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The processing requirements of both tea and coffee have spawned the evolution of several kinds of mechanical equipment, but for the last 50 years the evolution has been slow. Mechanical sieves, destoners, catadors, stalk removers have changed little from their original designs although over the same period, consumer expectations of quality and safety have risen to new heights. However, the introduction of optical sorters to these industries since the late 1980s has changed working practices and greatly helped processors in meeting consumer demands.

Second to harvesting, the most labour-intensive process in tea and coffee production is that of hand-picking, to remove damaged product and the foreign materials (stones, string, sticks) still remaining after the mechanical cleaning processes. Traditional hand-picking is an expert but low-paid occupation requiring experience, sharp eyesight and continuous concentration. It is also a slow process. For example, achieving a bagging capacity of one tonne per hour of good quality arabica coffee will require around 100 hand-pickers.

Electro-optical sorters were first introduced for cleaning cereal and rice grains where picking out the defects by hand was not feasible on an industrial scale. The principle was quite simple. The grains would flow in front of bright lights. A sensor which was tuned to the brightness of the grains would then produce a electrical signal if the brightness suddenly decreased as a black grain passed in front of it. This signal would in turn trigger a valve that fired a small blast of compressed air at the grains, hopefully knocking out that particular black one. As the science of electro-optics and image analysis developed through the 80’s and 90’s, so did the effectiveness of the sorter, and it started to appear in coffee mills. Early sorters, like manual labour, had low capacity and were able to identify and remove only completely black coffee beans, and in order to meet the production capacities of robusta mills, whole rooms full of sorters were required.

In the tea industry too, sorters were adapted to manage the different types and sizes of leaf, and became to some degree effective in removing major defects, for example stalks, and some foreign material. Again, the major limiting factors were capacity, with many sorters needed to achieve a capacity of even 500kg per hour, and precision, with much good tea being ejected along with the defects.

The diverse nature of tea presents many problems to the optical sorter designer. In most cases, after fermenting and drying, the main sorting of the leaf is achieved mechanically, through a process of sieving, mechanical gravity separation (separating materials of different densities) and electrostatic attraction, to separate the dry stalks from the leaf. Optical sorting relies on a steady, even flow of tea passing the sensors, which is sometimes difficult to achieve. Some teas have a particulate nature (every leaf separate) and can be handled easily, others have leaves that tangle together in clumps, making accurate rejection of the defective leaf impossible and in Japan, good green tea can vary in size from 1cm leaves down to dust. Not surprisingly then, the use of optical sorters in the tea industry varies considerably from country to country, Japan being the biggest user, followed by Sri Lanka and India.

Coffee is much easier to handle, but presents its own specific difficulties. Defective beans in robusta coffee tend to be much darker than the good beans, making electro-optical sorting quite feasible. Defective beans in arabica are, however, much more subtle and take experience to spot. Grassy green immature beans and reddish pulp-stained beans do not differ very much in colour from a good bean; the trained human eye can spot them more consistently than its electronic equivalent. Therefore optical sorters are far more widespread in robusta areas than Arabica areas, where the use is still limited.

Staying with coffee, why spend a large amount of money on expensive optical sorters (one of them can cost as much as the entire remainder of the coffee mill) when manual labour is cheap?

First, economics. Robusta processors work with very low profit margins, and so need high volumes to be profitable. While the driers, hullers and mechanical cleaning equipment can manage these volumes, usually 4-8 tonnes per hour, an army of hand-pickers struggles to manage 2-3 tonnes per hour, so the efficiency of the operation is compromised. Installing optical sorters before the hand-pickers, and using a reduced number of personnel only for final inspection can increase efficiency by 3 or 4 times and has become so essential in meeting tight contract deadlines that practically all robusta plants now depend on them.

Fortunately, the rapid technical advances during the 90’s have increased both the efficiency of sorters and the capacity. Secondly, the introduction of digital (CCD) cameras to replace the simple photodiode sensor made a step change in sorting, increasing capacity from less than 500kg per hour to as much as 12 tonnes per hour on one sorter, and at the same time improving the efficiency markedly. Whereas a pre-1995 sorter would reject several good beans with each bad one, processors should now expect as little as 10% good beans in the rejected ones.

The capacity increase brought with it savings in installation costs (no need for the whole room now!), and the price (per kg capacity) of the sorter has tumbled, so its use is now economically necessary.

The effect on the local economy of introducing such process changes should not go without comment. The justifications for using an optical sorter include quality, capacity and efficiency improvements, but also reduction in labour cost.

The sorter should operate 24 hours per day giving an absolutely consistent quality output, whereas humans, although having better eyes, become tired, distracted and inconsistent. It is inevitable that fewer personnel will be required to sort the coffee and other tasks or employment will have to be found.

On the other hand, people are still more valuable than electronics in the Arabica process. The modern optical sorter should now be capable of discriminating most of the subtle colour differences of Arabica beans and the small damage and worm-holes caused by insects, but in Asia, unlike South America, Arabica processing is usually small-scale, concentrating on the very high quality specialist market rather than high volume. Arabica coffee also reacts badly to mechanical handling, which appears to accelerate the natural bleaching process, so machines are avoided as much as possible. Low volume and very high precision favours the human eye, so while the use of optical sorters to sort Arabica is widespread in South America, it is rare in Asia.

The electro-optical sorting of coffee is very efficient. However, electronically sorting tea has provided greater and more diverse challenges.

Tea leaves come in different colours, sizes and shapes and may be mixed with other herbs or flowers. These physical characteristics do not lend themselves easily to automated processes, as consistency is difficult to achieve although sorter manufacturers have gone to considerable lengths to make the process effective. Hence the use of optical sorters is regionalised according to the nature of the tea, and is prevalent mainly in Japan and Sri Lanka, with increasing interest in Vietnam and China.

As with coffee, tea processors can be low or high volume, some concentrating on specialist leaf production, others high volume into teabags. The specialist leaf tea producers will be using manual and mechanical techniques to clean and grade the tea, concentrating on gentle handling and high yield of large leaves. The optical sorter is used to help remove stalks and damaged leaves – black and brown leaves from green tea and white and yellow leaves from black tea. It is also used to remove foreign material that has managed to remain in the tea throughout the mechanical cleaning processes. Typical of such unwanted material is insects, aluminium foil, string, plastic pieces and human hair.

In Japan, while the removal of foreign material is crucially important, the sorters are also used to supplement electrostatic separators in separating stalks from leaves throughout the complex rubbing and sieving process.

The selection of the optical sorter is governed by capacity (most manage very low capacities) and precision, particularly the need to remove all unwanted foreign materials. The diverse nature of such material means that extra sensing technology is desirable, augmenting colour defection with the use of infra-red sensors and shape analysis.

Deciding to invest in an optical sorter is a difficult decision for many processors and careful consideration of payback is very important. The costs, apart from the price of the sorter, include power and the provision of compressed air (using much more power than the sorter itself). Against that must be set the increased profit possibilities from improved efficiency, capacity and yield and, of course, the ability to raise quality and consistency.

The use of electronic sorting can move a producer into a whole new area of trade. Vietnam is now the world’s second largest Robusta coffee exporter but until recently it exported all its coffee un-sorted, at a price some 200 dollars per ton below the average market price. The introduction of electronic sorting in the major production plants has considerably increased consistency and increased the value of the coffee.

A case in point is to be found in Sumatra where PT Sari Makmur is one of the of the country’s principal coffee producers and exporters, retailing under the brand name Opal. The owner, Suryo Pranato, is very clear in stating that his success is down to his investment in advanced production equipment, especially electronic sorters. To achieve the required production rates several hundred people were required to sort by hand. The installation of the sorters dramatically increased production efficiency and the consistency of the coffee quality. He says, ‘The use of optical sorters has helped us meet customer quality demands and delivery dates. The sorters have eliminated much wastage and breakage and the payback for our investment has been so quick that we quickly installed two more after the first one.

No more need be said. The use of electro-optical sorters in both coffee and tea production is set to increase throughout the foreseeable future. 

Buhler is a global Technology Group and a system partner for the supply of plant, equipment, and process know-how in the fields of Food Processing, Chemical Engineering, and Die Casting, with some 6,200 employees worldwide.

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