Homemade Wine: When to Harvest Grapes

My father makes wine and it’s delicious! A lot of people here in Istria traditionally make wine in their wine basements. But my father, he is really into this. He takes wine making very seriously, taking care of every little detail.

September is the start of wine making process, I’ll start with a series of posts about this topic. The first challenge is to pick the best time to harvest grapes.


How do you know when the grapes are ready to be picked?

In Istria the grapes are usually picked in early September. My father keeps track for the last 11 Years and the earliest harvest we had was Sep 7 and the latest was Sep 28. Usually it’s somewhere in the middle.

There are a few things you need to consider when checking whether your grapes are not just ready to be picked, but whether it is the perfect moment to pick them:

1. Taste is maybe the best way to judge whether the grapes are ripe. There are many various types of grapes so it makes it much easier if you know how the grapes are supposed to taste. Anyway, they should be sweet and flavorful, and not too tart.

2. Color and shape is the second point to consider. The grapes should be full, all round and have a deep color. The individual berries should be easily picked. The white sorts will turn from green to yellow. While the red sorts will get a deeper color. But the looks are not enough, the grapes usually turn the right color but still need a few more weeks to develop sweetness.

Harvesting grapes - red grapesHarvesting grapes - white grapes

3. Leaves and seed color is another thing to look at. Seed will turn from white to tan to brown when the grapes are ripe. Leaves of ripe grapes are yellow or red, and their petioles turn brown and get wooden.

4. And the most important is his majesty… Sugar! All winemakers are obsessed with sugar because the strength of the alcohol and the dryness of the wine depends on sugar. First measurement is usually done 10-12 days before harvesting. This is usually repeated every few days but one thing needs to be taken into account – that at the beginning you will see around 0.1% increase every day. But this will increase towards the time when the grapes are ready to be picked. And the last few days are very important because then the smell and the taste comes out.

Usually sugar is measured in brix degrees (°Bx), percentage of sugar in a liquid. It depends on the sort. My father always targets the sugar ratio of 18% to get 12% of alcohol in wine, but some grape sorts are sweeter.


My father users the refractometer to measure the sugar. Refractometer is an optical tool that shows the ratio of dry materials. You put a drop of juice on the refractometer. The sugar in the juice will cause light to bend at a certain angle, depending on the quantity. The refractometer measures this angle and contains a scale corresponding the the quantity of dissolved sugar in the mixture.

The scale is typically given in °Brix measurement and  the Oechsle scale.

Refractometer scale
Refractometer scale

If the refractometer only shows Oechsle scale sugar ratio can than be calculated but usually the newer refractometer have a scale that lets you read the °Bx , and the Oe° (Oechsle degrees) on the scale from 0 to 170 Oe° and also the alcohol ratio.

Oechsle scale is widely used in German, Swiss and Luxembourgish wine making. If you multiply Oe with 0.266 and then deduct 3 and you’ll get the sugar ratio. 

Example: 90 Oe° x 0.266 = 23.94 – 3 = 20.94 % of sugar.

But enough with the technical stuff! It is important that you know that there are many different ways to measure sugar ratio, and many different scales and you need to be sure how your tool (in this case refractometer) is measuring it.

In the pictures below you can see how sugar is measured in juice of just picked grapes.

When deciding on harvesting time you should take a sample from different parts of your vineyard and put a drop of juice on the refractometer to measure the sugar. 

Harvesting grapes

Harvesting grapes
Putting a sample on refractometer
Harvesting grapes
Measuring sugar content – reading the scale

Acid is the sugar counterpart. Acid has influence on the color, balance and wine taste but also the growth and vitality of yest during fermentation and protecting wine from bacteria. If you harvest your grapes too early, the sugar level will be too low and the acid level will be too high. If you harvest them too late, the sugar level will be too high, the acid level will be too low and the tannins will have evolved to a point where they don’t provide the wine with its required structure. It sounds simple but actually it is not easy to determine the perfect timing.

The best time to harvest grapes is early in the morning while the air temperature is not too high. Also, it is better to harvest them when there’s dew.

Did you know…that Spain is the top producer of grapes for wine making (based on area planted km2).

So, let’s harvest the grapes and see what’s next! 🙂



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