brown raisins

How Raisins Are Made

Raisins are dried grapes, nothing more, and nothing less (most of the time). But, since grapes contain a lot of water, it requires some smart processing and proper quality control to make sure those grapes actually turn into raisins.

Raisins are preserved grapes

Raisins start out as grapes and technically, they can be made from any grape variety. Nevertheless, worldwide, the majority of raisins is made from a single variety: Thompson seedless. Aside from flavor and texture, an important decision factor for choosing a variety is how well the plant grows in a given climate.

What about currants? Currants are a type of raisin, made from a grape variety called current. They are not the same as redcurrants, which are no grapes at all.

As you might have experienced yourself, grapes don’t keep well. The moist fruit, with plenty of sugar and delicate skin, spoils in a matter of weeks, even under ideal storage conditions. The skins and overall structure of the fruit simply aren’t strong enough to keep them for long, unlike apples and bananas for instance.

As such, grapes need to be processed into something to ensure the crops don’t get lost. Making grapes into raisins is just one possible preservation method of grapes. Another, very popular one is of course making wine, or even more simple, making grape juice.

Reducing water activity

Fresh ripe grapes contain over 75% water and a water activity well above 0.9. Raisins on the other hand contain just 15% moisture and have a water activity closer to 0.5.

Remember that water activity is a measure for the amount of available water in a product. Pure water has a water activity of 1.0, a product with barely any available water has a value close to 0. Generally speaking, the higher the water activity of a product, the more easily it spoils. This is mostly because at higher water activity, it is easier for most microorganisms to grow and thrive. These microorganisms can spoil the food, or grapes in this case.

A lot of microorganisms that cause spoilage grow and thrive on a grape. Grapes are full of water, and also contain a good amount of sugar for the microorganisms to munch on. However, barely any microorganism can grow on raisins, there simply isn’t enough water available. This is the main reason why you can store raisins for such a long period of time.

fresh green grapes
Fresh grapes, with a water activity well above 0.9.

Raisin production = drying

For any raisin manufacturing process farmers need to wait until the grape has ripened sufficiently. It should contain enough sugars to be tasty. A common way to measure the sugar content of products like these is using Brix. Brix is a measure for the amount of sugar in a product and can be measured quite easily using a refractometer. For grapes producers are generally looking for a Brix value of at least 20.

To make raisins, it’s crucial to properly dry the grapes, but, in such a way that the final raisin doesn’t become rock solid. That is, not too much moisture should evaporate, nor should it evaporate too quickly. But it’s also crucial that enough moisture evaporates. If the raisins still contain too much water, they’ll still be vulnerable to mold and spoilage.

There are two main ways to dry grapes:

  1. Use the sun
  2. Use a mechanical way of drying

Sun drying raisins

If the climate in which the grapes are grown tends to be dry and warm at the point of harvest, raisins can be dried outside, in the sun. One of the largest raisin manufacturers in the world, California, has such a climate and a lot of raisins are sun dried here.

There are two ways to dry grapes in the sun:

  1. On the vine
  2. Not on the vine

To dry grapes on the vine, the branches that contain grapes are cut so the grapes no longer get water and nutrients from the plant. Instead, the branch as a whole will dry out, including the grapes. This process takes several weeks by the end of which they’re harvested using specialized equipment that can shake the raisins from the rest of the plant.

Drying on the vine takes some time. Ripening off the vine generally goes a little faster. In this process, the grapes are harvestesd and spread out on mats, often right next to the plant. There they remain for about 10 days, though the exact duration depends on the climate, basking in the sun.

Both processes use the heat and power of the sun to evaporate moisture from the grape. Because grapes have such a thin skin and contain mostly sugar and water, they do dry quite easily. Which method is used depends on a lot of factors such as climate, grape variety and more.

Mechanical drying

If the climate isn’t well suited, or if the weather in a particular year isn’t cooperating, manufacturers may also decide to use mechanical drying processes to dry their raisins. For this process the grapes need to be harvested first, before moving them to the processing location.

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To dry the grapes properly using machines, they require a pre-treatment. This pre-treatment helps the skin to lose moisture during drying. Several methods exist, one of them is a dip in an alkaline solution (alkaline is the opposite of acidic and has a pH-value >7).

In mechanical driers air is blown past the grapes to help the moisture leave the grape. This process is notably faster than sun drying. It generally takes less than 2 days for grapes to dry sufficiently. However, if not well controlled, the quality of raisins produced this way can be lower.

Having a closer look at raisins

So far, we’ve only discussed the moisture content of raisins. But there’s a lot more to raisins than just evaporating moisture. For one thing, raisins also change color quite drastically during the drying process.

Raisins don’t just dry, they also turn brown

Most raisins are made from green (or yellow) grapes, nevertheless, the final raisins are often brown. This is because of a chemical reaction that takes place within the grape during drying. During drying grapes are damaged and different ingredients within the grapes interact with each other. This allows a range of chemical reactions to take place.

Sliced apples turn brown over time, and so do pears, bananas and avocados. All of these turn brown for the same reason that raisins turn brown: enzymatic browning. The PPO (polyphenol oxidase) enzyme in these products triggers a reaction that causes the formation of molecules with a brown color.

The reaction can be stopped by adding a chemical, sulphur dioxide, to the raisins before the drying process. This is how golden, or yellow raisins, are made.

brown raisins

Why do raisins contain oil?

If you’d look at the label of a pack of raisins there’s a good chance you find that it doesn’t just contain raisins, there’s also (a little bit of) oil in there. So why do manufacturers add this?

It’s mostly to prevent clumping. Raisins are quite dry, but still contain moisture and a lot of sugar. This can cause the raisins to stick together quite easily. By coating the raisins in a thin layer of oil, they don’t stick to each other as easily. Secondly, it can help prevent the raisins from drying out even further by serving as a thin protective coating on the outside.

Over time sugar crystallization causes white spots to appear

During storage, raisins may develop white crunchy bits on the outside. These crunchy bits aren’t harmful, they’re just sugars!

Raisins contain a very high amount of sugars, over 50% of a raisin is sugar. Initially, that sugar is dissolved or on the inside of the raisin, not visible, though you sure can taste it of course. During storage, the outside of a raisin tends to dry out a little more. Moisture continues to evaporate, much as would happen for a lot of products when they’re left out in the air, thick of a dried out slice of bread.

At some point, so much moisture has evaporated that not all the sugar can remain dissolved. Instead, some of the sugar in the raisin will start to form crystals. It’s the same process that allows you to make rock sugar and what happens when making a crunchy cranberry pie. You will see these small write crystals on the outside of the raisins.

Not sure whether your white spots are indeed sugar crystals? An easy test to see whether they are crystals is to add a drop of water onto them. If they’re sugars, they will dissolve almost instantaneously.

References

California raisins, Technical specification, link

L. Peter Christensen, Raisin Production Manual, 2000, link (see chapter 6 to learn more about raisin grape varieties)

USDA, Food Data Central, Grapes, green, seedless, raw, FDC ID: 2263891, link

Aslan, Yakup & Hussein, Hawsar & Abdullah, Seerwan & Cavidoglu, Isa. (2019). Determination of Some Quality and Safety Parameters for Black Raisin Juice. 5. 58-76. 10.7176/JSTR/5-4-08.

J.P. Zoffoli, B.A. Latorre, Table grape (Vitis vinifera L.), Postharvest Biology and Technology of Tropical and Subtropical Fruits: Cocona to Mango, 2011

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