With selective hand-picking, cherry quality is paramount because the high cost of labour must he compensated by high product prices. Unless the scale of your hand-harvesting operation is large (unlikely in Australia), the technology required to extract immature cherry from the harvested cherry is not justified so you will be restricted to using basic conventional pulping equipment. This equipment, developed over many decades for use under traditional coffee production systems, ensures production of high quality coffee from prime cherry. However, it does not discriminate between immature and prime cherry so immature cherry composition must be kept to less than about 3% of the harvested sample in order to maintain quality. Hand-separating immature cherry from harvested samples before processing is common practice. Obviously, there is a trade-off between picker selectivity and productivity.
It is common and simple practice to remove overmature cherry by floating in water and separating it from the prime cherry before pulping. Under hand-picking regimes in Australia, it is unlikely that this coffee will be of sufficient quantity or quality to justify subsequent recovery. If the float separated product consistently comprises about 15% of hand-picked cherry, then recovery by sun-drying and dry-processing may be warranted. However, drying needs to be well-managed to avoid quality pitfalls. At such levels of overmature cherry, selective hand-picking regimes are more difficult to justify from an economic viewpoint.
Conventional pulping equipment
Conventional pulpers rely on the roughened surface of a rotating disc or drum to confine cherry against the pulping chop (disc type) or breastplate (drum type) until the cherry ruptures. Expelled beans escape the pulping chamber through an adjustable opening while the pulp is carried away on the surface of the disc or drum. Immature cherry does not pulp in the conventional sense; instead, it is broken or abraded by the disc or drum until it can escape from the pulping chamber with prime bean. Product contamination with immature cherry is directly proportional to the level of immature cherry in the harvest. Once combined with bean in this fashion, immature cherry is extremely difficult to separate physically.
Typical performance parameters (obtained from manufacturer’s specifications) and estimated water requirements of the main types of conventional pulpers are shown in the Table below. Additional water is required for floating, fermentation and washing. Conventional wet-processing systems typically use about 5 to 20 L/kg of cherry processed. This includes floating, siphoning (transport of cherry from float tank to pulper in water), pulping, grading, fermenting and washing. This may be reduced to about 2 to 10 L/ kg if dry fermentation is used. More recently, pulpers with minimal water requirements have been developed in Latin America.
Processing Less Selectively Hand Picked Coffee
The economic viability of hand-picking coffee can be improved significantly if picking is less selective, because picker productivity is increased. However, this requires the processing system to extract greater amounts of immature cherry from the harvested sample.
If your operation is very small, then this immature cherry may be separated by hand at the factory, though this partly offsets the advantage of less selective picking in the first place. Alternatively, manufacturers of traditional processing equipment offer attachments which allow pulpers to pass limited amounts of intact immature cherry These include rubber breastplates for drum pulpers and flexible pulping chop assemblies for disc pulpers. After pulping, the intact immature cherry is separated from prime bean by screening or sieving. This is done on the basis of size since intact immature cherry is significantly larger than the pulped prime bean. This equipment is usually supplied as a bolt-on attachment to standard pulpers.
Using these accessories, traditional pulping equipment can accommodate cherry comprising up to 10% immature cherry. Above these levels of immature cherry, parchment damage, breakage of immature cherry or loss of prime bean become excessive.
Mechanical Grading System
To improve the performance of such equipment, a mechanical grading system (called the Aagaard grader) is available. It is used in many coffee growing regions of the world although its installation is harder to justify for the small scale hand-picking industry in Australia. It is installed after pulping, and grades beans by their behaviour in water. Heavy beans, light beans and floater/un-pulped beans are separated. The pulper is usually set wide to minimise the risk of immature cherry being broken and so ensure the quality of the heavier prime grade. Floating and unpulped cherries are re-passed through a secondary pulper with the pulping chop or breastplate set more aggressively. The re-pass grade will contain most of the immature and floating bean and will consequently be of lower quality.
An alternative to traditional pulping equipment is the cherry classifier. This machine was developed in Costa Rica to accommodate cherry from less selective hand-picking. It separates immature cherry and pulp from prime cherry in one operation, though its ability to separate immature cherry is not much better than conventional equipment with accessories for immature cherry. It does not separate bean from pulp, so conventional pulpers are required for this purpose. Typically, the pulpers do not work as effectively in this application since the pulp is somewhat broken up in the classifier. Poor pulp separation is an inherent problem of this system. The classifier operates at capacities of about 3,000 kg cherry/h which is much greater than the requirements of Australian hand-picking regimes.
Recently some new processing equipment using a waterless pulper, grading screens and demucilager has been developed. This system is very compact and relatively cheap.