Crisping skin
Crunchy, crispy, crackly are sounds and textures much appreciated in our food. The terms don’t have agreed scientific definitions, although there may be differences in the frequencies of sound they produce (for example crispy making higher-pitched sounds than crunchy) but there is no consensus.
Our mouths can interpret a remarkable range of sensations. A partial list includes density, dryness, graininess, gumminess, hardness, heaviness, fattiness, roughness, slipperiness, smoothness and wetness — and then there is noisiness.
Sound is a major contributor to the perception of taste (if a potato chip didn’t make a sound would it taste like a potato chip?). It is carried to the ears by conduction through the teeth and skull, and can be remarkably loud. The vibrations activate the auditory nerve bypassing the usual way we perceive sound (from sound waves impinging on the eardrum). Chewing can also create ultrasound frequencies that we can’t hear but can feel.
We might have evolved to prefer crisp sounding food because it signifies, at least for some fruit and vegetables, freshness. The sound comes from pressure inside the plant cells that is released on biting, making a sound. Other sources of crispy-crunchy are dry foods such as crisps and crackers and wet-dry foods (french fries, battered food, or pies/tarts). And then there is skin — the ever-popular pork crackling or crispy chicken skin.
Skin is a network of long spring-shaped collagen fibres forming a two-dimensional mesh. The arrangement allows stretching in multiple directions. Skin has high water content (70–80%) in the form of an elastic gel within the collagen mesh. Underlying the skin, and depending on the animal, will be a protective fat layer. The fat is trapped in another fine collagen matrix, making it rubbery and difficult to render out.
Making a crackling involves four steps, gelatinizing the collagen, rendering the fat, drying out the skin and finally crisping and puffing it.
Achieving this with the skin attached to the meat is a challenge at all stages. The temperature needed to gelatinize the collagen (and later crisp it) is higher than we would like to cook the meat, and water in the meat interferes with the drying stage. With Peking duck, the skin is inflated with air to keep it away from the flesh. In a roast chicken, stuffing some herbs under the breast skin can help achieve something similar, or just free the skin from the flesh over the breasts/thighs by sliding fingers under it.
To optimise the cooking of skin, the most straightforward way is to remove it, cook it separately and serve it as a garnish. Pork crackling is traditionally served this way, although not usually separately cooked.
Step 1 — Gelatinisation
In this stage the collagen is broken down (hydrolised) in the presence of heat and water into smaller components, thus breaking the mesh and softening the skin. The spring-like collagen molecules tighten in the presence of heat before they gelatinize, thereby shrinking the skin. Keep this in mind and use larger pieces of skin than finally desired. Skin will shrink preferentially in one direction, usually at right angles to the direction of its underlying muscle fibres (skin is used to being stretched along the muscle fibres and so is more elastic in that direction). Gelatinisation does not occur at the sous vide temperatures typically used for cooking meat proteins.
Step 2 — Rendering
Rendering subcutaneous fat is made difficult because it is not in globules that will just melt and run away, but rather contained in its own collagen network. It is for this reason that pork skin is usually scored — to partially break this mesh and to allow multiple surfaces for fat to render out (scoring needs to be deep and to penetrate the fat). For pork, rendering (and gelatinizing) is most efficient in a pressure cooker. Excess remaining fat can be scraped off after rendering, but retain some because it can set into a sort of foam during the final crisping stage, creating an additional crunch to go with the crisp of the skin.
Step 3 — Dehydrating
Skin will dehydrate if left uncovered in the refrigerator for a few days. A low oven is also effective, or a purpose-designed dehydrator.
This stage is critical. The degree of dehydration needs to be such that most, but not all, of the water is removed. It is made a little easier because some water is tightly bound and difficult to remove (visceral water) so there will always be some remaining. The skin should be slightly, but not easily, bendable.
If this water balance is not right, nothing will happen at the crisping stage.
Step 4 — Crisping
Crisping requires fast and high heat. Under high heat the remaining water superheats but cannot immediately escape as steam because the dry and glassy skin contains it. At about the same time but a bit thereafter the skin softens under heat and becomes pliable. This reduces pressure on the water, which rapidly explodes as steam, expanding and puffing the skin.
If not heated enough, the skin will remain glassy and not be able to expand. If not heated quickly enough, the steam will slowly build up sufficient pressure to rupture the glassy skin and just escape without puffing. Too little water means there is not enough steam to puff, too much and the skin will not be able to be heated sufficiently quickly. Despite modernist approaches, this final step still requires trial and error.
Radiant heating is faster than an oven’s convective (air) heating, so the grill is an option. If using an oven, the temperature should be very high (~240C) to compensate for its inefficiency. Oil is fast, either deep-fried or in a layer in a pan, but messy. If it is thin (chicken/fish) the skin can be crisped in a electric toasted sandwich maker (the corrugations provide an interesting effect). The method doesn’t matter so much as understanding what needs to be achieved.
Suggestion
Pork: Pressure cook 1h; scrape of excess fat (but leave some); oven dehydrate at 120C until the skin is rigid but still bends; Crisp in hot oven (240C) or deep fry in pieces at 200C.
Chicken/fish: Sous-vide at 85C for 3h; dehydrate in an oven set to 150C for 30m (between two flat baking sheets to retain shape); Deep fry at 185C to crisp. Don’t wash the mucus off fish skin — it crisps nicely.
Skin can be taken to the dehydration stage, vacuum-packed to seal and frozen until needed.
Despite everything, I still find the process to be hit-or-miss.
