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Monitoring in Tea Production

By Nigel Melican

Because tea factories have, for so many years, been producing tea that always finds a market, monitoring for optimum efficiency and quality has not necessarily been built into the production process. But things are changing, and as packing companies are now buying more tea directly from the producers, entering into forward contracts, and specifying exactly the type of teas they want, it is becoming more important for manufacturers to be able to meet those specifications. International regulations controlling acceptable levels of pesticides and other residues, health and hygiene, among other things, also make measuring and monitoring an essential part of factory operations. Every tea factory manager should ask himself daily about his process conditions, If you are not measuring them, how are you going to control them?

The secret to the successful running of a tea production unit is to assess anything that could possibly go wrong, to isolate it, and then modify procedures to ensure that it will not go wrong in the future. If the risk areas are monitored and all procedures are followed correctly, then the risks are eliminated. Built in alarm systems that alert managers and workers when things threaten to malfunction will help to reduce errors.


The two aspects of monitoring needed for the growing crop are of the local climate and attacks by pests.

Tea yield and quality are very dependent on the weather, so the first stage of monitoring is the regular collection of data about climatic conditions. Every tea estate should have efficient equipment to measure temperature, humidity and rainfall, and should collect such information as part of the efficient running of the estate. Monitoring methods can range from visual reading to collection of weather records by satellite link. With modern technological methods of weather recording, and routine collection of data, it is often possible to predict such factors as the onset of insect infestation and changes in crop yield, which can be directly influenced by weather patterns. Methods such as plucking rounds, irrigation, fertilizer rates and applications of relevant pesticides can then be adapted to suit the prevailing conditions on a day-to-day basis.

The crop should be regularly monitored for attacks by pests. This is usually done through careful observation by the naked eye, and information should be shared with neighboring estates in order to avoid the rapid spread of infestations over a wider area.


As soon as the green leaf has been plucked, things can very easily start to go wrong unless temperature is carefully monitored and controlled.

First of all, tea should be processed as soon as possible after plucking, so the fresh leaf must reach the factory in as short a time as possible. Two hours is the recommended; three hours is the absolute limit. During transportation the leaf must also be kept cool or it will start to ferment (oxidize), producing tea with an unpleasant, stewy flavor. Simply throwing the sacks of tea into the back of a lorry in very hot conditions will create these kinds of problems. A better method is used by some estates: they hang the bags of leaf on lorries with racks. This allows air to circulate freely around the sacks and helps to keep the leaf cool. Some also use plastic stacking baskets to stop the leaves being crushed and becoming overheated.

It is much cheaper and more efficient to make sure that leaf is handled correctly than to introduce expensive refrigerated lorries. The leaf should be transported in sensible containers and the monitoring of the temperature of the leaf inside the sacks or baskets is essential.

If leaf temperature goes above 43C, the cell walls begin to break down, release enzymes and the catechins in the leaf begin to oxidize. What plantation workers often do not appreciate is the fact that when leaf is picked in a field where the ambient temperature is 40C and is loaded into a sack where the ambient temperature is also 40C, the temperature of the leaves nearest the outside of the sack will drop down to 28, 29, or 30, as water is evaporated from the leaf. But the water stops evaporating in the middle of the sack where the leaf is packed more tightly and no air is circulating, and here the temperature can rise to as high as 50C due to the heat of respiration.

To avoid such high temperatures, leaf should be packed loosely and air must be allowed to circulate freely. Packing can be controlled by monitoring the weight of each container and the heat inside the containers is easily monitored by inserting a thermometer probe into the sacks. Use of portable data loggers ensures that factory management has a permanent record of green leaf transport conditions.


Withering is normally a 16-20 hour process carried out under ambient conditions which can often fluctuate. It involves two processes. The first is the biochemical process during which the starch in the leaf is converted to sugar, and some of the proteins in the leaf turn to amino acids. The withering process prepares the juices in the leaf for the manufacture of tea. If leaf is taken directly from the field and processed without withering, the resulting liquor will have a harsh, grassy, and unpleasant flavor. This biochemical wither takes an absolute minimum of 12 hours and cannot be speeded up without losing quality-but it can be extended to 24 hours by cooling the leaf. As yet, there is no on-line test to monitor the optimum biochemical wither, but tea scientists are working on it.

The second part of the process is the physical wither during which water is removed from the leaf, in order to reduce the moisture content from 80% to 68%. This part of the process can be accelerated by blowing dry air through the leaf and by applying heat. To ensure best quality, there should at least be some checking of the temperature of the air the leaf is receiving. An air temperature of a steady 25-30C is ideal, but if it goes above 35C the leaf may become overheated and quality will fall off very rapidly. A temperature of 40C is definitely damaging. Alarm bells therefore need to sound-perhaps when air temperatures reach 33-35C-warning staff to turn down the heat.

Measuring of how much moisture has been lost from the withering leaf is traditionally carried out by examining and squeezing a handful of leaves. Checks are also run by weighing a bag of leaf into the withering trough and then taking the leaf bag out at intervals and weighing it again to see how much moisture has been evaporated during that time. The first of these monitoring methods depends on the skill and experience of the individual worker. In a factory of 50 troughs, the second method is laborious.

With orthodox leaf, the target moisture content after withering is between 50% and 75% moisture, depending on the type of manufacture. Assams are traditionally soft withered (65-75% moisture) and Ceylons are hard withered (50-60% moisture). In CTC leaf, a wither of between 68% and 70% is usual. Variations in the moisture content of leaf after the withering process are, however, often greater than variations in the moisture content of fresh leaf coming into the factory from the fields. This means that the withering process often widens the standard deviation of moisture content, instead of reducing it, in the withered leaf. This quite common problem can be expensive and complicated to solve. The best way to tackle it is to install Near Infra Red (NIR) moisture measurement. NIR equipment shines a light on the leaf and measures the amount of interference caused by water in the reflected light. Measurements are taken 10 times every second by a sensor head as the tea moves along on a conveyor belt. This is the only known way of accurately measuring moisture content instantaneously and on-line. Its use is standard in other related industries (tobacco, coffee, flour milling and food products).

NIR is expensive but can also reduce production costs at the later drying stage. Dryers work best when run under constant conditions. If the tea to be dried enters the dryer at a fixed rate with a fixed moisture content, the dryer will work more efficiently. As the moisture content varies, the dryer conditions also have to be varied to suit the tea, or factory managers have to accept that some tea will not be dried enough and other tea will be dried too much. The former compromise increases fuel costs and both compromises reduce tea quality. NIR moisture monitoring allows dryer feed rate to be varied automatically to ensure a constant load of water into the dryer. One liter of oil is required for every two kilograms of tea dried, and running the dryer at a constant setting optimizes evaporation, saves fuel, reduces costs and improves quality. These kinds of savings and improvements are particularly relevant to larger factories where up to four or five million kilos of tea are being dried annually.


Leaf maceration by cutting is swift and little monitoring is needed during the cutting process, but checks on increases in temperature in the Rotorvane and the CTC machines will give the factory manager an indication as to how efficiently his machinery is working. If the temperature rises too high, it is an indication that the nip is too tight. If there is no steady increase in temperature from one machine to the next, then one nip is insufficiently closed. In the ideal factory, this background information should be consistently monitored and logged. The actual temperature does not need to be checked all the time, but staff do need to be quickly aware if it is too low or too high.

The size of CTC cut or degree of maceration will usually be checked by eye, but monitoring the efficient use of machinery will help to ensure that costs are kept down and machinery is well maintained. The electric current drawn, the sounds the machine makes while operating, the vibrations, and the smells of the process, all add to the overall picture of how the factory is running and operators should be constantly aware of them.


More can go wrong during the natural oxidation (fermentation) stage of processing than in any other part of tea manufacture. This is because oxidation involves a very sensitive, exothermic reaction that produces heat and which can easily get out of hand and go too fast. Monitoring the time and temperature of fermentation is therefore crucial.

The modern method of temperature control is to blow cool moist air through the dhool (macerated or rolled withered leaf). Temperature, humidity and airflow all therefore need to be monitored.

The duration of oxidation at a given temperature determines the rate of change of green catechins to theaflavins and the amounts of theaflavins that are converted to thearubigins. Thus time determines the final briskness, strength, thickness and the flavor of the liquor. So, timing also needs to be monitored.

Given that during oxidation, cool air is blown from bottom to top, the dhool at the bottom of the layer is cooler than that at the top. Ideally, the dhool should be forked over at regular intervals to prevent the lower layer remaining under-fermented and the upper layer from over-fermenting. This can happen even more easily if the layer of dhool is too thick. It is possible to install continuous monitoring to check the temperature at different depths to see what the differential is. Too great a difference indicates either that the air is not moving fast enough through the dhool or that the air temperature is too high. The best thermometers to use for this 3-level temperature checking are stainless steel probes or electronic thermometers that are inserted into the dhool at appropriate depths on the oxidation bed.

On a continuous fermenter, which generally turns the tea throughout the process, it is important to monitor the inlet air temperature, the inlet air humidity, the temperature of the surrounding air, and the temperature of the dhool.

The length of time that the fermenter is set up for varies according to conditions and often needs resetting to suit the optimum oxidation time for the leaf on any particular day or hour. This varies according to the raw material, the time of the wither and the oxidation temperature. The only way that the factory manager can know the optimum fermentation time is to run his process line with the fermenter set at a particular time and save a sample of black tea from the dryer, then to repeat that process three more times at a range of fermentation times, until the optimum time of oxidation has been decided by liquoring and tasting the four samples. The problem with this method is that it takes four hours to run the test and, during that time, the temperature, humidity and general conditions in the oxidation room can change, often negating the purpose of the test. And yet, a factory can be losing between 5 and 7 US /kg of tea by being 10 minutes wrong with the fermenter setting.

To save time and to accurately find out the optimum setting for the fermenter, we recommend the use of the Teacraft Op-Tea-Mizer. This unique piece of QC equipment allows the factory to take samples from four timed points on the fermenter and to dry the tea under similar conditions in 20 minutes instead of the normal four hours mentioned above. The fermenter can then be set to its optimum time and ensure that the factory produces the best tea possible from any given line of dhool.


During drying, both temperature and airflow need to be monitored.

Most tea driers have some kind of system for indicating temperatures, but they are often inaccurate-sometimes by as much as 10 C- because they are old or they were wrongly calibrated in the first place. Temperature variations of only 3-4 can mean the difference between good and spoiled tea. Thermometers need to be absolutely accurate and they need to be in the right position in the drier to be of any use. If an accurate thermometer is in the wrong place in the drier, or is the wrong length, it is of no use at all.

Two aspects of airflow need to be checked-both the volume going into the drier and the air pressure inside the drier. Because in a fluid bed drier the air passes through the bedplate at high resistance, it goes through the holes into the chamber very quickly to fluidize the tea particles. As water is lost from each tea particle, its temperature increases proportionally and so, by measuring and comparing the temperature of the inlet air and the temperature of the exhaust air, it is possible to monitor the moisture content of the tea accurately within the drier.

Most modern fluid bed driers have built-in temperature recorders but, again, these are only any good if they are accurate. It is necessary to check and recalibrate thermometers to be absolutely sure that readings are accurate and to use a reliable platinum resistance thermometer as the factory standard.

In some driers, there can be a +/- 3% deviation in moisture content of the made tea. Final monitoring needs to be carried out as the dried tea exits the dryer in order to again check the moisture content. Ideally, NIR equipment should be used here for purposes of accuracy. Alternatively, tea samples need to be taken every 10-20 minutes to check the moisture content using a reliable moisture meter such as the Teacraft Thermoray. It is also advisable to check the moisture meter regularly for accuracy: ISO standards exist for this.


In the tea factory, checks on grading are traditionally done by eye. The grading machines (sifters) are set up with standard meshes which must not have any faults or large holes in, and a more exact way to check that the meshes are doing their job of separating the leaf into particles of regular sizes is to use particle size analysis. This passes a set amount of tea through the sieves in a set time to check that the grading profile is the same each day or each week. The grade profile is measured using a set of certified sieves and a laboratory sieve shaker to test that the size range of particles going into a grade is always the same. If it is not, then the settings and meshes of the factory sifters must be adjusted slightly.

The amount of emphasis put on grading depends very much on what packers and blenders are demanding from their suppliers. It is especially important for teabag manufacturers. If the density of a given tea is incorrect, it can affect the amount that goes into the tea bags and may mean that expensive adjustments have to be made to packing machinery.

To help with this problem, modern factories use tapped density volume meters to make sure that after grading, a given volume of tea has a certain weight. The old-fashioned way to check this is to use a measuring cylinder that is filled with tea, and then bang on a hard surface to see how the level of tea settles. However, there are too many variables for this method to efficiently measure density and it does not meet international standards, thus an automatic tapped density volumeter is recommended for consistent monitoring.


Once the made tea has been packed it is possible that it will absorb quality-destroying moisture during transportation and storage. Paper sacks, with their four layers of paper, foil and polythene, are a vast improvement on tea chests, but if sacks are not made to internationally acceptable standards, are not properly sealed at the valve or are ruptured in any way, the tea will not be totally protected from taints and moisture.

When sacks of tea arrive in the factory where they are to be blended, it is important to check the moisture content so that tea with a higher level of moisture content is used before tea with a lower level of moisture. Monitoring bulk moisture content can be carried out by plunging a probe moisture meter into the sacks or tea chests.


Tea companies may feel that all the tea they make will find a market and that they therefore do not have to worry very much about monitoring the various stages of the manufacturing process. But monitoring can help them to save money by making efficient use of machinery and equipment, and by improving and standardizing quality they will attain better prices in the marketplace.

The amount of monitoring needed varies from one factory to another and monitoring must be appropriate to the particular situation. Many factories would benefit from the use of simple equipment to run simple checks. But the most important aspects of manufacture that every factory should be concerned with are the monitoring of temperature, humidity, airflow, moisture content and density in the packing room.

One of the most useful pieces of equipment that can help tea manufacturers improve quality and reduce costs is the unique award-winning Teacraft ECM System for Miniature Manufacturing of Black Tea. This mini-scale tea factory can precisely simulate any process conditions and is being used by tea research institutes and forward-looking tea producers throughout the world. It can be set up to show how tea quality varies according to process conditions and helps the factory manager to set conditions that are ideal for the quality he is trying to achieve. Samples of teas made under a range of different process conditions can be sent to brokers who can immediately put a value on the tea and therefore tell the manufacturer how much he would make or save on the production of that tea in the factory situation.

Nigel Melican knows tea from nursery through field and factory to value added packing. He claims to have processed tea on every continent except Antarctica. After 27 years with Unilever Research, he set up Teacraft Ltd. in 1990 to provide advice and supply technology to the tea industry worldwide. Teacraft is based in England but the company works wherever tea grows.

Teacraft Ltd., P.O. Box 190, Kempston, Bedford, MK42 8DQ England. Tel: (44) (0) 1234 852121, Fax: (44) (0) 1234 853232, E-mail: teacraft@aol.com.

Tea & Coffee - November/December 2000


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