Mold and mycotoxin contamination of tea: contamination of processed tea with fungal mold and mycotoxins is not something that immediately springs to mind in the same way as for coffee. But risk is there and potentially wider due to the spectrum of plant ingredients used in herbal and medicinal teas, as well as Camellia sinensis for black and green tea.
Mycotoxin contamination of coffee by Ochratoxin A is a huge potential problem, though relatively straightforward. Commercial coffee uses beans from Coffea Arabica or Coffea robusta, two species with a correspondingly restricted range of fungal pathogens and contaminants. Aspergillus ochraceus and Penicillium verrucosum producing Ochratoxin "A" as their signature mycotoxin are the most important, affecting coffee cherry pre-harvest or later during storage and processing of green coffee.
A potentially complex contamination picture for tea could not be more different. The bulk of commercial tea (black and green) does come from one plant species (Camellia sinensis) but with significant "secondary" teas like South African Rooibos tea (red bush tea). This is harvested from a completely different plant species called Aspalathus linearis of the pea and bean family. But from here there follows an endless list of plants contributing roots, tree-bark, leaves, flowers, fruit or seeds to a correspondingly broad spectrum of herbal and medicinal teas, with a combined and correspondingly endless list of fungal contaminants. And they will persist because the ingredients are sun-dried and not artificially dried at high temperature like black tea.
Herbal teas may contain just one or two ingredients like chamomile or lemon and ginger, or be multi-ingredient functional tea preparations.
Field fungal pathogens infect plants while they are growing in the field (e.g. Exobasium vexans [blister blight disease] on Camellia sinensis) while others called post harvest pathogens manifest as on molds picked plant produce in store and transit especially under inappropriate conditions of temperature and humidity. Generally speaking warmer temperatures and higher humidity is most conducive to mold development during this post harvest period.
Complicating the picture are field fungal pathogens present on fresh green tea leaves or other harvested plant parts and brought in with the harvest. They may subsequently grow on processed tea ingredients and in some cases synthesise mycotoxins. Even without harvest-sourced contamination it is impossible for processed tea to avoid contact with spores of mold fungi. Fungi such as Penicillium and Aspergillus (blue-green mold) Rhizopus and Mucor (bread mold) are everywhere, growing on all available organic matter such as fruit, bread, cheese and leather, or in air water and covering all surfaces as spores.
Fungal pathogens attacking living plants are highly specific. They rapidly infect some plant species but do absolutely nothing to others. This means processors, shippers and wholesalers of herbal and medicinal tea products potentially face a vast range of different fungi, working on the basis that every plant species suffers attack by at least one specific fungus. Research carried out into potential contamination of herbal and medicinal teas, discussed in detail later, bears this out.
There are thousands of different fungi but only a small minority are mycotoxin synthesisers. That said, the few that are cause havoc with food safety because mycotoxins are some of the most potent poisons known. Many are carcinogenic and some genotoxic. Impaired liver, kidney and brain function are well known consequences of ingesting even minute quantities of mycotoxin. They are virtually indestructible meaning once in processed tea products they cannot be removed or destroyed so prevention is the only real way forward.
Tea (Camellia sinensis) used for medicinal, health or just pure pleasure reasons has a vintage going back thousands of years, but so do mycotoxins. Evidence from Middle Ages, Ancient Greece and even Ancient Egypt suggest people were poisoned by the mycotoxin made by the ergot fungus (Claviceps purpurea) of rye grains. More recently the so-called "Salem Witches" of 17th Century New England (U.S.) were thought to be ordinary women suffering hallucinogenic fits from eating rye bread contaminated with the same ergot mycotoxin.
So what are potential problems for tea and the tea industry from contamination with mold and mycotoxin? Mycotoxins and human health apart, even surface spoiling molds cause big problems. First is the obvious visual problem of mold growing on tea and are there because ingredients were insufficiently dried during processing or maintained in a properly dry state during storage transit and packing. This causes particles of dried tea leaves or other plant ingredients to stick together as well as displaying a surface covering of mold. But there are other potential problems only evident to the consumer.
Tea contaminated with mold smells musty and may not release its inherent aroma when the package is opened. Furthermore infusion of fungal-contaminated tea will leach out chemicals from the fungus that may mask or destroy the subtle favors (taste and aroma) for which teas are invariably drunk. And if there are mycotoxins present, even in minute amounts, on or inside the dry tea product then they can be leached out during infusion. Mycotoxins are harmful in just trace amounts (parts per million or even parts per billion) with obvious implications for human health.
Virtually all contemporary teas, and not only herbal and medicinal teas, are drunk for health giving properties (real or imagined). Manufacturers of standard breakfast teas, traditionally taken as a morning pick-me up or afternoon treat, are not slow to impress on consumers the high concentration of antioxidants, natural chemicals mopping up "free radicals" and helping fight human disease including cancer.
No food product should be contaminated with fungal mold and/or mycotoxin, but tea promoted for health giving properties should be one of the very last products to suffer such loss of integrity.
Fungal Molds and Mycotoxins in Tea
Compared with coffee there is little known or appreciated about potential for fungal mold and mycotoxin contamination in tea, whether Camellia sinensis for black tea and green tea or the hundreds of ingredients used in the wider range of herbal and medicinal teas. Results from the relatively few studies undertaken so far indicate the industry should look at this potential problem in wider context and greater depth.
One of the most recent, exhaustive studies on mold and mycotoxin contamination was carried out in the Czech Republic by Rezacova and Kubatova in 2005, who confirm that data on fungal contamination of teas is sporadic with only few scientists worldwide investigating the problem. They tested 40 samples of green, black and herbal teas from shops in Prague and found 81 different species of fungus. No mycotoxin production was identified but presence of Aspergillus species with known capacity for ochratoxin A and aflatoxin production led the authors to issue warnings, which sparked more interest and investigation is required.
Sultanate of Oman is one country concerned about maintaining purity and inherent favor of its teas. Researchers there investigated presence of fungal molds in black tea. Around 48 samples of four popular commercial brands of black tea (Camellia sinensis) were purchased from the local markets in the Muscat area and the tea surveyed for mycoflora (fungi).
Five fungal species were isolated, with Aspergillus niger predominant in all brands tested. Percentage contamination with Aspergillus niger ranged between 0.66% and 30.34%. Other fungi isolated were Aspergillus flavus, Penicillium spp and Pacelomyces spp, but average concentrations were much lower at 0.6%, 0.84% and 0.21%, respectively.
Statistically significant differences for Aspergillus contamination were found between batches, meaning presence fungus is a real problem and not down to a "one-off" accident. Aspergillus flavus is well known in food safety circles because some strains produce the acutely poisonous aflatoxin mycotoxin, but none of the 25 strains of Aspergillus flavus isolated from these black teas were found to be mycotoxin synthesisers.
Fungal contamination had apparently not degraded the tea because full analysis showed total ash, water-soluble ash and mineral concentration of samples to be within the British standard. But researchers claimed the results showed that black tea is contaminated by fungi, which could conceivably constitute health hazards for humans. They pointed out how post-harvest contamination of tea can be reduced or even eliminated if tea processing is conducted under the most hygienic of conditions.
An equivalent study nearby in Egypt, this time with regular black tea and decaffeinated black tea, found virtually all commonly used tea brands in the country were contaminated with fungi. Aspergillus and Penicillium were again among the most prevalent. Fungal counts were highest for teas with highest moisture content at purchase and rose if moisture content was raised and storage period lengthened. After 20 days storage at 45% moisture content, Aspergillus flavus and Aspergillus tamarii were easily the most predominant molds. Aflatoxin production by Aspergillus flavus (strain IMI 89717) was recorded on regular and decaffeinated black tea samples after 20 days storage at 45% moisture content, with highest levels recorded in decaffeinated black tea.
Analytical studies conducted by M. Halt at the Faculty of Food Technology at Osijek in Croatia looked specifically at the plant ingredients used for medicinal teas and also samples of herbal teas. They analyzed large numbers of medicinal plant materials and herbal tea samples to reveal a number of different fungi, the most common of which were Aspergillus, Penicillium, Mucor, Rhizopus, Alternaria, Cladosporium and Trichoderma.
Two of the fungi isolated (Alternaria and Cladosporium) are generally regarded as field pathogens, Trichoderma is a so-called "friendly fimgus" used in horticulture to biologically control plant pests while the remaining four are generally found as post harvest mold fungi. The Aspergillus flangus was Aspergillus flavus, well established as a synthesiser of aflatoxin responsible for regular episodes of serious poisoning in humans and farm animals, mostly due to contaminated cereals and peanuts. This fungus was actually present in 18% of the medicinal plant samples and 9% of the herbal tea samples.
Fungal contaminated medicinal plant samples were analyzed for a range of mycotoxins including ochratoxin A and zear alenone as well as aflatoxin. Ochratoxin A was found in 15% of the samples analyzed. The study concluded that medicinal plant material and probably herbal teas, especially if stored improperly, were susceptible to fungal mold growth and should therefore be routinely tested for presence of fungi and mycotoxins before entering the market. The 70 plus different plant ingredients used in medicinal or herbal teas tested included Menthae (mint), Rubus (raspberry/blackberry), Sambacus (elderflower) and Achillea (yarrow).
Plant parts other than leaves used to prepare herbal teas are at risk. Scientists constantly recommend development and use of analytical tests for roots including Valerian, Echinacea, ginger, liquorice, burdock, dandelion commonly used in such teas, because surveys show they are very much at risk from fungal growth and mycotoxin contamination.
Researchers in Portugal concentrated on a completely different group of mycotoxins called the Fumonisins produced by the fungus Fusarium moniliforme, but dangerous none the less because Fumonisns are implicated in human esophageal cancer. They looked at a wide range of tea products covering black tea and plants renowned for their medicinal properties and used in commercial herbal teas. Approximately 87 samples including 18 black tea samples and 69 samples of four different medicinal plants (chamomile, leaves of the orange tree, leaves and flowers of the linden tree and corn silk) used for herbal tea products were acquired from supermarkets in Lisbon.
Samples were analyzed for presence and level of fumonisin B1 (FB1) and fumonisin B2 (FB2) using a standard laboratory method called high-performance liquid chromatography. This gave a detection limit of 20[micro]g/kg for each of FB1 and FB2. FB1 was found in 65.5% of the samples with the highest number of positive samples in black tea (88.8%) and with levels ranging from 80 to 280[micro]g/kg.
Leaves of the orange tree had highest concentrations of FB1 (range, 350 to 700 [micro]g/kg) followed by leaves and flowers of the linden tree (20 to 200 [micro]g/kg). Corn silk and chamomile samples had the lowest contamination of FB1 with concentrations ranging from 50 to 150[micro]g/kg and 20 to 70[micro]g/kg, respectively. FB2 was not found in any samples tested whether black tea or ingredients in herbal tea products.
Potential Problem and No Easy Solution
Mold and mycotoxin contamination of tea ingredients (Camellia sinensis and herbal/medicinal teas), and by implication their infusions, is recognized by scientists but does not appear high on the priority list for regulatory authorities in the same way as for coffee, cereals, nuts and a host of other food products. Limits do exist in some countries but there is no hard and fast international standardization.
Problems are not perceived to be widespread or acutely serious, and sheer spectrum of ingredients from diverse origins within herbal tea preparations presents real logistic and economic barriers for regulators. The generally held view is that mycotoxin contamination of black tea is of low potential. This thesis is supported by the anti-fungal effect of natural chemicals contained in tea leaves and the very nature of black tea processing.
Leaves of Camellia sinensis are rich in polyphenols, the precursors for "tea chemicals" that impart color, body, taste and aroma to the infusion. Polyphenols occur widely in other plants, generally at lower concentrations than in tea, with anti-fungal properties that should deter growth of contaminating fungi and therefore production of mycotoxin. In addition, nature of black tea processing with extended high temperature drying is not conducive to survival of fungi. That said, if mycotoxin is already present on the leaves at harvest any amount of drying and firing will make minimum impact. Research shows Ochratoxin A survives the roasting stage of coffee manufacture. Nigel Melican from Teacraft (Bedford, England) points to a rapid high build up of microbes on picked tea leaves during an 18 hour withering stage, and how this transitory microbial growth is important in development of tea quality. However, once black tea has been dried at temperatures around 100[degrees]C all microbes good or bad will disappear. The secret says Nigel Melican is to maintain tea in this "zero microbe" state into storage, packing and transit. Bill Waddington of TeaSource (Minnesota, U.S.) concurs and describes how TeaSource tea comes in gold foil bags, which are 99.9% air-tight as long as the zip seal is zipped tightly shut.
Fungi are everywhere as spores, but need the right conditions for germination and growth. In the same way that tea, by virtue of the infusion process, relies on water at a suitable temperature (in this case boiling) so fungi require moist humid conditions and warm temperatures. This means tea must be stored, transported, packed and marketed in a suitably dry condition crucial to maintain a mold and mycotoxin free condition.
The research carried out in Egypt by Hasan and Abdel-Sater at Assiut University showed quite clearly how fungal contamination and mycotoxin production increased when tea was stored with increasing moisture content (15, 25, 35 and 45%), and further increased with longer periods of storage in these high moisture conditions. All commercial teas (20) tested in Egypt, of Indian, Indonesian, Kenyan and Sri Lankan origins, possessed moisture contents within a fairly narrow range of 5.2 to 6.8% when purchased from suppliers for testing.
But even these levels appear on the margins of acceptability, because FAO in 1995 established a standard for black tea, which included a specific paragraph on moisture content. The standard said black tea should be packed with a moisture content of less than 4% and warned of moisture absorption during transit to the port of export and shipment to the importing country.
Ten years ago a committee advising the European Commission on food safety said tea seemed to have a safe history of use but due to lack of data concerning associated fungi in tea, it had not been possible to identify health hazards arising from elevated moisture content and consequent mycotoxin contamination. In essence, it was saying that it did not know. At that time it suggested moisture levels in tea up to 10% seemed to provide an acceptable safety margin, but warned that a lower level of less than 6% may be needed to prevent deterioration (physical and chemical) in intrinsic tea quality.
It is worth remembering that black tea is generally exported from hotter humid climates to cooler drier climates. Tea is highly hygroscopic, which means it will absorb water from the air until equilibrium is reached. Shipping ports in tea-producing countries like Sri Lanka, India, Kenya and Indonesia experience hot humid conditions with high temperatures that give the atmosphere a high water holding capacity. Under such conditions dried black tea if not properly protected will absorb moisture quickly and to a high level before it comes into equilibrium with the ambient air.
Dried black tea is highly hygroscopic, says Nigel Melican, but moisture content will generally only rise to 7 to 8% under the influence of ambient air alone and experience shows 14% w/w (weight/weight) moisture content is needed before mold growth really gets going. Poor storage and transport involving presence of condensation is generally required to reach this level, says Melican. Condensation can be a problem for all commodities shipped from the hot humid tropics to colder temperate climates.
Last but not least is the logical question about molds and mycotoxin in aged tea for which molds and other microbes are actually encouraged during the post fermentation phase that imparts an intrinsic flavor to the tea. I received an inquiry from a medical man concerned about known presence of Aspergillus molds in such tea and possible presence of aflatoxins which among other things cause liver cancer. The reader pointed to alleged high levels of liver cancer in China and wondered whether there was any link.
Aspergillus fungi exist in aged teas but not all species of Aspergillus produce aflatoxins. Even within those species (like Aspergillus flavus) that do, only certain strains are mycotoxin synthesizers. Many reports suggest drinking infusions of aged tea actually protects against a range of illnesses including cancer due to the high and active concentration of chemicals derived from the polyphenols in fresh tea leaf.
Pesticide residue is an additional "polluter" of tea purity, but here lies a supreme irony in relation to mold and mycotoxin. Organic teas including black and green Camellia sinensis and herbal teas are now all the rage. Organic tea is by definition produced without using pesticides, which means it is logically more likely to be contaminated with fungal mold and mycotoxins.
Dr. Terry Mabbett has been covering the tea, coffee and cocoa industries for decades. He resides in England. He has worked in crop production and processing throughout the tropics- India, South East Asia, West Africa and the Caribbean--and in his home country of the U.K. Dr. Mabbett has been writing professionally for over 20 years.