I love sugar, especially when it's in the form of a caramel or a butterscotch-type "quick caramel" sauce, drizzled over cakes and ice cream, or baked apples. Unfortunately, sugar can be
In the kitchen, I think the number one problem with working with sugar is that it crystallizes, especially when you don't want it to, like when you are making quick caramel sauces. Fortunately, there are tricks that you can do so that your caramels and sauces don't turn gritty.
What is sugar?In the kitchen, the most common form of sugar is granulated sugar. People often assume that granulated sugar is glucose, but it's not. Glucose, along with fructose, are actually the building blocks that make up each molecule of sucrose that is granulated sugar. Brown sugar, like white sugar, is also mostly sucrose. It's important to note that in sucrose, glucose and fructose are chemically bonded.
The other type of sugar you need to be aware of is "invert sugar", such as corn syrup. Invert sugars are made when larger sugars, like sucrose, are broken down to their basic building blocks, glucose and fructose. What most people don't realize is that corn syrup actually comes from corn starch. Starch is the storage form of glucose in plants, and it's a long chain of glucose molecules bonded together. If you treat starch with either an enzyme (amylase) or an acid and a little heat, the starch chains break down into their building blocks: you obtain lots of glucose. Corn syrup is thus a mixture of glucose, fructose, and sucrose. Honey is also an invert sugar: bees drink flower nectar containing sucrose, and they secrete an enzyme (invertase) that breaks down the sucrose (digestive acids also help this process) to form glucose and fructose.
Why do we need to know about sucrose?When sucrose is present in high concentrations, like when you are making butterscotch that's loaded with brown sugar, the sucrose molecules tend to pile up and crystallize. The molecules just can't help but crystallize because there is so much sucrose around. The caramels and quick caramels can become powdery, or even gritty (if larger crystals form) because the sucrose is essentially precipitating/crystallizing out of the sauce.
How do you stop sugar from crystallizing?
The most common precaution to prevent crystallization in recipes for quick caramel sauces and caramel is to add an invert sugar to your recipe, like corn syrup or honey. Why? Remember invert sugars contain glucose and fructose. Sucrose has a harder time crystallizing when glucose and fructose are floating around in the saucepan because glucose and fructose prevent the sucrose molecules from piling up on each other and crystallizing. Invert sugars interfere with the crystallization of sucrose, and therefore sugar sauces and caramels are less likely to crystallize if you add a little bit of corn syrup or honey to your recipe.
If you are out of corn syrup and don't have honey on hand, you have a second option: add a squeeze of lemon juice. Lemon juice is acidic and therefore if you mix a little lemon juice with sucrose, and you heat the mixture, some of the sucrose will break down to its building blocks, i.e. glucose and fructose. By adding a little lemon juice to your sugar sauces and caramels, you are basically making a little invert sugar in your saucepan so that the sucrose, and your caramel, won't crystallize.
The proof is in the pudding
I hope I have helped clarify why some batches of quick caramel crystallize. Adding a little corn syrup to a batch of quick caramel makes sense on paper, but it's always nice to test out the options ourselves, so that's what I did.
It's hard to tell from the photos but the quick caramel prepared with no additives and a small amount of water is more opaque and less clear than all the others: this sauce has a powdery mouthfeel, reminiscent of brown sugar fudge. On the other hand, the caramel sauces made with either corn syrup or lemon juice are much clearer. The mouthfeel of both sauces is completely smooth, without any detectable powdery texture. Of course, the flavour of the sauce containing lemon juice was a little more citrusy, which personally I wasn't a fan of. So, lemon juice works to prevent sugar sauces from crystallizing, but perhaps the flavour might not be what you are looking for.
I tested out one more option that's not "in the books": dilution. From a practical standpoint, caramel sauces tend to crystallize because there's so much sucrose dissolved in so little water, so I doubled the amount of water to see what would happen if I made a more dilute quick caramel. Obviously the "diluted" quick caramel was a little waterier, especially next to the other batches, but from a crystallization standpoint, this sauce didn't become powdery or gritty. Of course, this option won't be of much help if you are making a real caramel because most of the water evaporates as the sugar boils, but for a quick caramel sauce, it's definitely another option worth considering. In fact, my grandmother's quick caramel sauce recipe contains double the water than the one I served with the baked apples.
Crystallization is a science
I find it ironic because as a chemist, I spent a lot of time trying to force my products to crystallize, and yet in the kitchen, we usually strive for the opposite in a perfectly smooth caramel sauce. Luckily, we have a few tricks to choose from so that we never have to face a batch of caramel turned to a solid mass of gritty sugar.