They stand right next to each other in the supermarket aisle. On the one side, beautiful plump, brown raisins. On the other, seemingly the same raisins, but of a yellow, some say golden colour. When buying them you can’t help but try to remember which of the two you needed. Did it even matter?
Are golden raisins made from a different grape variety than brown ones? And so would they taste different?
Have the golden ones been washed or have the brown ones been baked? Would that impact my recipe?
Luckily, this is an luxury issue since both raisins really are very similar. They can be made from the same grape and can (but don’t have to) be processed almost identically, with the exception of one step: “bleaching”. Even though those differences aren’t huge, knowing what they really are should help you choose next time you’re standing right in front of them.
Raisins start out as grapes
Every raisin starts out as a grape. There exist a lot of different grape varieties, some are most suited for wine or juices, whereas others are better for table grapes or for making into raisins. A common grape variety used for making raisins are the Thompson seedless.
After harvesting the grapes are dried to turn them into raisins. As a result they shrivel up and lose a large part of their overall weight. Whereas fresh grapes contain around 75% moisture, the finished dried raisin contains less then 15%.
Transforming grapes into raisins is just another way to preserve the grapes for times to come, just like wine making is. Grapes themselves only remain fresh for a couple of days when stored properly whereas raisins can be kept for months on end without risk of spoiling.
The low moisture content makes it impossible for most microorganisms to grow on the raisins. For reference, the water activity of grapes is well above 0.9, whereas that of raisins in around 0.5-0.6 (and differs per variety and processing parameters).
Sun drying raisins
Raisins can be dried within a couple of weeks using solely solar power. The raisins are spread out on mats or beds and within a couple of weeks they will be sufficiently dry to store and process further.
Browning of raisins
Raisins don’t just loose moisture though during drying, they also change change. Chemical reactions, sped up by the heat of the sun, cause the raisins to turn brown. One of those reactions is an enzymatic browning process that is very similar to that happening in sliced apples or bananas. The reaction is triggered by the enzyme PPO (polyphenol oxidase).
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Since sun drying is a natural, slightly uncontrolled process, the colour of the final raisins can change. Faster, hotter drying tends to give darker brown raisins whereas slower, cooler drying gives a lighter coloured raisin.
Instead of using the sun to dry the grapes into raisins, manufacturers may also use mechanical drying processes. The raisins first receive a pre-treatment which helps the skin to lose moisture during the drying process. This treatment can consists of a dip in an alkaline solution.
Once they’ve been pre-treated the raisins can be dried in tunnels be blowing air past the raisins. Instead of a few weeks, these raisins will be sufficiently dry in less two days!
Making them golden
No matter which drying method you use on your raisins, they will turn brown over time. To keep the raisins a light, golden colour, they need to be treated with sulphur dioxide (SO2). Sulphur dioxide prevents the chemical browning reactions from occurring. The exact mechanism and reaction for this is unknown but it is known to inactivate the PPO enzyme completely and irreversibly.
Golden vs regular raisins
Despite the use of sulphur dioxide (or a sulphite) to keep the golden raisins golden there does not have to be any difference between golden and regular raisins! If they are both dried using the same drying process, they will be very similar.
Most of the textural differences between raisins are due to differences in the grape variety used or differences in drying processes. Golden raisins are always dried mechanically since they need that alternative treatment and regular raisins are more generally dried using just the sun. These two processes can definitely give differences in raisins texture. Some raisins will be a little plumper whereas others may turn our drier or even a little caramelized (if it became really warm)!
What about those white crunchy bits?
Raisins may develop white crunchy bits on the outside of the raisin, especially if you store them for extended periods of time. These crunchy bits aren’t harmful at all: they are sugars.
Raisins contain a large amount of sugars, over 50% of a raisin is sugar. As long as there is enough moisture in the raisin, all those sugars will dissolve in the water present. However, if the raisin continues to dry on the outside, some of those sugars may come out of solution and crystallize (it’s also what happens when making a crunchy cranberry pie). They will form small white crystals on the outside of the raisins.
An easy test to see whether they are indeed crystals is to add a drop of water on the white area. The sugars will dissolve almost immediately!
California raisins, Technical specification, link
L. Peter Christensen, Raisin Production Manual, 2000, link
Sayavedra-Soto, Luis Alberto, Inhibition of polyphenol oxidase by sulfur dioxide, 1983, link
J.P. Zoffoli, B.A. Latorre, Table grape (Vitis vinifera L.), Postharvest Biology and Technology of Tropical and Subtropical Fruits: Cocona to Mango, 2011