Grape processing: from picking to sorting

Grape processing: from picking to sorting

by Maurizio Broggi September 28, 2023

In the first part of our article series on pre-fermentation steps, Westgarth Wines wine specialist Maurizio Broggi discusses transportation from vineyard to winery, use of containers, and sorting.

After deciding when to harvest, the next decision the winemaker must make concerns the receiving and processing of the grapes to extract the juice that will be fermented into wine. Although transportation may seem a straightforward stage of the wine production process, appropriate procedures during transport are essential to deliver healthy, undamaged grapes to the winery. Once delivered, the winemaker has a number of options available to process the fruit prior to fermentation, which differ for white and red winemaking and according to the style and quality level of the wine.

Transportation from the vineyard to the winery

Once the grapes are harvested, they need to be rapidly but carefully delivered to the winery to avoid physical damage and loss of quality. The damage and oxidation grapes can experience during transportation depends on container size, temperature, and transit time.

Grapes contain two oxidative enzymes known as laccase and tyrosinase. When berries are damaged, juice is released and exposed to oxygen in the atmosphere. The two oxidative enzymes accelerate the reaction with oxygen resulting in the oxidation of grape phenolics causing brown juice. Furthermore, damaged berries allow microorganisms such as wild yeasts, fungi, and bacteria to attack grapes, causing microbial spoilage that may result in off-flavors and haze. For white grapes, avoiding breakage of the skins during transport is especially critical to avoid the extraction of unwanted phenolics from the skins.

How the fruit is delivered to the winery depends on the type of harvest (hand vs. machine harvest) and the distance between the vineyard and the winery.

Grape containers

The size and shape of containers for grapes may range from small, shallow crates for hand-harvested grapes, to large containers that can be tipped directly into the receiving bin at the winery. The bigger and deeper the container, the more grapes will be crushed at the bottom. Consequently, bigger, deeper containers will likely result in some level of damage to berries and subsequent release of juice due to the weight of the grapes on top. Smaller, shallower containers are ideal for keeping the grapes intact and preventing berry damage.

Sparkling wine producers, for example, typically adopt small baskets that can be stacked directly on trailers or dumped into larger bins. Medium-size wineries often adopt half-ton bins. For larger operations, such as for machine-harvested grapes, fruit may be directly loaded into 10-ton containers. Whichever the container and transportation methods are adopted, it is critical to minimize the number of transfers of fruit to keep grapes as intact as possible.

Obviously, reducing the duration of transport between the vineyard and the processing facility minimizes the risk of physical damage, oxidation, and microbial attack. Temperature is also a key factor as the higher the temperature, the more significant the effects of oxidation and microbial spoilage.

Prevention practices

There are several practices adopted to protect freshly harvested grapes from oxidation and spoilage from microorganisms. After picking, grapes are often dusted with potassium metabisulfite, a chemical compound that slows oxidation and prevents microbial attack. Since warmer temperatures increase the speed of oxidation and microbial growth, harvesting at night with cooler temperatures minimizes the negative effects of temperature. Dry ice (carbon dioxide in solid form) can also be added to cool down the grapes. Transport over long distances can be safely done with enclosed, refrigerated tankers that are maintained anaerobically by blanketing fruit with carbon dioxide.

Once the fruit arrives at the winery, it must be processed as soon as possible. If there are delays before processing, grapes should be kept chilled to avoid loss of quality. The grapes are then dumped into some form of hopper, then transferred via belts, slides, or screw conveyors to the next grape processing equipment.

Sorting

The production of high-quality wine begins in the vineyard by harvesting healthy, undamaged, ripe grapes. Sorting (or triage) is the process of removing lesser-quality grapes, which may include green, raisined, underripe, damaged, or rotten grapes. Rejected grapes may end up in lesser wines or discarded altogether. Sorting also allows the removal of any MOG (material other than grapes) such as stems, leaves, or any other debris accidentally picked with the grapes.

Sorting is one of the key processes that allows winemakers to improve quality by selecting the best possible grapes. The level of sorting can range from none or minimal for inexpensive wines, to ultra-rigorous for premium and fine wines. Due to the cost involved in this operation, rigorous sorting is adopted for high-quality wines where the higher price per bottle justifies higher labor and equipment costs. Gianfranco Soldera, the outspoken winemaker and owner of the world-famous Case Basse estate in Montalcino, was well-known for his staunch dedication to absolute quality, which translated into painstakingly selecting only perfectly ripe and healthy single berries for his 100% Sangiovese Toscana.

How sorting works

Sorting can be done by inspecting whole bunches or single berries (after destemming). It can be carried out using specific equipment ranging from simple hand-sorting tables to more technological solutions such as shaker tables and digital optical sorting systems. The traditional way to sort fruit is done manually by operators who visually and physically inspect grapes on a moving belt or sorting table. Depending on the size of the crop and winery, the number of people involved may range from a couple to a dozen at a time.

Automated sorting machines that inspect single berries are also widely used. The basic models of sorting equipment select berries based on their physical properties such as berry size, weight, and shape. The most advanced sorting technologies today are digital optical sorting machines. This system is equipped with a computer that takes photographs of single berries, immediately ejecting, via blasts of air, any underripe, raisined, or rotten berries. The berries are selected based on parameters such as color, shape, and size that are set up by the winemaker. At Dominus, one of Napa Valley’s most prestigious estates, they are passionate advocates of rigorous sorting, which is believed fundamental in achieving the purest expression of their terroir. Here, hand-harvested grape clusters initially go through manual sorting followed by an optical sorting system that selects only the most perfect berries.







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