Study on the Drying Technology of Natural Color

Pigment is a general term for dyes, pigments and all substances that can absorb light waves in the range of 400–700 nm [1]. Pigments can be divided into synthetic pigments and natural colors. Since W illian invented the first synthetic pigment, aniline violet, in 1856 [2], synthetic pigments have been used in large quantities. They have the advantages of being bright in color, strong in coloring power, highly stable, odorless and tasteless, easily soluble, easy to mix, and low in cost, and are therefore also used as food coloring. Food coloring is an edible substance that colors food to improve its hue and color, and it is a major category of food additives [3].

However, synthetic pigments are mostly tar-based substances that have no nutritional value and are harmful to the human body. Some synthetic pigments pose a risk of cancer if consumed in excess, so the safety of synthetic pigments is seriously questionable. Natural colors, on the other hand, are mainly extracted from plants, animals and microorganisms. Compared to synthetic pigments, natural colors are safer and have physiological activity. They also have certain nutritional effects and pharmacological functions, so the development of natural colors is particularly important [4].

However, due to its high sensitivity to light, heat and pH, as well as its susceptibility to oxidation, reduction and microbial action, Natural Color is easily affected by external conditions during processing and circulation, leading to oxidation and decomposition. In addition, the presence of coexisting components causes some Natural Colors to develop peculiar odors and smells [5], which seriously affects the color value per unit product and shelf life of Natural Color.

As an important part of the processing of natural colors, the development and application of drying technology is an important way to solve these problems. For example, Du Minhua et al. [6] used vacuum freezing technology to process strawberry puree, which greatly reduced the loss rate of strawberry pigments and VC and better preserving the nutrients and color of the food. Jin Feng et al. [7] used spray drying technology to microencapsulate corn pigments, and Valduga et al. [8] extracted anthocyanins from grape pomace and microencapsulated the extract to obtain a powdered Natural Color, effectively solving problems with the processing, preservation and reproduction of its nutrients and natural flavor. Although a variety of drying techniques have been used in the processing of Natural Color, there has been relatively little research on the application of low-temperature vacuum drying in this area. However, the drying characteristics of low temperature and vacuum are very conducive to ensuring the quality and yield of pigment processing, which makes the research on low-temperature vacuum drying of Natural Color of great practical significance.

1 Natural Color extraction and processing

Natural Color is mainly used for coloring or changing the color of food to stimulate and increase people's appetite. In addition, Natural Color has certain pharmacological and nutritional functions, such as turmeric's anticancer effect, safflower yellow's antihypertensive effect, paprika red's antioxidant effect, red yeast rice's hypolipidemic effect, and tea chlorophyll's blood lipid regulating effect [9], so it is widely used in the food, pharmaceutical, and cosmetics industries. In addition, chlorophyll can also be used in fats, soaps and oils and waxes, etc. [10].

The quality of Natural Color is mainly reflected in the color value per unit product, product shelf life and effective ingredients. The processing process mainly includes crushing, extraction, separation and purification, concentration and drying. Due to the instability of Natural Color, each process in the processing will affect its product quality. How to use high-tech to improve conventional technology or develop new technology has become an important direction of research. Cao Yanping [11] believes that Natural Color can currently be studied from the perspectives of extraction technology, separation and purification technology, and pigment structure identification and performance research.

In the study of extraction technology, in addition to the traditional solvent method, researchers have also studied and developed a series of high-tech methods such as ultrasonic extraction, microwave extraction, supercritical extraction, multi-stage or continuous extraction, high-pressure extraction, and enzyme-assisted extraction. for example, Beatriz et al. [12] used supercritical CO2 to extract lycopene from the skin and seeds of tomatoes; Katherin et al. [13] studied the effect of extraction conditions on the extraction of lycopene from watermelons using supercritical fluids; Chun et al. [14] also studied the effect of supercritical fluid extraction parameters on the yield and antioxidant properties of lycopene; Maier et al. [15] studied the enzymatic method for extracting polyphenols from grape pomace, etc.

In the study of separation and purification: in addition to the silica gel and alumina that were used in the early days, activated carbon is the most commonly used and inexpensive adsorbent, and its separation effect is also relatively good. In addition, recent research on emerging technologies such as chromatography resins and gels, as well as high-speed countercurrent chromatography, ultrafiltration and nanofiltration membrane technology, have also been successfully applied to the separation, purification and concentration of pigments.

Natural color products are mainly available in powder and liquid form. Even if the quality is high after separation, problems such as oxidation and decomposition still exist during circulation, especially for important natural color ingredients, which are more difficult to exist and store stably for a long time. In addition, most pigment products are in liquid or paste form, which is not conducive to storage and transportation. In actual production, it is difficult to quantify products in solution state when they are used, and the unit color value is low, and the shelf life is short, generally 12 to 18 months. Usually, the unit product color value of powders is high and the shelf life is long, so drying is an important way to solve this problem. However, different drying methods will also directly affect the quality of the product. Many researchers at home and abroad have also carried out a large number of experiments on the post-processing of pigments using various drying techniques. The drying methods involve spray drying, vacuum drying, microwave drying, etc.

2 Application and comparison of drying methods in the further processing of Natural Color

2.1 Application of drying methods in the further processing of pigments

Different drying methods directly affect the performance, form, quality and energy consumption during production of Natural Color. Different drying methods have been applied in the further processing of pigments, but the degree of impact on product quality varies.

2.1.1 Spray drying and microencapsulation

Spray drying is a drying method in which a single process atomizes solutions, emulsions, suspensions and slurries and evaporates the solvent by contacting with hot air to obtain a powder, granular, hollow ball or agglomerated dried product [16]. However, the use of high temperatures and air in spray drying greatly affects the quality of heat-sensitive materials such as Natural Color during processing. The process of using spray drying technology to enclose solid and liquid substances in a tiny, semi-permeable or closed capsule is called microencapsulation. This technology can prevent the active ingredients in the preparation from oxidizing, hydrolyzing and volatilizing.

Jin Feng et al. [7] studied the preparation process of corn pigment microcapsules and obtained the optimal composition of the wall material of pigment microcapsules: maltodextrin-microporous starch with a mass ratio of 1:1, 10% pigment, and 40% total solids. The optimal process for spray drying is: inlet air temperature 140°C, outlet air temperature 80°C. Zhong Yaoguang et al. [17] used spray drying to study the microencapsulation of NFH pigments. The results showed that the wall material was maltose (55%), hβ-CD (25% w/w) C and gum arabic (20%); the feed flow rate for spray drying was 50 mL/min; the inlet air temperature was 200°C and the outlet air temperature was 80°C. Seda et al. [18] microencapsulated anthocyanins extracted from black radish, and found that the optimum inlet air temperature was 160°C. The product quality was evaluated in terms of pigment content and antioxidant properties.

Although microencapsulation using spray drying can effectively solve problems such as processing, preservation and reproduction of nutrients and natural flavor components, and is used in the food additive industry, including spices, natural colors, seasonings, etc., the application of microencapsulated pigment products is limited due to the effect of the coating material on the color when mixing colors.

2.1.2 Microwave drying

Microwave, as an electromagnetic wave, refers to an ultra-high frequency electromagnetic wave with a frequency of 0.3–300 GHz, or a wavelength of 1–1000 mm [19]. It can generate a high-frequency electromagnetic field. Polar molecules in the dielectric material continuously change their polar orientation with the frequency of the electromagnetic field in the electromagnetic field, causing the molecules to vibrate back and forth and generate frictional heat to achieve the purpose of drying. It is mainly used for drying extracts after material extraction and concentration. It is an energy-saving and consumption-reducing technology that features fast drying speed, high efficiency and low cost. This technology can be used for drying and sterilization, extraction and concentration, puffing and low-temperature dehydration.

Liu Chunquan et al. [20] studied the dehydration test of microwave drying purple sweet potato chips, obtained the dehydration law of microwave drying purple sweet potato chips, established a microwave drying model for purple sweet potato chips, and also investigated the effect of microwave drying on purple sweet potato pigment. The results showed that the product pigment content was higher when the microwave power was 700W, the slice thickness was 6mm, and the pre-drying time was 20-50s. The three factors that affect the pigment content of the product: radiation power, slice thickness and pre-drying time were studied. Meng Xianghe et al. [21] studied the effect of microwave on the color of processed fruit products, discussed the changes in color and pigment composition after processing, and showed through liquid chromatography that microwave treatment does not change the structure or quality of carotene, but causes a loss of total carotene degradation. It was also found that microwave heating of kiwifruit causes a significant decrease in chlorophyll a and b.

This shows that because microwave drying usually uses a drying temperature of 80-100°C, there is also a phenomenon where the color of the dried product is significantly lower than that of the raw material.

2.1.3 Vacuum drying 

Vacuum drying is the dehydration and drying of materials with a high moisture content at a low temperature and under vacuum, including vacuum freeze drying and low temperature vacuum drying. It has the following characteristics: (1) It is carried out at low temperatures and is suitable for heat-sensitive substances. For example, proteins, microorganisms, and the like will not denature or lose their biological activity; at the same time, low temperatures will reduce the loss of some volatile components in the material, making it suitable for drying some chemical products, pharmaceuticals, and foods. 2) Drying is carried out under vacuum, with very little oxygen, so some thermally sensitive substances that are easily oxidized and afraid of high temperatures are protected.

Du Minhua et al. [6] used a linear weighted combination method to optimize the vacuum freeze-drying process of kudzu fruit pulp, and obtained the optimal process parameters: Material: The maximum surface temperature during analysis is 48℃, the initial drying chamber pressure during sublimation is 26Pa, the thickness of the charge is 7mm, the loss rate of VC and strawberry pigment is 6% and 38%, and the freeze-drying time is 18h. Ma Wenping et al. [22] initially studied the vacuum freeze-drying technology of wolfberry pigment. Because wolfberry pigment is a heat-sensitive material, it was found in the drying test of fresh wolfberry that the quality of the product will be affected when the temperature exceeds 50℃. Therefore, the separated fresh fruit pigment of medlar was used to make crude powder of medlar pigment by freeze drying. The sensory indicators such as product color, tissue morphology, odor and impurities, as well as the physical and chemical indicators such as β-carotene content, were all very satisfactory.

Low-temperature vacuum drying has similar drying conditions to freeze drying and is also widely used in many fields. For example, it is used in the food industry for the production of dried litchi, dried longan [23], ginseng [24], high-VC red dates [25], etc.; in agricultural production, it is used in the production of rice [26], corn [27] and other grains, while there are few research results on its direct use in pigment drying.

2.2 Comparison of several drying methods in Natural Color processing

Although there have been many studies on the use of drying technology to process natural colors, it is still worth studying which drying method is more suitable for further processing of pigments. Lü Yinghua et al. [28] used three different methods of freeze drying, spray drying and hot air drying to further process mulberry pigment, and compared the quality of the dried powder. From the comparison of sensory, physical and chemical and hygiene indicators, the results showed that vacuum freeze drying can better maintain the color and bioactive components of mulberry pigment.

Table 1 compares the different drying methods for Natural Color. It can be seen that: (1) the inlet air temperature for spray drying is between 120 and 200°C, and the processing is exposed to air, so it is easily oxidized. Although the drying time is very short, it will still affect the yield and quality of the pigment. Seda et al. [19] believe that high inlet and outlet air temperatures will affect the yield of anthocyanins. In addition, after the pigment product is spray-dried and microencapsulated, when it is mixed with other food additives at a later stage, it will affect the color value to a certain extent. (2) The temperature of microwave drying is not as high as that of spray drying (generally 60-100°C), but there are still problems with the thermal stability and oxidation of pigments [21]. This method is suitable for some pigments that are resistant to heat and oxidation, but it is not universal. (3) Vacuum freeze drying has a very low temperature and is carried out under a vacuum, so it is suitable for further processing of the pigment. However, its biggest problem is that it consumes a lot of energy and takes a long time to work. 4) The conditions for low-temperature vacuum drying are low temperature (20-60°C), vacuum, and less freezing than freeze-drying, so the energy consumption is lower and the drying time is shorter.

The above comparison and analysis show that low-temperature vacuum drying is a more suitable and effective method for Natural Color drying and is worthy of further research.

3 Low-temperature vacuum drying of pigments

3.1 Mechanism of low-temperature vacuum drying of pigments

During low-temperature vacuum drying, the phase change temperature of water under low pressure is lower than that under normal pressure [29], so the moisture ratio is more likely to vaporize than under normal pressure. As shown in Figure 1, after the material is heated, the internal moisture quickly vaporizes, and there is a large pressure difference between the inside of the material and the surface. The pressure gradient is in the same direction as the moisture transfer, and under the action of the pressure gradient, the moisture quickly moves to the surface, allowing the water vapor to enter the gas phase in the surrounding environment and be pumped away by the vacuum pump. From the mechanism of low-temperature vacuum drying, it can be seen that for vacuum drying, the pressure gradient is in the same direction as the moisture transfer. It is not easy for the surface of the material to harden and crack, and compared with vacuum freeze-drying, it has the characteristics of fast drying rate, short drying time, and low equipment operating cost [30].

3.2 Model study of pigment low-temperature vacuum drying

3.2.1 Mathematical model simulation in drying technology

As one of the focuses of drying technology research, mathematical model simulation and analysis has the following advantages over experimental research:

1) low cost; 2) fast; 3) detailed and comprehensive results and information; 4) can simulate ideal conditions; 5) can also simulate actual conditions. Therefore, the application of mathematical methods in drying has attracted the attention of many scholars. Xu Ying et al. [31] studied the freeze drying of yellow clams and established a heat and mass transfer model for yellow clams; John et al. [32] studied the drying model of grape seeds, etc. However, the lack of experimental data or the difficulty of measuring drying process parameters directly affects the accuracy of mathematical models. Huang Lixin et al. [33] summarized the main mathematical models and analysis methods used in the field of drying, and pointed out that with the development of computer technology and the development and application of large-scale commercial software, the accuracy of drying process simulation results has been significantly improved. Therefore, the research work on mathematical model simulation in drying technology will develop even better on the original foundation.

3.2.2 Thin-layer drying model simulation

Low-temperature vacuum drying usually involves placing the pigment solution to be dried in a thin layer on a heated plate, and then placing the entire plate under high vacuum to complete the drying of the entire solution. Therefore, low-temperature vacuum disc drying is also a kind of thin-layer drying [34]. At present, the mathematical simulation process of thin layer drying generally includes the following steps: 1) select several commonly used mathematical models; 2) obtain data from experiments; 3) use the data obtained from the experiment to determine, through engineering mathematical methods, a model that best fits the experimental results; 4) verify the model equation.

Regarding the thin layer drying model, Sharma et al. [35] gave a more comprehensive semi-theoretical empirical model and is listed in Table 2. The N ew ton model, also known as the L ewis model, is a moisture movement model based on N ew ton's cooling law; the P age model adds an exponent to the time based on the N ew ton model, which is a purely empirical model. such as the low-temperature rice model by Li Dong et al. [36] and the infrared drying chitin model by Ou Chunyan et al. [37]; Henderson and Pabis is also known as the single-diffusion model, which is based on Fick's second law.

Many researchers at home and abroad have also done a lot of work using these thin-layer drying models, such as Zeng Libin et al. [38]'s hot air drying silver carp model, Goyal et al. [39]'s mathematical model for thin-layer drying of plums, and Debabandya et al. [40]'s wheat model. However, these models are generally based on assumptions that differ greatly from reality, resulting in model deficiencies. Some models also ignore the effect of initial moisture content on drying rate, and are not suitable for drying characteristics of materials with high moisture content that require long preheating times [41].

Wang Zhiwen et al. [42] studied a one-dimensional theoretical model for vacuum drying, introduced the theory of vacuum drying for sheet-like and spherically symmetric materials, derived the differential equations of the heat and mass transfer problem, and used theoretical analysis to obtain the time-varying moisture content characteristics of the material and estimate the end of drying. However, some errors will be generated due to the assumptions in the theoretical model, such as setting the thermal diffusivity as a constant.

Most of these models are semi-theoretical empirical models that cannot accurately reflect the internal moisture migration process of materials, nor do they specifically describe the change of the internal moisture diffusion coefficient of materials with drying time. Therefore, the thin-layer theoretical model needs to be further studied.

4 Outlook

Today, with the rapid development of the food industry, the prospects for the development of natural colors are very broad. However, how to overcome problems such as the oxidation and decomposition of pigments is still an important research and application topic in the processing and refining of natural colors. The role of drying technology in the deep processing of natural colors is beyond doubt, but how to further change its limitations is also the direction of future research.

The analysis results show that some methods that have been used in pigment processing, such as spray drying, microwave drying and vacuum freeze drying, still have shortcomings. At the same time, it is shown that low-temperature vacuum drying is very suitable for drying Natural Color. Since there has been little research in this area at home and abroad, it is worth further in-depth research and exploration.

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