How to Sample Grapes

When it is getting close to harvest, early August for some wine grapes in southern Missouri, you need to collect a berry sample from the vineyard to help determine when to harvest the grapes. It is important that the sample be taken properly, because a small number of berries must accurately reflect the entire crop.

You must sample each variety of grape separately, even if you plan to blend juice at the end. Take up to 200 berries per sample, equal amounts from both sides of each row (e.g., if your rows run north and south, take 100 berries from the east sides of the rows and 100 berries from the west sides of the rows). The berries should be picked at random, so you can walk down the row and pluck an equal number of berries from each side without looking too closely (try not to "select" only ripe berries). Put the berries in a plastic bag and bring them to room temperature before sampling. If you will be in the vineyard for a long time, put the bags of berries in a cooler.

The beginning winemaker may only have the apparatus to sample for sugar. If you use a hydrometer to measure sugar, you will need a larger sample of juice than if you use a refractometer. The more advanced home winemaker may also test for pH and acidity. If you are just measuring sugar or sugar and pH, you can crush the berries with your hands by squeezing the plastic bag and then letting the juice run out. If you plan to test for acid as well, extract the juice by processing it through a "squeezo" type food strainer (photo above) or crush the berries in a plastic bag with your hands, strain the juice from the pulp in cheesecloth, and then wring out the pulp or pomace in the cheesecloth. Particularly when dealing with slip-skin American grapes, it is important to crush the berries thoroughly to get an accurate acid measurement. If you need to store the berries or the juice in the refrigerator before processing and/or running tests, you need to bring them to room temperature before testing.