Comparison of the effects of different drying methods on volatile compounds in Qaidam Goji berries based on GC-IMS
Lycium barbarum L. is a deciduous shrub belonging to the Lycium genus (Lycium L.) in the Solanaceae family. It is widely distributed in the northwest region of China and is a valuable “medicinal and edible” plant resource. Goji berries have plump granules, rich juice, and a slightly sweet taste. They are rich in various active ingredients such as betaine, flavonoids, goji polysaccharides, carotenoids, riboflavin, and ascorbic acid. Goji berries, as a traditional Chinese medicine, are known as “goji berries”. According to the Compendium of Materia Medica, goji berries have the effects of improving vision, clearing the liver, nourishing yin and kidney, strengthening yang, lowering blood sugar, anti-aging, and anti fatigue; Tibetan medicinal wolfberry can treat headaches, forgetfulness, insomnia, abnormal emotions, and gynecological diseases caused by “heart heat”; Modern medicine has proven that it has the effects of enhancing immunity, reducing blood sugar, antioxidation, delaying aging, promoting tumor cell apoptosis, and lowering blood lipids. Due to the high medicinal value of goji berries, their fresh fruits are susceptible to microbial infection and difficult to preserve for a long time. Currently, goji berries are used in medicine in the form of dried fruits. The common production processes for goji berries include sun drying, hot air drying, microwave drying, and vacuum freeze-drying.
Odor is still one of the important means to judge the quality of medicinal materials, and the volatile components contained in medicinal materials give them a unique odor. Therefore, volatile organic compounds can be used as an important indicator to evaluate the quality of medicinal materials. Research has found that different drying methods can have varying degrees of impact on certain chemical components in goji berries, leading to significant changes in volatile organic compounds in goji berries. Qaidam Goji Berry is one of the unique Chinese medicinal herbs found in the Qinghai Tibet Plateau, mainly produced in the Qaidam Basin. Due to the unique environment of the plateau and the influence of geographical factors such as altitude and climate, Qaidam Goji Berry has significant differences in shape, size, color, and nutritional types and contents compared to other Goji berries. Previously, Qu et al. used gas chromatography-mass spectrometry (GC-MS) to detect volatile flavor compounds in fresh, sun dried, dried, and freeze-dried goji berries from Ningxia, and conducted principal component analysis on various flavor components. Nine volatile components, including alcohols, heterocycles, hydrocarbons, esters, ethers, acids, aldehydes, phenols, and ketones, were detected from four samples, which together give goji berries a unique flavor. After drying, goji berries showed a significant increase in volatile flavor compounds. Although studies have confirmed that different drying methods have an impact on the volatile components in Ningxia wolfberry, there are few reports on the effects of different drying methods on the volatile substances in Qaidam wolfberry. At the same time, due to the drawbacks of GC-MS technology, such as long operation time and complex sample pretreatment, it is urgent to establish a suitable and rapid modern detection method for better research on the volatile components of goji berries.
Gas chromatography ion mobility spectrometry (GC-IMS) is a newly emerging volatile substance analysis and detection technology in recent years. It combines gas chromatography with ion mobility spectrometry for rapid detection of volatile components in samples. Compared with GC-MS technology, this technology has the advantages of faster sensitivity, simpler operation, and more stable analysis results. At the same time, it does not require any special sample pretreatment to quickly detect trace volatile organic compounds. GC-IMS technology can currently be applied to identify volatile organic compounds in traditional Chinese medicine, providing a theoretical reference for the quality evaluation and production of medicinal materials. Shi et al. used GC-IMS technology to detect the odor of Platycodon grandiflorus leaves and different degrees of roasted Platycodon grandiflorus charcoal; Wang et al. compared the effects of different drying methods on the volatile components of Panax ginseng using GC-IMS technology; Lin et al. used GC-IMS technology to monitor changes in volatile compounds in bitter almonds under accelerated destruction conditions; Liang et al. identified volatile flavor compounds in Xinhui tangerine peel from different years using GC-IMS technology. Therefore, this study used GC-IMS technology to detect and analyze the volatile substances and main components in fresh Qaidam Goji berries, natural shade dried, hot air dried, freeze-dried, and microwave dried Goji berries. The effects of different drying methods on Goji berry quality were compared, in order to provide a reference for the quality evaluation and processing of Qaidam Goji berries in the future.

This study is the first to apply GC-IMS technology to analyze the volatile components of Qaidam wolfberry under different drying methods. By using natural shade drying, hot air drying, vacuum freeze-drying, and microwave drying methods to process fresh Qaidam goji berries, we found that the goji berries after natural shade drying, hot air drying, and freeze-drying still maintained their bright color and slightly sweet taste, while the goji berries after microwave drying were crispy, small, and slightly yellow in color. However, due to the subjective influence of individual sensory evaluation, GC-IMS technology was used to objectively analyze the odor of goji berry samples and compare the effects of different drying methods on volatile organic compounds in goji berries. Through this technology, a total of 52 volatile organic compounds were identified from goji berry samples, among which aldehydes, ketones, and alcohols were the main components of goji berry volatile organic compounds. Qu et al. used GC-MS to detect similar results, but compared with GC-MS analysis results, this study detected more compounds. This may be due to the accurate detection of low levels of volatile organic compounds by GC-IMS technology or the differences in volatile components between Qaidam goji berry and Ningxia goji berry. Although GC-MS technology is the preferred method for analyzing volatile organic compounds, its sample preparation is complex, data analysis is cumbersome, and its ability to separate substances with similar or identical components is limited. GC-IMS technology combines the advantages of both GC and IMS technologies, eliminating the need for pre-treatment of samples, shortening the analysis cycle, and accurately analyzing low levels of volatile organic compounds. Therefore, GC-IMS technology has great advantages in the analysis of volatile organic compounds.

This study found that compared with fresh goji berries, the volatile compounds in goji berries obtained after different drying methods showed significant changes, indicating that different drying methods can significantly affect the volatile organic flavor compounds in goji berries. Comparing four types of dried goji berries, it was found that the content of volatile organic flavor compounds in vacuum freeze-dried goji berries was similar to that in fresh goji berries, but significantly different from other goji berries. However, the content of volatile organic compounds in dried goji berries after drying and microwave drying was very similar. The environmental temperature during the drying process of goji berries may have a significant impact on the types and contents of volatile organic compounds in goji berry samples. As one of the traditional Chinese medicinal materials, goji berries inevitably contain many volatile components with poor stability. When the temperature increases, these components are decomposed by heat, which may also be one of the reasons for the similar content of volatile organic compounds in goji berries after drying and microwave drying. From the fingerprint spectrum, it can be seen that the content of alcohol compounds decreases after drying and microwave drying, possibly due to the lower boiling point of alcohol compounds, which evaporate with increasing temperature. In addition, some alcohol compounds are prone to esterification or oxidation to produce ketones and aldehydes under high temperature conditions, which may be one of the reasons for the increase in the content of certain aldehydes and ketones in dried goji berry samples. The content of ester compounds in goji berries significantly decreased after microwave drying. The content of aldehydes, ketones, and alcohols in the freeze-dried goji berries is higher than that in goji berries treated with other drying methods. From the results, it can be seen that this method can maintain the types and contents of volatile organic flavor compounds in goji berries at the level closest to fresh goji berries. This may be due to the fact that freeze-drying is at a lower temperature, which slows down the occurrence of chemical reactions in the fruit and preserves unstable volatile components.

This study found that microwave drying and hot air drying have the greatest impact on the volatile organic compounds in Qaidam Goji berries, while freeze-drying can preserve the volatile substances in Goji berries to the greatest extent possible. In addition, microwave drying and freeze-drying require high processing equipment, which increases the cost of Goji berry processing, while natural shade drying requires high environmental requirements. Therefore, although the volatile organic compounds in Goji berries after hot air drying are lost significantly, due to its low cost and ability to be processed in large quantities, it is still the main method of Goji berry drying at present. In summary, this study established a GC-IMS rapid analysis method for volatile components in Qaidam Goji berries under different drying methods. By comparing the effects of different drying methods on the appearance, types, and content of volatile components in Qaidam Goji berries, it was determined that microwave drying method is not suitable for the processing of dried Goji berries. Freeze drying method is the most effective way to preserve volatile organic compounds in Goji berries to the greatest extent possible, but how to reduce its processing cost has become an urgent problem to be solved in the future.

In summary, this study established a method for analyzing volatile organic compounds in Qaidam Goji berries using GC-IMS, and compared the effects of different drying methods on volatile organic compounds in Goji berries. The results showed significant changes in volatile organic compounds in dried Goji berries, indicating that freeze-drying can maximize the retention of volatile organic compounds in Goji berries and is more suitable for drying and processing Goji berries.