When harvesting, it is important to limit oxidation, microbial development and contamination.

Oxidation

Oxidation of berries will cause changes to colour and the loss of phenolics & aroma compounds. It may also stimulate microbial contamination. It is affected by:

• Contact with air: some harvest trailers will blanket the vintage with carbon dioxide or nitrogen.
• Temperature: best to harvest grapes at low temperatures
• Time: it is important to process berries as rapidly as possible, particularly if they have been damaged in the harvesting process
• Berry integrity: enzymes within the berry maintain sufficient activity to oxidise their components, but this activity is regulated (& moderated) by cellular compartmentation if the grape is undamaged.
• Grey rot: Botrytis cinerea releases an enzyme (laccase) that catalyses oxidation very effectively and is difficult to control in the winery.

Microbial development

This is affected by:

• The health state of the grapes: rotten grapes need much more careful, rapid and cool handling than healthy grapes.
• Hygiene: grape must is an ideal medium for the growth of many yeasts, bacteria and moulds. It is essential to keep harvesting and transporting equipment and machinery clean.
• Temperature & time: cool & quick is ideal
• Berry integrity

Contamination

This may occur from:

• Rainfall
• Leaves and petioles dropping into picking bins
• Picking shears in picking bins
• Other MOG (Matter Other than Grapes) harvested by mechanical harvesters
• Soil on the underside of harvesting bins
• Hydraulic oil from the harvesting machine
• Metal or plastic taints from harvesting equipment

In some cases sulphite is added to the fruit in the vineyards, to protect it from delays in processing. This is often required with mechanically harvested fruit that can suffer from juicing due to mechanical damage. Up to 100 mg/l addition of Potassium metabisulphite (PMS), can decrease the effects of oxidation and slow microbial spoilage.

Some wine styles, such as sparkling and carbonic maceration, require berries, and thus bunches, to be brought to the winery intact.