Elsevier

Food Chemistry

Volume 394, 15 November 2022, 133557
Food Chemistry

Chemometric origin classification of Chinese garlic using sulfur-containing compounds, assisted by stable isotopes and bioelements

https://doi.org/10.1016/j.foodchem.2022.133557Get rights and content

Highlights

  • Isotopes, sulfur-containing compounds, and bioelements in garlic are characterized.

  • Sulfur-containing compound and δ34S from different regions are investigated.

  • Good discrimination accuracy obtained by chemometrics for Chinese garlics.

Abstract

Geographical origin discrimination of agro-products is essential to guarantee food safety and fair trade. Garlic samples cultivated in six provinces or major production regions in China were characterized for stable isotopes (δ13C, δ2H, δ18O, δ15N, and δ34S), bioelemental contents (% C, % N and % S), and sulfur-containing compounds (8 organosulfur components and 2 amino acids). Results showed that many of the 18 analyzed garlic variables had significant differences among production regions. Some sulfur-containing compounds found in garlic from different provinces had a strong correlation with sulfur isotopes, suggesting garlic sulfur isotopes were also affected by geographical origin. Two supervised pattern recognition models (PLS-DA and k-NN) were developed using stable isotopes, elemental contents, and sulfur-containing compounds, and had a discrimination accuracy of 93.4 % and 87.8 %, respectively. Chemometric classification models using multi-isotopes, elements and sulfur-containing compounds provides a useful method to authenticate Chinese garlic origins.

Introduction

Garlic (Allium sativum L.) is a commonly grown crop since ancient times. Globally, it is widely consumed as a vegetable or seasoning in various forms (dried granules, powdered, minced or whole) due to its distinctive nutritional properties and appealing flavor. In addition, it is homeopathically used for disease treatment and prevention in some Asian countries. China is not only the largest global garlic producer, but also the biggest consumer and exporter. In 2019, 23.3 million tons of garlic were harvested, and 1.76 million tons were exported, accounting for around 80% of total global consumption (FAOSTAT, 2021). The main Chinese garlic cultivation regions are concentrated in north, northeast, and southwest of China. The northern regions include Shandong, Henan, Jiangsu, and Hebei provinces, where production accounts for approximately 80 % of output in China (Zhao et al., 2018).

Geographical origin verification of food claims has more important in recent times because food origin assurance provides an essential guarantee of safety and quality, especially in instances where food labelling is non-existent or untrustworthy. Development of clear and trusted methods for geographical identification also ensures trade fairness. Garlic is rich in various bioactive molecules or compounds, and to date, most origin verification methods are based on specific compounds, such as phenolic compounds (Ahmad et al., 2020), metabolomic components (Hrbek et al., 2018), and multi-element analysis (D'Archivio et al., 2019). Other analytical techniques, for instance, infrared spectroscopy have been used to trace garlic origin (Biancolillo et al., 2020). Physical properties such as moisture and pH were also considered in origin evaluations (Ahn, et al., 2019).

It is noted that garlic is richer in sulfur-containing compounds, containing organosulfur compounds (OSCs), and free S-amino acids. OSCs are major flavor and bioactive substances in garlic, which includes γ-glutamyl peptides, S-alk(en)yl-l-cysteine sulfoxides (ACSOs), and their intermediates or enzymatic products like allicin (Martins et al, 2016). Montaño et al. (2011) investigated γ-glutamyl peptides and ACSOs in garlic samples harvested on the four different locations in Spain. It concluded that garlic from different growing locations resulting in significant differences of OSCs amount. However, Hassan et al. (2015) found that allicin had similar amount in garlic cultivated in Egypt and China. Liu et al. (2020) determined 29 compounds in garlic and found alliin, methiin, γ-l-glutamyl-S-allyl-l-cysteine (GSAC), and arginine were the most abundant compounds in Chinese samples.

Although different from traditional molecular analytical techniques, stable isotopes are an essential tool for studying geographical origin of biological products, food traceability and verification research, and often applied to trace agro-products or herbs (Lyu et al., 2021). Stable isotopes not only provide geographical information about agro-products, but also enable scientists to learn about complex plant relationships with soil and human influences (Brand & Coplen, 2012). More recently, garlic origin studies from European and Asian countries have employed multi-isotope analyses. Opatic et al. (2017) characterized Slovenian organic garlic using δ13C, δ15N, δ18O, and δ34S values, and reported mean values as −26.9 ‰, 6.5 ‰, 1.8 ‰, and 3.0 ‰, respectively. It is worth noting in this study, δ18O values were measured using the CO2 gas equilibration method, while mean δ15N values were used to provide evidence of organic farming practices. An Italian study showed that mean garlic δ13C, δ15N, δ2H, δ18O, and δ34S values were around −25.7 ‰, 1.2 ‰, −48.0 ‰, 31.3 ‰, and 1.5 ‰, respectively. Significant differences were obtained for sulfur isotopes among different growing regions in this Mediterranean coastally-dominated country (Pianezze et al., 2019). In Korea, mean δ13C, δ15N, δ2H, δ18O, and δ34S values were −27.1 ‰, 3.7 ‰, −57.0 ‰, 27.6 ‰, and −1.7 ‰, respectively (Choi et al., 2020, Choi et al., 2021). In particular, Korean garlic sulfur isotopes were more negative than values previously reported for other countries. Liu et al. (2018) compared Argentinian garlic isotope values with some Asian countries and proposed that δ2H and δ18O values could discriminate origin differences between these two continents, and noticed differences between different mainland China regions, including Hebei, Sichuan, and Fujian provinces, and Taiwan. δ15N and δ18O values were different between mainland China and Taiwan garlic; mean δ13C and δ15N values of Chinese garlic were −27.5 ‰ and 1.7 ‰, respectively, δ2H values of Chinese garlic had a wide range of values from −70 ‰ to −35 ‰, and δ18O values were centered around 30.0 ‰. In comparison, Taiwanese garlic had mean δ13C and δ15N values of −27.5 ‰ and 5.0 ‰, respectively and a smaller δ2H range from −55 ‰ to −42 ‰, with δ18O values centered around 26.5 ‰. In addition, Choi et al. (2021) investigated the main Chinese garlic production regions (Henan, Jiangsu, and Shandong provinces). The overall mean C, N, O, and S contents were 40.8 %, 2.4 %, 45.7 %, and 0.8 %, respectively and mean δ13C, δ15N, δ18O, and δ34S values were −28.1 ‰, 1.8 ‰, 27.5 ‰, and 5.5 ‰, respectively. Our previous study investigated PGI Chinese Jinxiang garlic and made comparisons with two other Chinese provinces and several Asian countries (Nie et al., 2021) to improve Chinese garlic origin verification. δ34S values were found to be a useful addition to characterize garlic origin, while Liu et al. (2020) also found that sulfur-containing compounds were common components in garlic, and that they varied greatly among different regions. Moreover, sulfur isotopes and various sulfur-containing compounds of garlic have not previously been used to reconstruct a geographical verification model.

The aim of present study was to characterize garlic samples harvested in major Chinese production regions using multi-isotopes (δ13C, δ15N, δ2H, δ18O, and δ34S), bioelements (%C, %N, and %S), and sulfur-containing compounds (8 organo-sulfur compounds and 2 amino acids). The relationship between garlic sulfur content, sulfur isotopes, sulfur-containing compound abundance, and the sulfur content of sulfur-containing compounds was also explored based on geographical origin information. Finally, chemometrics were employed to construct a discrimination model capable of classifying garlic from different production regions across China to prevent geographical fraud. This study improves the geographical identification database of Chinese garlic and provides the framework for an origin verification system for Chinese garlic.

Section snippets

Garlic collection

Garlic samples were collected from the main garlic production belt in each region. A map of the sample sites and detailed information are shown in Fig. 1 and Table S1, respectively. At each site, the garlic samples were collected from different major farms, and around 500 g of a sample was obtained from each farm. Briefly, a total of 242 garlic samples were harvested from six provinces in China. 72 samples were collected from Heilongjiang (HLJ, n = 40) and Liaoning (LN, n = 32), located in

Bioelemental, stable isotope, and sulfur-containing compound analysis

In this present study, bioelements (% C, % N, and % S), stable isotopes (δ13C, δ2H, δ18O, δ15N, and δ34S), and some sulfur-containing compounds (8 organosulfur components and 2 amino acids) found in garlic were investigated. The range, median, mean, and standard deviation of each analyte are listed in Table 1, and the relationship between these variables and geographical factors (altitude, longitude, and latitude) are shown in Table S4.

Conclusion

A suite of bioelements, stable isotopes, and sulfur-containing compounds were characterized in 242 garlic samples collected from six major production regions across China. The study showed that garlic from different regions across China has different isotope and elemental spatial patterns which could be exploited to improve traceability and authentication of Chinese regional garlic. Garlic from HLJ and LN in northeastern China had lower % C, but higher % N and % S than other regions. Garlic

CRediT authorship contribution statement

Jing Nie: Conceptualization, Investigation, Resources, Visualization, Writing – original draft. Rui Weng: Investigation, Resources, Methodology, Writing – original draft. Chunlin Li: Software, Writing – original draft. Xiuhua Liu: Formal analysis, Writing – original draft. Fang Wang: Validation, Resources. Karyne M. Rogers: Visualization. Yongzhong Qian: Writing – review & editing. Yongzhi Zhang: Project administration, Writing – review & editing. Yuwei Yuan: Supervision, Funding acquisition,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This study was supported by funding from an IAEA Coordinated Research Project [Grant No. D52042] and Special Fund of Discipline Construction for Traceability of Agricultural Product (2021-ZAAS).

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