A flavour perspective of Tiepishihu (Dendrobium officinale) – an emerging food ingredient from popular traditional Chinese medicinal plants: a review

Summary Many Dendrobium orchid stems are used in Traditional Chinese Medicine (TCM). The most popular and premium species is Dendrobium officinale, and its stem in TCM is called Tiepishihu. Tiepishihu has a sweet flavour and is an ingredient in Chinese tea and desserts. There is no comprehensive understanding of its flavour compounds. It is, therefore, essential to understand compounds responsible for its flavour, and how they are formed. This review assesses twelve diverse studies in Tiepishihu flavour (2013–2022). Thirty aroma compounds were compared – furfural and nonanal were identified as common compounds. Four of seven essential amino acids were taste‐active, with lysine being the most potent. Pre‐harvest factors such as environment impact specific aroma compounds. Post‐harvest processing methods, including drying and grinding, can control Tiepishihu's flavour. Methodological consistency is a challenge, but controlling Tiepishihu's flavour could increase its commercial value as a food ingredient.


Introduction
Dendrobium is a genus of epiphytic orchid belonging to the Orchidaceae family that grows in East Asia.This genus consists of over a thousand species of orchids, with more than seventy species found exclusively in China (Cheng et al., 2019).Dendrobium have fragrant and waxy flowers with lanceolate leaves, and its stems are thick and articulated.Over half of the Dendrobium species are used in Traditional Chinese Medicine (TCM) and are referred to as 'Shihu' (Cakova et al., 2017;Chinese Pharmacopoeia, 2022).Stems from one species in particular, Dendrobium officinale, referred to as 'Tiepishihu' (Fig. 1) is well respected as one of the most expensive herbs from the Dendrobium genus (Ye et al., 2017).The cost for the highest quality of Tiepishihu in 2002 (Ding et al., 2002) was 3000 USD per kg.The use of Tiepishihu dates back to the oldest recorded Chinese herbal collection, Shennong , and has been traded and consumed for over a thousand years (Teoh, 2016).Tiepishihu is also a component of Chinese tea and modern cuisine, known for its "sweet" flavour (Zhu et al., 2018).
In TCM, Tiepishihu is used to treat minor ailments like indigestion, dehydration, and fever (Tang et al., 2017).At present, much of the research has centered around the chemical profiling of Tiepishihu and the identification of possible bioactive compounds that explain the herb's traditional medicinal effects.Bioactive compounds (polysaccharides, phenols, and alkaloids) have been identified in Tiepishihu by many studies (including Cakova et al., 2017;Huang et al., 2015), including an O-acetylglucomannan polysaccharide (dendronan) which has been identified in Tiepishihu and is believed to be the key bioactive compound.
Commercially Tiepishihu is an ingredient in nutraceutical beverages and health foods in China (Meng et al., 2019).Tiepishihu is developing a wider range of consumer applications, including use in stir-fries and sauces, teas, juices, wine, and desserts (Cakova et al., 2017).The presence of glucomannans and pectin could give Tiepishihu water absorbing, thickening, and emulsifying properties when used as a food ingredient (Devaraj et al., 2019).
Wild Dendrobium is listed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora as an 'endangered' plant but high demand in China has encouraged research centres to promote the restoration and conservation of Dendrobium (Tang et al., 2017).Dendrobium can be successfully grown in a wide range of environments and under climate-controlled greenhouses is improving.Increased demand has meant that China has been sourcing Dendrobium from nearby countries like Laos, Myanmar, Nepal and Vietnam, which is also helping to restore wild Chinese Dendrobium (Hinsley et al., 2018).Methods such as backpropagation neural networks and programmable logic controllers (Ding et al., 2018) can be used to maintain the wild conditions of Dendrobium.Greenhouse cultivation may be therefore key to ensuring a stable supply to sustain the growing global demand of Tiepishihu.
For wider consumers to accept Tiepishihu as a food ingredient, it is important to have an appealing flavour that is both consistent and stable.There have been previous studies that have shown the volatile composition of Tiepishihu, but little has been done in terms of flavour characterisation.This review will focus on Tiepishihu because it is in popular demand in China, it is seen as a more premium Dendrobium product, and it allows direct comparison between studies of one species.Therefore, a comprehensive understanding of its flavour properties is useful to evaluate how future commercial growth environments reflect the flavour properties of traditionally grown Tiepishihu.To achieve this, this review examined all the available scientific literature (2013-2022) related to flavour-active compounds found in Tiepishihu and proposed comprehensive strategies for comprehending, controlling, and optimising flavour through various means, including genetic and epigenetic modifications, environmental adjustments, traditional agricultural practices, and simple post-harvest processing techniques.

Flavour profile of Tiepishihu
This review covers two main groups of flavour compounds reported in Tiepishihu: volatile compounds that can be perceived by olfactory receptors in the nose, and non-volatile compounds that are tastants predominantly perceived by gustatory receptors on the tongue.

Volatile aroma compounds in Tiepishihu
Identification of volatile compounds in Tiepishihu is of importance to flavour analysis.To date, more than 250 volatile compounds have been reported in Tiepishihu.Three studies on its volatile analysis and aroma compounds were included and compared in Table 1.Different numbers of aroma compounds from Tiepishihu were reported: from eleven (Ma et al., 2018) to over thirty (Chen et al., 2016;Dong et al., 2020).When all studies were compared, a total of thirty aroma compounds were listed (Table 1), which could be classified as aldehydes (8), alcohols (9), acids (4), ketones (4), and others (5).Main compounds (in bold) were chosen as the top three relative percentage content for each study.For example, furfural (bready),

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Flavour perspective of Tiepishihua review A. Kirkwood et al.
eucalyptol (herbal), and hexanal (green) are the main compounds from Chen et al. (2016).Comparing all three studies, only two aroma compounds were found to be in common: furfural, nonanal (aldehydic).The remaining twenty eight compounds varied significantly between reports, apart from the different pre-harvest conditions among samples, the main variations between the studies could be due to different methods in sample preparation and extraction, analytical instrument parameters, and methods for compound identification and authentication.This review summarised three major sources of variations when comparing these studies: First, different sample preparations might have led to different aroma compounds being reported.The study by Chen et al. (2016) defined the samples as freshly picked stems, whilst this information was not provided by others.Indication of sample freshness is important as changes to aroma composition might occur throughout shelf-life and exposure to light (Sigrist et al., 2002).There was also variation in replicate numbers Chen et al. (2016) and Ma et al. (2018) used two to three replicates for Gas Chromatography-Mass Spectrometry (GC-MS) analysis, other studies did not report the number of replicates.Second, each study used a different aroma extraction technique: headspace sampling (Chen et al., 2016), headspace-solid-phase microextraction (HS-SPME) (Ma et al., 2018), and thermal desorption (Dong et al., 2020.Each technique has advantages and disadvantages.For example, HS-SPME, as a quick and solvent-free technique, can isolate volatiles and semi-volatiles, but only semi-quantitative data can be

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Flavour perspective of Tiepishihua review A. Kirkwood et al.
obtained.Whilst solvent extraction can provide more quantitative data with established solid methods, this technique can be more time-consuming than the HS-SPME technique (Hu et al., 2019).Finally, differences in data analyses could lead to challenges in comparing results.For example, some of the three studies employed the use of (i) mass spectral libraries and retention indices (Ma et al., 2018) and (ii) mass spectral libraries, retention indices, and authentic standards (Chen et al., 2016), whilst another did not comment on the identification methods used (Dong et al., 2020).Lack of comprehensive assignment could lead to reporting incorrect aroma compounds.Whilst mass spectral libraries and retention indices can tentatively assign a compound's identity, authentic standards are normally required for full confirmation.
There was also variation in the method of data presentation, for example, some authors expressed data as percentage relative content (Dong et al., 2020), and others expressed data as a percentage total peak area (Chen et al., 2016;Ma et al., 2018).To ensure maximum comparability across studies, the relative content (%) was used herein and compared in Tables 1 and 2.
Overall, previous work has provided preliminary results on Tiepishihu's aroma profile.However, due to the mentioned variations between the studies, it is difficult to fully understand the flavour profile of Tiepishihu.There are no Tiepishihu (or even Shihu for that matter) studies to date that have determined odour activity, so the impact of aroma compounds cannot be considered.For odour activity, this could include using Gas Chromatography-Olfactometry (Lester et al., 2021).

Non-volatile tastants in Tiepishihu
Based on the recently reported results in Tiepishihu, this section of the review focuses on amino acids, polyphenols, and sugars that could potentially contribute to the taste of Tiepishihu.

Amino acids and polyphenols
Amino acids (AA) are crucial for nutrition, health, but also for their diverse taste properties, for example sweetness, albeit less potent than sugars.Essential AA must be obtained from food as the body cannot synthesise them.Currently, seven out of the nine essential AA have been reported in Tiepishihu (Yuan et al., 2019).The taste properties of these seven AA vary from flat to sweet and bitter (Table 3).This review has calculated dose-over-threshold factors in accordance with the method described by Scharbert & Hofmann (2005).Dose-over-threshold (DOT), included in Table 3, evaluates the taste contribution of separate taste compounds.When the ratio of dose over threshold factor is ≥1.0, the compound is 'taste active'.The present review found that four amino acids International Journal of Food Science and Technology 2023 (threonine, methionine, isoleucine and lysine had a DOT ≥1.0 for all stem ages.Tiepishihu contained threonine at the highest abundance from the literature, but threonine has a relatively low DOT value, when compared to lysine, which has the highest DOT.It may be speculated that the "bitter, complex, salty, sweet" taste of lysine contributes to the sweet taste of Tiepishihu.Polyphenols are also of interest due to their bitter, astringent, and antioxidant properties (Cakova et al., 2017).The only possible polyphenol tastant to be found in Tiepishihu is quercetin.Quercetin is found in a wide variety of fruit and vegetable produce and is known for its bitter flavour and cardioprotective effects (Patel et al., 2018).An optimised extraction by Zhu et al. (2019) quantified quercetin (2.506-2.594lg/ g) in Tiepishihu.The taste threshold for quercetin (in 5% ethanol), presented in different units, is 4.53 lg/ mL (Dresel et al., 2015).As the DOT factor is ≤1.0, quercetin is not likely to be taste-active in the Tiepishihu extract by Zhu et al. (2019).

Sugars
A range of free sugars have been detected in Tiepishihu, and it would be valid to speculate that sugars could contribute to the 'sweet' nature of the extract.A study by Jin et al. (2016) analysed derivatised methanolic extracts of Tiepishihu using GC-MS.Among the twenty-six sugars and glycosides reported, it is worth noting that sucrose, glucose, galactose, and fructose were identified.The presence of these well-established sweet-tasting monosaccharide and disaccharide sugars is likely to contribute to the sweet nature previously used to describe Tiepishihu, but future work into quantitative analysis could indicate whether these sugars are present at taste-active concentrations.

Other possible tastants
There may be more tastants in Tiepishihu, for example, alkaloids (dendrobine, Cheng et al., 2019), coumarins (Jin et al., 2016), and aromatic acids (ferulic acid, Ye et al., 2017).At similar concentrations, which contribute taste/flavour to other plant systems.However, the lack of quantitative data and taste threshold references makes it difficult to evaluate their contribution to Tiepishihu's taste.
Overall, many non-volatile compounds can possess gustatory properties, and from the available comprehensive data, essential AA may be contributing to the taste of Tiepishihu.Only essential AA have been measured and reported, however, non-essential AA also have taste properties.Sugars, amino acids, polyphenols, sugars, alkaloids, coumarins, and aromatic acids may contribute to taste, and therefore, future work should involve quantification and taste threshold studies.

Pre-harvest
The major pre-harvesting factors that may contribute to the flavour of Tiepishihu covered in this review are intrinsic factors (plant age; stem colour) and extrinsic factors (geography, altitude).The results from different studies (Yuan et al., 2019;Hu et al., 2020) are assessed, and their impact on Tiepishihu's flavour profile is summarised.

Intrinsic factors
Plant age.Dendrobium stems lignify as they age, providing structural support and hydrophobicity for the plant to grow tall and absorb nutrients (RHS Plants, 2022;Zhao, 2016).Yuan et al. (2019) quantified seven essential AA in the stems of 2-and 3year-old D. officinale and the results showed a lower concentration (1.2%-50.0%decrease) of AA between 2-year-old and 3-year-old stems (Table 3).Threonine and valine content almost halved between two and three years.There was a significant decrease of five out of seven AA (threonine, valine, leucine, isoleucine, phenylalanine) after three years (P < 0.05), which was also the case for polysaccharide content (Yuan et al., 2019).Lignification could explain this reduction in phenylalanine as lignin biosynthesis starts with deamination of phenylalanine (Zhao, 2016).In terms of AA aroma generation, the stems of Dendrobium grown for different lengths of time may possess different aroma properties, so further study could follow from the work by Yuan et al. (2019).
Stem colour.Stem colour may have an impact on the aroma profile of Tiepishihu.Apart from the volatile analyses of red and green Tiepishihu stems by Hu et al. (2020), no other sources mention stem colours.Dendrobium candidum, on the other hand, is better known for its green and red stems-the red ones are said to be more expensive and a predictor of good quality (Jia et al., 2021).Anthocyanins, specifically, cyanin and cyanidin, have been found to give the red pigmentation in D. candidum, which is mediated through the transcription factor DcTT8 (Jia et al., 2021).The extrinsic factors that affect its expression of anthocyanins has have not yet been devised, but the drought has been shown to cause transcription factorinduced anthocyanin expression in apples (An et al., 2020).Extrinsic factors are not the only reason for red pigmented stems: in some plants, like Hedera helix, red stems are characteristic of juvenile plants (Hackett, 2002).In D. officinale, interestingly, camphor (minty) was only detected in purple stems in all four locations but not green ones (3).The mechanism International Journal of Food Science and Technology 2023 between stem colour and camphor production cannot be proposed, but it could be that camphor (a pest deterrent) is generated in response to stress.Green stems from Fujian had a unique aroma compoundhexadecanal (cardboard), and dihydroactinidolide (floral, rose) only detected in purple stem from Yunnan.There is no statistical difference in the relative content of volatiles in different coloured stems from the same provinces.It is not known whether the same anthocyanins in D. candidum leads to purple stems of Tiepishihu, and therefore, further work is required to determine: (i) the pigmented compounds in Tiepishihu, (ii) factors contributing to pigmentation (such as drought), and (iii) whether colouration causes a difference in flavour quality.

Extrinsic factors
Geography.Dendrobium is mainly distributed in the sub-tropical regions in China, including Guizhou, Yunnan, and Guangxi (Xiong et al., 2022).The impact of region on the volatile composition of Tiepishihu has been reported by Hu et al. (2020).The study profiled the composition of dried stems from four provinces in China using qualitative GC-MS and reported 101 volatiles from the six types of samples analysed.Further analysis into the supplementary of Hu et al. (2020) showed the results of eleven aroma compounds in total (Table 2).Six out of eleven aroma compounds are common between different locations, and four compounds appeared in all samples: octadecanal (oily), phytol (floral), myristic acid (waxy), and pentadecanoic acid (waxy).Among the compounds in bold with higher relative content for each sample, phytol had the highest levels (0.95%-3.23%).In comparing four locations, Tiepishihu from Yunnan had a larger number of aroma compounds detected with two unique fruity, floral aroma compounds (hexahydrofarnesyl acetone and dihydroactinidolide) and ethyl palmitate (waxy) only appeared in samples from Fujian.Different locations in China offer various environmental stresses for growing Dendrobium due to altitude, temperature, soil pH and composition, UV exposure, and climate.Volatile composition changes by geography offer consumers more choices and preferences.
Altitude.Altitude affects herb growth and extract quality via temperature, humidity, wind, water, and sunlight differences.Dendrobium is typically grown at an altitude of 1600 m (Guo et al., 2020) above sea level in parts of China (tropics and subtropics).In a study (Do Carmo et al., 2020), a significant (P < 0.05) positive relationship was found between the altitudes for growing Arabica coffee (Coffea arabica) and resulting aroma with higher sensory scores for aroma, flavour, and acidity in coffee growth at a higher altitude (1050 vs. 850 m).It has been shown previously that environmental conditions at higher altitudes can modulate the transpiration and photosynthesis rates of coffee plants, which leads to a more delicate coffee aroma (Hameed et al., 2020).Therefore, the aroma qualities in different plant extracts can be affected either positively or negatively by growth altitude.These differences are plant-specific, and there is a gap in the knowledge of the effect of altitude on the quantitative aroma composition and quality of Tiepishihu products.

Post-harvest process
Traditional post-harvest processes involved in Tiepishihu production can be split into five stages: cutting, fresh storage, drying, twisting, and grinding (Fig. 2).Dendrobium stems are harvested and separated, then stored short or long-term.Short-term storage is in bamboo baskets or cool sand.For long-term International Journal of Food Science and Technology 2023 storage, stems are rinsed and freeze-, sun-, or ovendried.Dried Tiepishihu comes in various forms, as shown in Fig. 2 (Teoh, 2016).For long-term storage, stems are rinsed with water and then freeze-, sun-, or oven-dried, for preservation.The dried and twisted form for Tiepishihu is named Tiepifengdou (Cao et al., 2020).Tiepishihu flavour may be controlled by these post-harvesting factors, such as different drying and grinding conditions.

Drying
No studies have evaluated how each post-harvest step can affect the flavour profiles of Tiepishihu.Drying can play a particularly important role, because it reduces water content, limits microbial growth, and inhibits enzymatic reactions, which in turn leads to longer shelf life (Orphanides et al., 2015).Drying methods can influence the aroma compositions of herbal extracts, such as basil and coriander (Ghasemi Ghasemi Pirbalouti et al., 2013;Ghasemi Pirbalouti et al., 2017), however, the optimal drying parameters are not clear with Tiepishihu.Some studies (Chen et al., 2016;Dong et al., 2020) indicated that Tiepishihu was freeze-dried and oven-dried at 40 °C, respectively.Freeze-drying seems to reduce oxygenated terpenes and sesquiterpenes in spearmint (Mentha spicata) leaves, due to the expansion of surface layer cells.Qualitative data of Tiepishihu makes it difficult to know these have been lost (Consuelo D ıaz-Maroto et al., 2003).The presence of furfural in freeze-dried Tiepishihu stem (Chen et al., 2016) is interesting since furfural formation is typically associated with thermal processingthrough dehydration of pentose sugars (Spinnler, 2011).A higher drying temperature of 65 °C (for 2 days) in the study by Ma et al. (2018) may explain the fewer compounds identified compared to others, potentially through volatilisation.
One study described a 'standard method' for Tiepishihu drying at 50 °C for 24 h (Cao et al., 2020), one for as long as 15 days (at 50 °C) (Hu et al., 2020) and another for 7 days (at 30 °C) (Pan et al., 2015), but these studies did not apply volatile analyses, and so the impact on aroma properties is unknown.The present review, therefore, recommend standardising drying protocols.

Grinding
Dried Tiepishihu is also processed into a ground form, which is often sold on the Chinese market as a tea ingredient.Superfine grinding of Tiepishihu increases the solubilities of its constituents (polysaccharides, proteins, and antioxidants) (Meng et al., 2019).Importantly, superfine Tiepishihu powder (optimal particle size of 30 lm) requires less time to dissolve compared to higher particle sizes (Meng et al., 2019).This can have a positive impact on the consumer, as shortening preparation time is convenient.No study has shown the impact of Tiepishihu particle size on flavour extraction, however, with horseradish (Armoracia rusticana) powders, ball milling for longer times (15-60 min) increases isothiocyanate content (Wang et al., 2020).From a mechanistic perspective, grinding for longer results in lower particle size and increased surface area.This can lead to reactions (i.e., enzymatic hydrolysis), which can alter the composition and proportion of volatile compounds.They found that refrigerated ball-milling (6 °C) could preserve volatile aroma compounds (Wang et al., 2020).

Conclusion
Tiepishihu, the stems of Dendrobium officinale, is an important growing premium Traditional Chinese Medicine with the potential to be a food ingredient of international commercial importance.This review summarises recent literature on the flavour composition of Tiepishihu.
From a flavour perspective, recent studies on volatile and non-volatile compounds present in Tiepishihu were compared.Thirty volatiles were identified from previous studies such as furfural and nonanal.This review article International Journal of Food Science and Technology 2023 identified that AA are the only taste-active components in Tiepishihu (for example, lysine).Sugars like glucose might be contributing to the sweet taste of Tiepishihu, but in these studies, many lacked quantification so its true taste impact might be overlooked.
Factors that impact flavour composition include pre-harvesting factors (plant age, geography, and altitude regions for Dendrobium growth) and postharvesting factors (i.e., drying and grinding conditions).Plant age affects the amino acid content in Dendrobium stems, with older stems showing lower concentrations of amino acids.Stem colour may also impact the aroma profile, with certain compounds detected only in specific-coloured stems.Different drying methods and conditions can alter the aroma compositions, but the optimal parameters for Tiepishihu are still unclear.Reliable and consistent methods were recommended for future investigation of flavour composition so that the impact of cultivation and processing methods on the flavour of Tiepishihu products can be better understood.
Due to gaps identified in the current studies, this review proposed improved methodologies to quantify and identify the key odour-active compounds (through GC-Olfactometry) in future studies.More studies on advanced and novel food processing techniques will help create innovative Tiepishihu products of high quality with desirable flavours, enabling their wider food applications in the global market.Yuan, Y., Yu, M., Zhang, B., Liu, X. & Zhang, J. (2019).Comparative nutritional characteristics of the three major Chinese Dendrobium species with different growth years.PLoS One, 14, e0222666.Yuan et al. ( 2019) is useful to the present review since it provided quantitative data on the concentrations of essential amino acids in 2-and 3-year old Dendrobium officinale stems.The present review article was able to compare this to other literature and correlate the levels with taste thresholds, laying the foundation for further research on the contribution of amino acid levels on taste properties of Dendrobium stems grown for different lengths of time.Zhao, Q. (2016). Lignification: flexibility, biosynthesis andregulation. Trends in Plant Science, 21, 713-721. Zhu, H., Li, P., Dai, Z. et al. (2018).Functions of plant growth substances in the growth of Dendrobium officinale Kimura et Migo. Medicinal Plant, 9, 48-51. Zhu, Y., Yu, J., Jiao, C. et al. (2019)

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Flavour perspective of Tiepishihua review A. Kirkwood et al.

Figure 1
Figure 1 Author's photographs of Dendrobium officinale in plant and dried Tiepishihu form.

Ó
2023 The Authors.International Journal of Food Science & Technology published by John Wiley & Sons Ltd on behalf of Institute of Food, Science and Technology (IFSTTF).

Figure 2
Figure 2 Author's representation and classification of known postharvesting Shihu techniques based on Teoh (2016) and Cao et al. (2020).Products are underlined.

Table 1
Summary of aroma compounds isolated from Dendrobium officinale stems from three published studies(1, 2, 3) Relative concentration was reported from published data: 1-Chen et al. (2016), 2-Ma et al. (2018) and 3-Dong et al. (2020).Characters and number in bold indicated main compounds (top three peak areas).Table excluded compounds that were a) not volatile at atmospheric pressure, i.e. above 300 molecular weight MW, and b) had no recognised aroma properties in literature.# Odour references from The Good Scents Company (2022).Ó 2023 The Authors.International Journal of Food Science & Technology published by John Wiley & Sons Ltd on behalf of Institute of Food, Science and Technology (IFSTTF).

Table 2
Aroma compounds isolated from purple (P) and green (G)-coloured D. officinale stems from four provinces in China: Zhejiang (Z) at East, Fujian (F) at Southeast, Yunnan (Y) at Southwest and Jiangxi (J) at Southeast Characters and number in bold indicated main compounds (top two peak areas).Compounds were identified using NIST Mass Spectral Libraries and identified using Retention Indices in literature.Table excluded compounds that were a) not volatile at atmospheric pressure, i.e. above 300 molecular weight MW, and b) had no recognised aroma properties in literature.
(Hu et al., 2020) from The Good Scents Company (2022).#Relative cntent (% of peak area) was reported in published data(Hu et al., 2020).Ó 2023 The Authors.International Journal of Food Science & Technology published by John Wiley & Sons Ltd on behalf of Institute of Food, Science and Technology (IFSTTF).

Table 3
Summary of seven essential amino acids identified in stems of Dendrobium officinale (2-3 years old) with their taste descriptions, thresholds and dose-over-threshold (DOT) values for species Ó 2023 The Authors.International Journal of Food Science & Technology published by John Wiley & Sons Ltd on behalf of Institute of Food, Science and Technology (IFSTTF).
Ó 2023 The Authors.International Journal of Food Science & Technology published by John Wiley & Sons Ltd on behalf of Institute of Food, Science and Technology (IFSTTF).