Machine harvesting is more often than not associated with mass-produced wines. Conversely, producers who are keen to stress the artisanal credentials of their wines will mention hand harvesting on back labels. But a new study calls the whole area into question, concluding that machine harvesting is, in fact, preferable to hand picking in some instances.

Paul Kilmartin, associate professor of wine science at the University of Auckland, New Zealand, has discovered that machine harvesting is actually conducive to making classic Marlborough sauvignon blanc. The mechanized process means that aroma-generating "thiols" – 3-mercaptohexanol (3MH) and 3-mercaptohexylacetate (3MHA) – which are responsible for its characteristic passion fruit characters, green aromas and sweaty notes, are enhanced.

“This has broken the traditional view that we can get the best wines from hand harvesting,” says Kilmartin, who, along with his colleagues in the School of Chemical Sciences, has recently completed a study on Marlborough sauvignon blanc.

“Machine harvesting seems to be quite important in obtaining thiols,” he explains. "In France, particularly in Sancerre, and in South Africa, grapes tend to be hand picked, generating a different style of wine.” Indeed, Sancerre is renowned for producing restrained styles of sauvignon blanc, with descriptors of gooseberry, elderflower and wet stone commonly used, while South Africa makes grassy examples of the variety.

However, machine harvesting has its critics. Berries are often ruptured by mechanized harvesters, which can lead to oxidation of the juice. This results in a brown color and a loss of freshness if an antioxidant is not added. Machine harvesters can also strip a wine of its delicacy. In addition, the machines are less selective than humans, often shaking leaves, bugs and other undesirables into the picking bins, which can make it into the fermentor if producers are not careful.

But Kilmartin is not claiming that mechanized harvesting is better than hand picking. Rather, his research concentrates on the fact that levels of thiols are five to ten times higher when grapes are machine harvested. This means that it's preferable for winemakers looking to create a highly aromatic style filled with passion fruit and green capsicum.

There is a caveat. While passion fruit and sweaty armpit aromas are encouraged by mechanical harvesting, New Zealand also has a greater propensity of these thiols. Kilmartin isn’t sure why. It could be the soil, the intense sunlight, or the temperature swings between day and night that are found in Marlborough. “The experiments have not yet been done,” says the scientist.

Kilmartin’s research into hand versus machine has so far been restricted to sauvignon blanc. His findings may not be applicable to other varieties – particularly those that are typically whole-bunch pressed, such as pinot noir, or white varieties with thick skins, including pinot gris and gewürztraminer. For example, the characteristic aromas of bubblegum and pear drop and the soft, juicy style of Beaujolais wines rely upon a fermentation taking place inside whole berries, and thus require hand harvesting.

Researchers at the University of California in 1973 found that grape varieties including semillon, trebbiano, grenache and zinfandel lost a lot of juice when they were machine harvested, and were prone to oxidation, losing their fruity vibrancy. Other studies have concluded that mechanization has little negative impact on wine quality.

All in all, the message is that some skepticism is required when reading references to hand harvesting on back labels. Hand picking is not as significant as a winery’s public relations team might have you believe. And if you are in the market for a sauvignon blanc from Marlborough with aromas of passion fruit and capsicum, you might even want to steer clear of the hand-picked wines.