Plants traditionally used to make Cantonese slow-cooked soup in China

Background Lǎo huǒ liàng tāng (Cantonese slow-cooked soup, CSCS) is popular in Guangdong, China, and is consumed by Cantonese people worldwide as a delicious appetizer. Because CSCS serves as an important part of family healthcare, medicinal plants and plant-derived products are major components of CSCS. However, a collated record of the diverse plant species and an ethnobotanical investigation of CSCS is lacking. Because of globalization along with a renewed interest in botanical and food therapy, CSCS has attracted a growing attention in soup by industries, scientists, and consumers. This study represents the first attempt to document the plant species used for CSCS in Guangdong, China, and the associated ethnomedical function of plants, including their local names, part(s) used, flavors, nature, preparation before cooking, habitats, and conservation status. Methods In 2014–2017, participatory approaches, open-ended conversations, and semi-structured interviews were conducted with 63 local people and 48 soup restaurant owners (111 interviews) to better understand the biocultural context of CSCS, emphasizing ethnobotanical uses of plants in Guangdong Province, China. Product samples and voucher specimens were collected for taxonomic identification. Mention Index (QI), frequency of use index (FUI), and economic index (EI) were adopted to evaluate the significance of each plant in the food supply. Results A total of 97 plant species belonging to 46 families and 90 genera were recorded as having been used in CSCS in the study area. Recorded menus consisted of one or several plant species, with each one used for different purposes. They were classified into 11 functions, with clearing heat being the most common medicinal function. Of the 97 species, 19 grew only in the wild, 8 species were both wild and cultivated, and 70 species were cultivated. Roots and fruits were the most commonly used plant parts in the preparation of CSCS. According to the national evaluation criteria, six of these species are listed on “China’s red list” including two endangered, two critically endangered, one near-threatened, and one vulnerable species. The QI, FUI, and EI of the 97 species in the study varied between 0.09 and 1, 0.23 and 9.95, and 0.45 and 6.58, respectively. Conclusions As an important part of Cantonese culture, CSCS has been popularized as a local cuisine with a healthcare function. CSCS also reflects the plant species richness and cultural diversity of Guangdong Province. Future research on the safety and efficacy of CSCS as well as on ecological and cultural conservation efforts is needed for the sustainable growth of China’s botanical and medicinal plant industry.


Background
In China, many communities have developed their own specific local type of soup, such as Simmer Soup in Hunan and Hubei provinces, Hot and Sour Soup in Sichuan province, Mutton Soup in Shandong province, and Cantonese slow-cooked soup (CSCS) in Guangdong Province. Among these soups, CSCS has the greatest number of varieties, and in general, it is well known locally and in foreign countries. As the name implies, CSCS is made with different kinds of ingredients from time to time and is cooked in a covered pot; the pot is allowed to simmer slowly at a low boil on a very low flame for an extended time. CSCS is a relatively low-fat, highly nutritious, and easily absorbed soup, used as a type of delicious appetizer, and has long been a form of traditional food therapy used by Cantonese people.
For a long time, CSCS and cooling herbal teas have epitomized Guangzhou food and drink culture [1]. The origin of CSCS can be traced back to 3500 years ago when it was used as an early form of Chinese herbal medicine [2]. Why? The heat and humidity of Guangdong inevitably penetrate the human body, making people feel very uncomfortable. Because Guangdong features a rich level of biodiversity, Chinese medicinal herbs are available for the Cantonese people to reduce a person's internal body heat or mitigate the humidity, but pure Chinese herbal tea is very bitter. Medicinal effects without this bitterness were desired. How was this problem solved? Clever Cantonese people added the medicinal herbs, such as ginger (Zingiber officinale Rosc.), which is used as an antinauseant [3], Lophatherum gracile Brongn., used to cure mouth and tongue sores [4], Zea mays L., used to induce diuresis [5], and the seeds of Euryale ferox Salisb. ex Konig & Sims, which are used to cure kidney problems, to delicious soups [6].
Cantonese people have brought CSCS into many places where they live, such as Hong Kong, Macao, Taiwan, and other places in Southeast Asia as well as to the Chinatowns of different cities worldwide. CSCS provides a competitive advantage for immigrant Cantonese who markets this product in many places. However, many kinds of CSCS exist, so how does one select the right soup? Because many people lack an awareness of the use of traditional Chinese medicine (TCM) in support of human health, the development of CSCS has become disorganized. In addition, as food security has improved in recent years, international attention has been drawn to food therapy and food safety. As a result, the various types of CSCS need to be analyzed, so that the soup materials can be categorized according to their functions, part(s) used, preparation methods used before decoction and their nature (classified as hot, warm, cool, cold or neutral), and flavor. If these soup materials are classified and used correctly, the opportunity to develop Chinese medicine and expand food variety will emerge. In addition, it is imperative that steps are taken to preserve the heritage we have in TCM along with developing and protecting the nature of CSCS.

Study area
The coastal province of Guangdong is bounded by five southern Chinese provinces along with Hong Kong and Macao. Guangdong Province has a unique style with various dialects, customs, traditions, and historical culture. Guangdong covers an area of 179,800 km 2 and has 56 ethnic minorities with the Lingnan culture being generally representative. The total population of Han nationality is 102 million in 2013, accounted for 97.46% of Guangdong Province; the population of Zhuang, Yao, Tujia, Miao, and Dong nationality accounts for 86% of the total ethnic minorities' population in Guangdong. The tropical and subtropical climates have a rich flora that thrives on a variety of geological features.
While Guangzhou serves as the capital of Guangdong Province, Shenzhen labels itself as an "emerging migrant city" (Fig. 1). Five villages (Mi Gang, Shi Hu, Luo Tang, Long Gang, and Bao An) and 48 restaurants in Guangzhou and Shenzhen were selected as the study sites. The criteria for selecting study sites, including soup chain stores and delivery outlets, were that the sites had a rich variety of CSCS materials so that the soup-drinking culture should be well preserved.

Ethnobotanical surveys
Our research findings are based on ethnobotanical surveys carried out from 2014 to 2017 with the objective of investigating, documenting, and interpreting which herbs Cantonese consumed in soup used to cure and prevent diseases and how these people produced and consumed these plants. A total of 63 local women in the five villages and 48 soup restaurant owners, both men and women, at other locations (111 interviews) were selected using participatory approaches, openended conversations, and semi-structured interviews ( Fig. 1) [7]. The restaurant owners, all of whom had lived almost all of their lives in Guangdong, the local female residents over 40 years old, and could cook CSCS were invited to participate as informants; they all readily accepted the invitation to be interviewed. The study was carried out following the International Society of Ethnobiology Code of Ethics; all participants were informed of our intent prior to the start of the interviews. Prior to conducting interviews, we bought or took photos of plant materials used in restaurants in order to conduct a cross-validation of plant identifications in the village; in addition, informants were presented with freshly pressed or gathered plant materials, so the species identifications could be confirmed. During all interviews, the interviewees were asked to speak freely about herb materials to allow us to acquire a list of those species used in making CSCS. In addition, when the interviewees permitted it, samples were collected with the help of local guides. Interviewees were given the option to answer the following questions about each plant species: (1) Have you ever used this plant for making CSCS or not? (2) How often do you eat it? (3) Did you sell or buy this plant? (4) Where do you gather this plant? (5) What plant part do you use of this plant? (6) What is the function of this plant in CSCS? (7) How do you prepare this plant for decoction? (8) How do you obtain plants to cure and prevent diseases in your daily life? (9) Which plants have you used during your lifetime to make CSCS, etc.? Finally, group discussions with key informants were organized separately from the 63 interviews in the five villages. Notes and photos were taken to record the relevant information provided by the informants; digital voice recorders and cameras were used to record the plants and activities of informants as they gathered plants in the field. We collected three specimens of each wild plant. Plants cultivated in home gardens were not used as voucher specimens. For those gathered from the wild and then planted in kitchen gardens, we also gathered the same species of plants from the wild. Voucher specimens of all wild plants available during field investigations were deposited in the Ethnobotany Lab of Minzu University of China. Plant identification was based on the Flora of China, and a review of specimens at PE, the herbarium of the Institute of Botany, Chinese Academy of Sciences, Beijing, China.

Statistical analysis
Mention Index (QI), frequency of use index (FUI), and Economic Index (EI) were adopted to develop and evaluate a cultural importance index for each plant species [8]. We use questions 1, 2, and 3 above to calculate the QI, FUI, and EI for each species where QI = number of mentions/ number of informants. For FUI and EI, the final value of each plant is the average of all informant responses. For the details of the calculation method see Table 1. Table 2 lists the ethnobotanical information for each plant, including scientific name, Chinese name, Cantonese   name, function, part(s) used, flavor, nature, preparation before decoction, habitat, conservation need, QI, FUI, and EI based on those defined by the Chinese Pharmacopoeia (National Pharmacopoeia Committee, 2010) and our ethnobotanical surveys. We analyzed the relationship between plant nature, flavor, and function with Apriori and Excel [9]; Apriori is a frequently used item set algorithm that is used for mining association rules. Weka 3.7 software was used to run the association rules program.

Diversity of plants used in CSCS
Our ethnobotanical surveys documented 113 kinds of plants or plant parts, as defined below, used as ingredients in CSCS (Table 4), including ingredients from 97 species in 90 genera and 46 families ( Table 2). In terms of the number of species, the eight species found in each caused the Gramineae and Rosaceae to rank first, followed by seven species each in the Fabaceae and Liliaceae (Fig. 2). The ingredients used in CSCS refer not only to whole herbaceous plants but also to the leaf, bark, root, seed, fruit, stem, bulb, juice, stigma, and flower. Root and fruit were used most commonly. Among these 97 species, 28 and 26 species were collected for the harvesting of roots and fruits, respectively (Fig. 3). Local people also prefer to preserve plants by drying for later use as food materials. We checked the status of the plant species used in CSCS by following the evaluation criteria established by the International Union for Conservation of Nature (Table 2). Six species are listed on the "China red list"; Dendrobium officinale Kimura et Migo and Cistanche deserticola Ma are CR (critically endangered), Dendrobium wilsonii Rolfe is listed as EN (endangered), and Atractylodes macrocephala Koidz. is VU (vulnerable). The remaining 91 species are in the "least concern" category. For the six endangered or vulnerable wild species, reasonable cultivation protocols and ex situ conservation methods need to be established as soon as possible.
The QI of the 97 species in the present study varied between 9 and 100%. The QIs of 46 species were ranked at 0-50% (~47.4%) and were 51-99% for 42 species (4 3.3%); only nine species had a QI of 100% (~9.3%). The nine species mentioned above are familiar to almost everyone and include Lycopersicon esculentum Mill., The FUI varied between 0.23 and 9.95. Nineteen species (~19.6%) were used more than once a month (FUI > 5). Table 2 clearly shows that the most frequently mentioned species were also the most commonly used, with the exceptions of R. rugosa (FUI = 3.83) and S. sinense (FUI = 4.14). Fifty-one species were used only occasionally in some years (FUI < 2.5); they are relatively somewhat difficult to obtain either by collection or through commerce.
The EI varied between 0.45 and 6.58. Also, 89 species had an appreciable economic importance (EI > 1). Lycium chinense Mill. had the highest EI value (EI = 6.58); L. chinense is very significant because most people like to add it to CSCS to flavor the soup and nourish the body.

Function and five elements of plants
Chinese people attached great importance to the therapeutic role of food during the early stages of the development of Chinese medicine [10]. CSCS has the concomitant function of serving as both food and medicine based on past experience and the theory of TCM. In the present study, the medicinal functions of CSCS can be classified into 11 categories (Table 2 and Fig. 4). In TCM, "qi" is considered to be a natural energy and  [11]. If a qi deficiency exists in the spleen, a person will be tired and experience a loss of appetite. If a qi deficiency occurs in the lung, a person will experience shortness of breath and cough, have pale skin color, and sweat spontaneously. The Cantonese often relieve these types of imbalances by adjusting the circulation of qi using food therapy. During our field surveys, we found 24 species involved in tonifying a person's qi (Fig. 4). In addition, some kinds of CSCS have significant effects in promoting digestion, dampness, and circulation as well as in tonifying a person's yang; these soups will have an astringent, soothing effect on the nerves and brain while replenishing the blood. Chinese philosophy considers yin and yang to be the two complementing principles of life; yin has the female characteristics of earth, cold, and darkness, and yang has the male characteristics of heaven, heat, and light. Any one person has both yin and yang, and these characteristics need to be balanced to maintain good health.
Although the precepts of Chinese food therapy are neither systematic nor identical in different times and places, some basic concepts have been isolated. The Chinese Pharmacopoeia (2010) classifies herbs as having five natures and five flavors based on the five elements theory, which determines their usage. The five natures (hot, warm, cool, cold, and neutral) are summarized mainly from the body's response after Chinese herbs are consumed [12]. In addition, herbs are classified into five flavors by their efficacy, using their true taste [12] including sweet, bitter, sour, pungent, and salty [13]. The five elements theory is similar with the concept of organoleptic characteristics introduced by Nina Etkin [14], how people use taste to select food or medicine; the taste of plants can define the curing properties or specific diseases of food or medicine, for example, sour tastes are useful for curing fever and nausea. In this study, each herb was linked with the theory of the five elements ( Table 3). The natures of these 97 species range from cold (22 species), cool (16), neutral (27), warm (31), and hot (only Durio zibethinus Murr.). Because Guangdong is located in the East Asia monsoon region, it experiences a hot and humid climate. Practitioners of TCM hold that inner heat will accumulate in summer, and this causes many types of illness. However, many people prefer to select cool and cold herbs to clear heat from the body. In our study, the most common function of herbs related to their nature was clearing heat, with 31 plant species having this function. Also, a small amount of a hot herb was often used in CSCS, which is consistent with the ethnobotanical survey conducted here.
We analyzed the relationship between plant nature and function. Thirty-eight species having a cool or cold nature, among which 23 species are mainly used to clear heat, account for 60.5%. In addition, we can see that the warm herbs are mainly used to tonify qi and yang, which accounts for 61.3% of all herbs analyzed (Fig. 4). Aside from tonifying qi and clearing heat, neutral herbs are mainly used to aid digestion and as astringents. However, no definite corresponding relationship was found to exist between nature and function.
In TCM, an herb with a sour taste would be assumed to be astringent; an herb with a bitter taste would be useful to eliminate dampness; pungent substances are thought to induce sweat; sweetness is supplementing, harmonizing, and moistening; and saltiness can soften hard masses [13]. In this study, 75, 26, 14, 18, and 3 species were classified as sweet, bitter, sour, pungent, and salty, respectively. We tried to find the corresponding relationship between flavor and function. Here, we indicated that sweet, bitter, and pungent herbs can be used as astringents and not just sour herbs. In addition, the main functions of sour herbs are clearing heat, stopping cough, and helping digestion. The main functions of sweet herbs are clearing heat, tonifying qi, nourishing yin, and stopping a cough. The main functions of bitter herbs are clearing heat, tonifying qi, nourishing yin, stopping a cough, and promoting circulation. The main functions of pungent herbs are clearing heat and tonifying qi (Fig. 5). Generally, clearing heat is the main  function of all herbs. It seems that there is no obvious connection between flavor and function. In fact, substances may also have more than one flavor. For example, Angelica sinensis (Oliv.) Diels is sweet and pungent, Lycopersicon esculentum Mill. is sweet and sour, and Scrophularia ningpoensis Hemsl. is sweet, bitter, and salty. In addition, each herb has its unique nature. Chinese herbal nature is an important part of TCM theory; a single characteristic (a nature, flavor, element, function, etc.) or two such characteristics cannot reveal the internal law of a particular herb systematically. Also, the 97 species discussed here cannot fully reveal the internal law; additional species will need to be analyzed. The relationship between each of the five elements of an herb and its function needs to be studied comprehensively, with the discussion not only confined to CSCS materials. In addition, we should combine the flavor and nature of an herb to explain the complicated relationship between the five elements and function and not separate flavor from nature. Modern scientific research has been undertaken on the five natures since 1960, mainly in China and Japan, with a focus on pharmacodynamic and substantial foundational research [12]. So far, no definitive compatibility of the five elements and function has been found for Chinese medicinal herbs. The experience from ethnobotanical research related to CSCS will supply some materials for studying the relationship between the five elements and function; this will help to match ingredients with different symptoms.

How to choose a type of soup?
Cantonese people usually match ingredients based on the symptoms, medicinal effect, seasonal changes, physical quality, job, age, and gender of a patient to help people keep physically fit and to prevent and cure diseases.
(1) Soup choice depends on the symptoms: Herbs comprise most of the ingredients in CSCS. Chinese medicine emphasizes matching the remedy to the case [12], so the Cantonese will choose different soups for patients with different symptoms. For example, a mix of Ziziphus jujuba Mill. and Angelica sinensis (Oliv.) Diels or Leonurus japonicus Houtt. was used to regulate menstruation; a mix of Ligusticum chuanxiong Hort. and Dumasia hirsuta Craib was used to stop coughing and replenish qi; see Table 2     jujuba, and R. sativus to nourish the yin and tonify the blood. Middle-aged men may be given Psoralea corylifolia Linn. and E. ulmoides to tonify the yang. The metabolism of an elderly person may gradually weaken; thus, elderly people should use Z. jujuba and Dioscorea fordii Prain et Burkill to invigorate the spleen and stomach as well as to tonify the qi and the blood. CSCS can help to keep a person in good health; however, not everyone will want to eat it because of some taboos related to their diets. Herbs not only have nutrients but also numerous chemical components, some of which are known to have biological actions; however, others may have a potential to influence human physiology and pathophysiology, but this area of research remains unexplored [10]. Codonopsis pilosula (Franch.) Nannf. can tonify the qi, replenish the blood [17], and has antitumor activity [18,19]. Astragalus membranaceus (Fisch.) Bunge can tonify the qi and the yang [20,21]. Panax quinquefolius Linn. possesses certain effects on tonifying the qi and nourishing the yin [22] and is active against human breast cancer [23]. Polygonatum cyrtonema Hua can tonify the qi, nourish the yin [24], and has anti-HIV properties [25]. Eucommia ulmoides Oliver can tonify the yang [26,27] and improve the human immune system [28]. In this survey, the Cantonese told us that pregnant women should not eat much Armeniaca vulgaris Lam., Coix lacryma-jobi L., and Hordeum vulgare L. They told us that people who are deficient in cold of the spleen and stomach should not eat much V. radiata. Panax ginseng C. A. Mey. and Panax quinquefolius Linn. cannot be eaten with R. sativus and C. pinnatifida. People who suffer from superfluity syndrome or warm syndrome cannot have C. pilosula. Patients with acute illnesses cannot have A. membranaceus. The Cantonese also told us that people with frequent diarrhea should not eat P. cyrtonema, and those with kidney ailments should not eat E. ulmoides.

Conclusions
The number of groups of people with less than robust health continues to increase. In the long course of development of CSCSs, the quintessence of TCM has been adopted. Different soups have different functions. CSCS has four dimensions: social, functional, cultural, and economic. As soup materials, traditional knowledge of various plants used in CSCS was documented, including local plant name, function, part(s) used, flavor, nature, preparation before decoction, habitat, and cultural significance indices. Knowledge of these herbs used in food therapy will provide a broad socio-anthropological context related to eating. The relationships among the nature, flavor, and function of herbs seem to be related to each other but are not absolute, which will be a key point of consideration in TCM. In addition, these theories of CSCS will provide the essential basis for the analyses and clinical usage of Chinese herbs.