Learn the science behind:
They stand right next to each other in the store: golden, or yellow, brown raisins. They seem identical, except for their color. Is that true? Is their color the only difference? And if so, why even are most raisins brown, not yellow?
Yes, golden raisins are pretty much identical to brown, or black colored raisins. The only difference is that golden raisins have been processed differently, to maintain their light yellow color.
Golden raisins are yellow
Golden raisins are ‘regular’ raisins, with the only difference that golden raisins are yellow in color, instead of dark brown. As a matter of fact, brown and yellow raisins can be made from the exact same grape variety. The only difference is how they’ve been processed.
Naturally, grapes turn brown during drying
Raisins are dried grapes. Most raisins are made from white, or green grapes. By drying the grapes, they shrivel and shrink and lose a lot of their moisture. However, during this process they also turn brown.
This browning process is a natural process. During drying, the cells in grapes break down, enabling moisture to leave the grape. However, this breakdown also causes molecules that are normally ‘locked’ away to be released and interact with one another.
One of the reactions taking place during the drying process is enzymatic browning. In this series of chemical reaction an enzyme, PPO (polyphenol oxidase) catalyzes a reaction with phenolic compounds that leads to the formation of brown molecules. It is a very common chemical reaction in freshly cut fruits, such as apples, pears, bananas, but also avocados,. They all turn brown once they’re sliced because of this reaction and so do raisins during drying.
Prevent browning before drying
In order to make golden raisins, the grapes need to be pretreated to ‘turn off’ the PPO enzyme.
There are a lot of ways to turn off the PPO enzyme, you could use heat or acids for instance. However, these will all also affect the texture and flavor of the grape and the resulting raisin. It’s why they aren’t commonly used to do so. Instead, to prevent raisins from turning brown, they grapes are pre-treated with sulphur dioxide.
Sulphur dioxide (SO2) can shut down the PPO enzyme by forming complexed with either the enzyme or some of the resulting molecules. This ensures they can’t continue to react into brown molecules.
Golden raisins aren’t dried on the field
Sulphur dioxide is applied to the fresh grapes for a few hours, before they are transferred to the drying process. This process needs to happen off the field. It’s why golden, or yellow, raisins can’t be dried on the field and especially not while they’re still on the vine (whereas brown ones can).
Sulphur treated raisins can then be dried in two ways:
- Sun-dried: the treated raisins are again brought outside and spread out to dry in the sun. This is the least energy intensive, and thus often cheapest method. But, it does need a warm, dry climate and will take several weeks.
- Mechanical drying: the raisins are dried using specialized drying equipment. Manufacturers can control this process very tightly. Raisins dried this way can be more plump and moist, simply because they haven’t been dried too much by the sun. This process does require more energy, and tends to be more expensive.
Mechanical drying needs to be gentle
Despite having used sulphur dioxide, raisins can still turn brown during drying if they’re dried at too high temperatures. At these higher temperatures yet again other chemical reactions, such as caramelization start to play a role. Since grapes are full of sugars, they are very prone to burning and browning under high heat. It’s why drying needs to happen at moderate temperatures.
Are brown or golden raisins ‘better’?
Since their only difference is their color, one isn’t necessarily ‘better’ than the other. It all depends on how you want to use your raisins. A few considerations to keep in mind:
- Golden raisins are not suitable for people with a sulphur allergy
- Does the color of your raisins matter in your application? For instance, some baked goods look better with darker raisins for a little bit of contrast, though in an apple pie you might want to use golden ones?
- Since the sulphur dioxide has inhibited enzymatic browning. Golden raisins do tend to contain more phenolic compounds.
- Do you prefer sun-dried vs. mechanical dried? Mechanical dries raisins tend to be a little plumper, but, that process does cost more energy.
And then it’s up to you which one you prefer. At least you now know that they are pretty similar!
Lante, Anna & Tinello, Federica & Lomolino, Giovanna. (2016). The Use of Polyphenol Oxidase Activity to Identify a Potential Raisin Variety. Food Biotechnology. 30. 98-109. 10.1080/08905436.2016.1166125. link
Sayavedra-Soto, Luis Alberto, Inhibition of polyphenol oxidase by sulfur dioxide, 1983, link
Science-Direct, Enzymatic browning, K. Prabhakar, E.N. Mallika, in Encyclopedia of Food Microbiology (Second Edition), 2014, link
Sun-Maid creative team, Sun-Maid Raisins and Dried Fruit, 2011, link