotoxic and a category 2 aquatic chronic toxic substance . Reducing the amount of hexane used in oil processing while keeping the same extraction performance has become desirable and is a key issue for industries for economic and ecological reasons.
Over the past few years, much interest has been paid to the applications of ultrasound (US) in food science and technology [7–10]. Applications of US generally involve processes that can enhance rates, improve quality and/or safety, and reduce processing time. A growing trend is the possible application of US for the extraction and promotion of natural compounds of interest  for industry [12,13]. The benefits of US are attributed to acoustic cavitation: micro-bubbles created in a liquid phase when subjecting a mixture to US will grow and oscillate quickly before collapsing due to pressure changes [14,15]. These violent implosions will fragment or disrupt the surface of the solid matrix, enhancing mass transfer and accelerating diffusion. Several studies suggest that ultrasonic technology is an effective tool for vegetable oil extraction from seeds [16–19].
Materials and methods