As the primary source of wine, the quality of grapes play a crucial role. Vineyard management is its first step, but climatic factors, vine diseases and the harvest method that is used, can cause that the grape quality is not uniform when it is delivered at the cellar. Grape sorting is used to eliminate these quality differences.
A survey of 2016 found that 30% of Australian cellars apply some or other form of grape sorting. The manual sorting of grape bunches was the method applied most and smaller cellars have also attended more to grape sorting. Belt sorting, optical sorting and vibrating belt sorting are other sorting methods that are also applied. The combination of harvesting, destemming and sorting in a single equipment is also applied. The destemming and sorting actions both have the aim to remove contaminants from the grapes. The method of destemming determines the condition of the grapes, which will influence the ease of grape sorting.
Optical sorting equipment is available from different providers. It does not only use the visible colour spectrum that humans can see, but also uses near infrared, which can with the aid of chlorophyll activation and fluorescence, detect plant material. The operation of an optical sorter depends on both the soft and hardware, which use shape and colour together to differentiate between objects. Optical sorters usually have a belt with valves over the belt to remove undesirable material. When the undesirable material is observed, the specific valve is opened and the material is removed by a stream of air. Other types use a vacuum to suck out the undesirable material, as soon as it is observed. The latter method can remove up to 400 contaminants per minute.
Densimetric sorting uses density as the basis of sorting. It is sensible, because the degree of ripeness correlates with density. Its advantage is that not only are contaminants removed from the grapes, but ripeness differentiation can also be applied. One of the methods is to transfer the grapes to a drum, which broken berries and debris are attached to and the intact berries fall into a bath containing a sugar solution of a certain density. The riper berries fall to the bottom and the unripe berries and debris remain on the surface. The two separated streams move over a strainer, where the solution is drained and circulated for reuse. Another method uses a draining conveyor to transfer the riper berries from the bottom of the solution. Apart from the sorting of grapes, densimetric systems can also be used to remove dust from berries or, in combination with a cooling system, the sugar solution can be used for the cooling of the grapes. The condition of the grapes before their sorting plays an important role in the efficiency of densimetric systems and can also gradually change the density of the sugar solution.
Belt sorting consists of a conveyor belt with rollers and has already been applied since 2008. In comparison with other sorting systems outputs of up to 50 tons grapes per hour can be obtained with it.
All leading suppliers of mechanical harvesting machines offer systems that combine destemming and sorting. One of the problems associated with these systems is that more juice occurs in the receiving bins, because destemming is a mechanical action which can damage the grapes. In the case of red grapes this does not present a problem, but in the case of white grapes it can have a detrimental quality effect. It is even debatable whether grape sorting is beneficial for white grapes, because juice and skin separation as well as the pressing of the skins are applied shortly after the delivery of the grapes. Sauvignon blanc grapes may be an exception, because skin contact is applied in most cases.
It can be accepted that grape sorting equipment will be developed further in the future (Nordestgaard, 2019).
Grape sorting can be done manually or electronically.
Nordestgaard, S., 2019. Latest advances in grape sorting technology. Australian and New Zealand Grapegrower and Winemaker, March 2019: 50 – 55.