Glasses are at a transition point between a solid, which has an ordered interlocked arrangement, and a liquid, whose constituents are free to flow around each other (however painstakingly). For lack of a better description, a glass is an amorphous solid whose constituent particles are jammed together and cannot easily flow, but that does not form the regular structure of a solid.
A windowpane is a genuine glass. Light passes through glass because its constituents are too small to diffract light, and because they are not held in a regular lattice that is large enough to do so. Lead crystal is clear because it is not a crystal – the lead is an impurity that interferes with crystallisation – it is a glass. Solids (including table sugar) are generally translucent or opaque if they form crystalline structures that diffract light.
Table sugar is pure sucrose, and can be a solid or a glass (or a liquid). Each sucrose molecule is made up of one glucose and one fructose molecule bound together (sucrose is a disaccharide, and glucose and fructose are monosaccharides). Table sugar at room temperature fits the definition of a solid – the sucrose molecules bond to form a hexagonal three-dimensional very regular structure, rather like a 3D honeycomb.
When heat is applied, these bonds begin to break, and the sugar is now a free-flowing liquid. With time and heat the sucrose molecules themselves start to break apart. This is caramelisation. The fragments form complex arrangements among themselves creating a remarkable range of new flavours from something that had only one flavour. If sucrose is melted in the presence of a little water, it also partly breaks down (hydrolises) into its components – glucose and fructose.
It used to be thought that first sugar melts and then it caramelises. But it is now known that some caramelisation can occur before melting, and that despite sucrose being a simple chemical, it has no defined melting point or carmelisation point, but rather overlapping ranges.
If liquid (and lightly caramelized) sugar is cooled in the absence of seed crystals, it will not return to its crystalline form, but instead form a glass. This is partly because the caramelisation produced new molecules that don’t form regular crystalline structures, and because heat broke some sucrose into glucose and fructose, which again disrupts crystalisation.
For the home cook to make a sugar glass, there are just a few easy steps. Melt the sugar with a little water, heat it to just before the first colour changes associated with caramelisation, cool it to set as a glass, powder the glass, sprinkle it in a thin layer on a flat surface, add any flavours, melt again in an oven, and cool. The powdering step is to break apart any residual crystalline structures, and to better control the thickness and evenness of the glass.
Two examples of sugar glasses follow.
1. Spiced glass
This spiced glass is designed to be part of a savoury course. Sucrose, of course, is sweet, but sugars come in many forms (they are just simple carbohydrates). Isomalt is a sugar that has about half the sweetness of sucrose, but that shares its melting properties. In this recipe, half of the sucrose is replaced by isomalt to reduce sweetness. Isomalt can be purchased online, or all sucrose can be used. It may be necessary to increase the spiciness to compensate for the sweetness in that case.
100g sugar, 100g isomalt, 25g water (i.e. 4:4:1)
Have a non-stick surface set up. I recommend using silicone baking mats, which are reusable and quite heat tolerant.
Heat sugars together to 160C, stirring constantly. A point-and-shoot thermometer is the least messy option.
This temperature is just under the point of caramelisation where colour changes start to occur. The sugar should be white.
Pour quickly and evenly onto baking mat and allow to cool.
Break into shards and grind to a powder (e.g. in a coffee grinder dedicated to spice grinding – this is very useful to have available).
Place the cleaned silicone mat on a baking sheet, and dust the powder through a sieve onto the mat in an even layer about the thickness of the desired glass. If desired, cardboard stencils could be placed on the mat before dusting to set shapes.
Sprinkle spice flavourings evenly over powder. For example coarsely grind: coriander seeds (4g), Sichuan peppercorns (3g), fennel seeds (2g) and black pepper (1g). Sieve out the fine powder from the coarser particles, and use one or the other (or don’t bother and just use as is) to dust over the sugar. The coarse particles will be more dramatic, but the picture below has used the fine particles.
Place the tray in 160C oven to melt again (a few minutes).
Remove and quickly score with a knife into shapes (e.g. rectangles or triangles) before the glass cools (if a stencil was not used). Scoring after melting is, I think, the easiest option. Alternately, allow to cool and break into random shards.
In the picture, a rectangle has been used for effect in a piece of sous vide pork belly.
[Recipe adapted from: Nathan Myhrvold, Modernist Cuisine, 2011, www.modernistcuisine.com]
2. Handkerchief glass
In this application, the glass will be placed on a food item, and melted to take the shape of the food, then allowed to set again as a shaped glass. To do this, some of the sugar is replaced with fondant and glucose syrup, which will give the glass a greater fluidity when heated and draped.
120g fondant, 60g glucose syrup, 60g isomalt or sugar (i.e. 2:1:1 ratio)
Proceed as for the previous example up to the first solidification (glass stage).
Grind 75g of set glass, 3g sweet smoke paprika and 1g cayenne pepper (or a ratio to taste) to a powder.
Dust over a non-stick sheet (baking paper), and melt in 160C oven.
Remove and score as before into square or circular shapes, or use a stencil at the dusting stage.
When hardened, balance glass on a selected food item (e.g. a square of sous vide pork belly topped with pickled carrot and cucumber and red pepper), and place briefly under a low grill to melt the glass until it is draped nicely over the food.
Remove from heat and allow to set, trim edges with scissors. If possible, trim edges or score them before fully set to avoid brittle breaks. I needed a bit more practice I think.
[Recipe from Grant Achatz, Alinea, 10 Speed Press, Berkely, 2008, p:328.]
Of course, any preferred spice mix could be used for any glass.