Drying technology combined with microwave-assisted extraction: A 3-in-1 processing solution

16/06/2021

Before the limitations of drying methods for agricultural products, medicinal herbs and extraction methods that cause loss of precious components from raw materials, the research team of PhD. Phuong .Thi Thuy Pham (Department of Process Equipment, Institute of Chemical Technology, Vietnam Academy of Science and Technology) has created an environmentally friendly method, saving energy, shortening processing time and reducing loss of medicinal ingredients during processing

Solutions take advantage of raw materials

Solutions for drying agricultural products and medicinal herbs are still applied such as heat pump drying equipment, methods of using microwave energy to dry frozen biological materials, etc. Although these methods have many advantages. points, but all cause a large loss of volatile medicinal ingredients in the starting materials. Besides, in Vietnam today, essential oils are extracted mainly by steam distillation. Although this method has a simple process, easy to manufacture, easy to operate equipment and low investment cost, it has many disadvantages such as not being applicable to materials with low - essential oils content. The obtained essential oil may be degraded if it contains components that are easily hydrolysed…

Members of the research team of drying technology combined with microwave assisted extraction. Group of Process and Equipment Department, Institute of Chemical Technology, Vietnam Academy of Science and Technology

That's part of the reason PhD. Phuong was looking for a solution to those weaknesses. Besides, she also wants to optimize it in the direction of "zero-waste goal in processing and preserving agricultural products and medicinal herbs". What is zero-waste in processing and preserving agricultural and medicinal products? According to her, because fresh herbs cannot be kept for a long time, if we want to bring them to the market for consumption, we need to dry them or extract valuable compounds out of them. "However, when drying agricultural products and medicinal herbs, a part of biological active ingredients will be attracted to moisture and lost. Meanwhile, the normal extraction process will produce waste products, which are solid residues and a large amount of solvent," explained PhD. Phuong. Therefore, the research team wanted to find a technology to collect all 3 types of products: dried products, extracts and essential oils, without wasting any valuable components of the product.

Even finding such a waste-free method is difficult, but also satisfying in terms of quality. "In order to evaluate whether the drying process of agricultural products and medicinal herbs is effective, it is not only about the rate of moisture loss in the material, but also the quality of the products after drying, such as: the ability to preserve All flavors, colors, nutrients, vitamins and bioactive substances in the material are not destroyed during the drying process, as well as the energy costs of that drying process”, PhD Method of explanation.

She realized that microwave assisted extraction technology is a promising option. "These technological application methods have low energy costs and short extraction times. In particular, recently, the method and equipment for microwave extraction combining vapor diffusion and gravity have been widely used in many studies to extract essential oils and biologically active substances at scale of the laboratory”.

However, the extraction device in this method uses a perforated disc or microfilter to hold the material inside the microwavable material container and allow the extracted components to permeate. "Therefore, the device is only suitable for materials that are large in size and not easily broken, such as orange peels, tangerines, green vegetables…”, said PhD. Phuong. On the contrary, when operating with small and fragile materials such as lemongrass and many other medicinal herbs, it is easy for the material to fall out of the extraction chamber under gravity, which causes the The extracted fluid or the resulting essential oil is contaminated with impurities, or the filter is clogged. In addition, the biggest limitation of the solventless microwave extractor is that the extraction efficiency is not equal to that of steam entrainment distillation. Therefore, medicinal herbs after microwave extraction usually still have medicinal properties.

Therefore, although the research team has determined that they will combine the drying process with the extraction of essential oils and extracts, which uses a drying device combined with microwave extraction, in order to thoroughly recover ingredients with medicinal properties; but they will still have to work around the limitations of previous microwave-assisted extraction devices.

Fabrication of equipment for Drying technology combined with microwave-assisted extraction

In order not to make mistakes of previous solutions, the scientists decided to create a drying device combined with microwave-assisted extraction. Specifically, this device consists of a microwave oven with a microwave generator, a microwave energy diffuser and a cooling fan. Notably, this device is drilled at least one hole in the ceiling of the oven to bring the temperature sensor inside to control the temperature during the drying process and a hole in the bottom of the oven to connect with the outside of the oven.

With that, "we put a drying chamber in the microwave. However, this drying chamber has a structure consisting of a chamber lid with a sealing device and with at least one hole for the thermometer, and a steam duct, in which a hole is cut to collect the vapor mixture generated during the drying process, and it is connected to an external condenser to condense the vapor mixture released from the drying chamber”, described PhD. Phuong.

In addition, the research team also installed a vacuum system connected to the extractor tank, which is characterized by the combination of ejector and pump to create vacuum for the drying chamber and supply cold water to the condenser. The team also added a heat pump system to cool the condenser feedwater.

Because the drying chamber has a lid, it is more convenient to load materials and take out the product after the drying process. In addition, the arrangement of the steam pipe inside the drying chamber instead of the perforated disc or the micro filter supporting the material, has helped to solve the situation that previous studies have encountered, that is, the material is dropped or stuck in the perforated disc or filter.

Dried ginger product after extracting essential oil

First, the research team will preliminarily process agricultural products and medicinal herbs depending on the nature of each type of material and the requirements of each product. After preliminary processing, raw materials are supplied to the drying chamber. The research team conducted microwave irradiation of the material, conducted the drying process at a fixed temperature and under vacuum conditions. During the drying process, the steam and other components separated from the material are released from the drying chamber by gravity. The team then condenses the vapor mixture and collects the extract; at the same time, separate the essential oil from the extract in the case of essential oils in the extract. As a result, agricultural products and medicinal herbs have been dried to a level sufficient to preserve and recover the mixture of extracts or essential oils during the drying process. Notably, this method is more energy-efficient than previous methods, as well as because it does not use solvents, it is completely environmentally friendly.

In one experiment, PhD. Phuong used fresh ginger from Hau Giang, washed them thoroughly with clean water and then ground them into small pieces with a blender. Through this method, the team extracted valuable compounds in the form of dried ginger, ginger essential oil and ginger juice. The rotary device combined with the microwave oven has ensured a uniformly dried ginger product, with a ginger essential oil content greater than 0.33% and moisture less than 10%. Notably, the quality of ginger essential oil obtained by this method has higher antibacterial activity than when using steam distillation method, while the physical properties of both essential oil products are same.

The amount of ginger essential oil obtained by this method is only half of the amount of ginger essential oil obtained by steam distillation, however, the remaining essential oil is not lost. This amount of essential oil is left in the dried ginger part. This dried ginger residue has high essential oil content, low moisture content (less than 10%), and is rich in fiber, which is considered as a dried ginger by-product. Besides, another product is ginger juice, which is rich in antioxidant ingredients. Meanwhile, both solid and liquid residues of ginger in distillation are generally considered waste. In addition, the energy required to produce 1.0 kg of ginger essential oil by this method is four times lower than that required by the distillation method.

Because of these improvements, methods and equipment for processing agricultural products and medicinal herbs by combining drying and extraction processes have been approved by the National Office of Intellectual Property of Vietnam patented patent number 1-0028413 was published on May 25, 2021.

Although the research team has had initial success in terms of research, but PhD.Thuy Phuong admits that "I am not good at technology transfer, so currently this device has not been transferred, only serving for the group's research work." However, she still hopes that one day, the equipment the research team has developed for many years will reach the hands of those who need it to be able to create products of high economic value, as well as investment cost savings.

Source: Joint article between National Office of Intellectual Property of Vietnam and Science & Development Newspaper

Drying technology combined with microwave-assisted extraction: A 3-in-1 processing solution

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