IS IT THICK?
The texture of food is very important This booklet has a focus on the viscosity of foods. Viscosity means the ability to flow. If a mixture is viscous, it will not flow well, it will be thick. How thick or viscous a food product is affects its texture in terms of consistency. Food Layers Another application of the word thickness is applied to depth. Even when beginning to cook the thickness of a product (biscuits, bread slices, meat slices) or part of a product (custard layers, pastry, crumble topping) is an obvious one to ask questions about.
When you are cooking, preparing parts of a recipe or analysing a product (Product analysis) you may ask yourself:
Another application of the word thickness is applied to depth of a layer of food in a dish. When beginning to cook the thickness of a product (biscuits, bread slices, meat slices) or part of a product (custard layers, pastry, crumble topping) is an obvious one to consider and to ask questions about.
Plain wheat flour can be used to thicken. It can be used as an ingredient, a thickening agent, for example, two tablespoons of flour in a stew recipe. Meat, tossed in the flour before frying off, is covered in stock. During cooking the stock thickens to create a gravy sauce.
It can be used as a roux as in making a roux sauce as a white sauce or béchamel sauce It can be used in a beurre manié, a kneaded paste of flour and butter, usually in equal parts and used in soups and juices to thicken them.
Maize flour or corn flour is gluten-free and is a finer starch grain. It can easily be blended with cold water, milk or stocks to form a solution. When the solution is heated thickening occurs and a clear, glossy sauce results.
Arrowroot Is a white powder starch that will thicken to form a clear gel when blended with liquids and heated. The shiny see-through sauce makes it suited to glazing fruits because it allows the colour from fruits to show through.
THE THICKENING PROCESS: GELATINISATION
Gelatinisation is the scientific term for the thickening process when starch ingredients are used.
Raw starch is not digestible and therefore it requires cooking.
Starch is formed from amylase and amylopectin. When starch is in a watery solution it forms a suspension that will separate out if it is not stirred. When heating a starch suspension the starch grains absorb the liquid and swell. On further heating the starch grains burst and begin to thicken the sauce. At just under 100⁰C all the starch grains burst, and the starch becomes digestible. Tasting a sauce is an important test to check if the starch has gelatinised.
Thickened sauces, custards and puddings rely on the proportion of thickening agent or ingredient used to the amount of liquid. A sauce can be a runny pouring sauces or a thicker coating sauce.
Classic sauces such as Velouté, Béchamel and Suprême are starch thickened by gelatinisation.
GELATINISATION OF STARCHES
Potatoes are not digestible when they are raw. They need cooking to make the starch they contain available for use as an energy giving food. Boiled potatoes are cooked until the starches are gelatinised. Some potatoes, Maris Piper, Cara, King Edward, and Kerr’s Pink are particularly good for boiling because the starches they contain become fluffy. Desiree, Estima or Vivaldi have starches that cook to a smooth texture. Salad potatoes, Charlotte, Maris Peer, Anya or Nicola remain firmer after cooking.
Oat grains processed to make rolled oats will thicken milk (or water) to create porridge. Alternately it can be used as an ingredient in baked products, such as biscuits, crumble topping, muesli and granola.
LONG GRAIN RICE
Long grain rice is almost pure starch and needs to be cooked to make it digestible. Basmati, Patna and Thai rice grains are popular. Rice papers are made from rice flour and water dried to create a semi-translucent brittle sheets. When soaked in water they soften and can be used to wrap other ingredients. Used in Thai and Vietnamese cooking to make many versions of ‘spring rolls’.
GELATINISATION OF STARCHY GRAINS
Round Rice called pudding rice is used to make traditional rice pudding. It is a round grain rice, as is the risotto rice, Arborio rice. During cooking the rice grains swell, absorb milk or stock and gradually gelatinise and become digestible. Paella rice is called Bomba, or Grano largo because it is fat and starchy. Glutinous rice is popular in China and Asia. It is called sticky rice and can be eaten sweetened or plain boiled. Sushi rice is also short, fat and starchy which means when gelatinised it sticks together and can be wrapped to create attractive Sushi.
Rice flour is a gluten-free thickener that can also be made into noodles, wraps and puddings.
Noodles can be made from wheat, egg, or rice and can be long and thin, or flat or very fine.
Pasta usually made from Durum wheat. It needs to be cooked and in many meals the pasta is the main carbohydrate e.g. Lasagne or Spaghetti-alla-Carbonara. Pasta such as Stelline, Conchiglietti or Risoni are so small they can be used to thicken soups and stews.
Gelatinisation is a basic process that is needed in many cooked starchy food dishes
MORE STARCHY THICKENING AGENTS
Potato flour starch is fine and gluten-free and will thicken soups, stews and gravy.
Polenta is corn used to make a thick starchy mixture served in place of potatoes or rice. It can be flavoured then fried or baked.
Hominy grits ‘Grits’ are processed maize grains used in USA and Mexican soups and stews.
Cornmeal can be ground coarse to fine depending on how it is to be used eg. Tortilla, cornbreads Couscous is pale yellow semolina (ground up durum wheat) starch. It is popular in Morocco and North Africa as an accompaniment in place of rice, pasta or potato.
Bulgur wheat can also be called cracked wheat. It needs to be boiled to soften the starch.
Quinoa is a nutritious grain that needs boiling to soften it before using in salads, stuffing, pilau or deli-pots.
Semolina is made from durum wheat, ground wither coarsely or fine. It can be made into a milk pudding or used in combination with flour to give crunch to biscuits.
Starch can be treated by steam to enable it to cook more easily or be used in a different way. Speedier cooking helps the consumer and many pasta shapes are now labelled ‘Quick-cook’.
Starch can be modified to make it ‘hold a set’ for longer in a sauce and delay loss of structure and fluid leakage known as ‘retrogradation.’
Modifications can also be made in order that starch gels are clearer and more stable, withstanding shaking during transport. This is better for consumer enjoyment of the food product.
Modification by hydrolysis creates corn syrup or hydrolysed starch seen on ingredient listings. Corn syrup is a sweetening and bulking ingredient that also controls thickening used in confectionery, frozen desserts and bakery products.
Rice can be modified using high pressure steam to partially cook it, make it safe and give it an extended shelf life. It is vacuum packed and can be stored at ambient temperature (room temp).
MODIFIED STARCH PRODUCTS AND GUMS
Thickening granules can be dropped into hot liquids without the fear of lumps forming.
Gravy granules are flavoured, modified starch mixtures that simplify preparing gravy.
Cold-mix desserts thicken in cold liquid. The starch is pre-gelatinised to give an ‘instant’ thickening.
Xanthan Gum is a natural starch produced from a bacterium, used in gluten free cookery to aid ‘holding together’ ingredients. It is widely used in manufactured cold desserts and yogurts to help the set and stability. Xanthan gum mixtures are thixotropic, which means they will slacken when shaken or stirred but firm up again. This gives excellent mouthfeel to products.
Xanthan gum is used alongside Locust bean gum, Carob gum and Guar gum in many sauces, ketchups and cold desserts.
Coagulation is the term used when proteins are affected by heat or enzymes. Egg whites when cooked turn from opaque to white and from runny to solid. Milk coagulates as it forms yogurt or cheese. Coagulation is also known as ‘denaturation’ meaning the food protein function and structure is changed and these changes cannot be reversed.
Enzymes contained in cheese cultures (or starters) are used to cause the milk proteins to clump together forming curds. The acidity of a milky mixture increases until the enzymes no longer function. Curds are salted and left to mature until the cheese is ripened.
Making a purée thickens juicy mixtures. Blenders and food processors reduce food pieces to minute particles very rapidly. A purée is a colloidal mixture of solid within a liquid, such as pastes, passata or purée soups.
Products such as fromage frais has a texture similar to yogurt. It is made from skimmed milk which is coagulated with a ‘culture’ to form curds. These curds are mixed with more cream to increase the fat content. On cooling, fromage frais becomes firmer but retains a light and fresh flavour.
Setting is an interesting change of state from liquid to solid. A sol becomes a gel.
Setting ingredients such as carrageenan, gelatine, agar or modified starches help to hold a set and enable food to withstand transport and carrying home from the shops.
Most hot starch thickened sauces ‘set’ when they are left to cool.
Pectin is found in underipe fruits and in citrus fruits and apples. It is used to thicken fruit syrups into jams and jellies. Sugar, acid and pectin combine to form a gel.
Carrageenan is a seaweed extraction used in quick set jelly mix, milk shakes and products that require smooth, thicker textures
Gelatine or Agar (vegetarian) helps to set dissolved liquids to create a colloidal structure holding the liquid in a ‘set’ known as a gel.
Lecithin in egg yolks is a natural emulsifier. Egg yolks are therefore excellent emulsifiers. Substances that would not normally mix, such as oils and watery liquids, will disperse together and remain in a homogenised thicker state, mayonnaise for example. Stabilisers are used to help sustain emulsions, as seen in salad dressings and frozen and chilled desserts.
OTHER PROCESSES THAT AFFECT THE TEXTURE OF FOOD
Fermented foods are increasing in popularity. Fermentation is the breakdown (solid to liquid) of organic food matter by bacteria or yeasts. Fermentation is not an easy process to control as it is sometimes lengthy and dependent on many factors. Popular fermented soya bean products are Tofu, Tempeh and TVP.
Tofu is produced from soya beans made into milk by boiling and mashing. It is highly nutritious. It is mild and bland and therefore mixes well with highly flavoured and spiced dishes. Tofu can be smoked to give it a stronger flavour, firmer texture and a brown colour.
Kefir is fermented dairy milk, traditionally goats’ milk. It provides healthy support for the immune system and the gut.
Soy sauce is fermented from soya beans to become either paler runny soy sauce or with added sugar syrup to make thick and sticky soy sauce.
Hoisin sauce is also fermented from soya beans with added garlic, sugar, salt and spices.
Oyster sauce is thick, rich flavoured and dark in colour. It is fermented soya with oyster sauce, caramel and salt. Oyster sauce is used in Chinese cooking.
Black bean sauce is fermented black soya bean, salt, sugar and spices.
Fish sauce is pungent and salty made from fermented fish or shrimps.
Rice vinegar is made from rice wine fermented to make a sharp and pungent vinegar.
Cider vinegar is made from fermented apple juice to create a pale, golden and clear acidic vinegar.
Yogurt is a fermented milk product using lactobacillus thermophilus or other similar helpful bacteria to thicken the milk. Yogurt can be produced from cow’s milk, ewe’s milk or other mammals.
Many yogurt desserts primarily thickened by bacterial cultures also need chemical help from stabilisers to create thick enough products to withstand transport to our shops. Greek yogurt contains more fat and is usually thicker and creamier in mouthfeel and texture.
Set yogurt is manufactured in the tub in which it is sold.
Yogurt is made from homogenised milk.
Homogenisation ensures that all the fat globules in the milk are evenly distributed.
The milk can be enriched with milk solids which coagulate during heating and thicken the yogurt.
If a yogurt is heat treated after fermentation it will not contain ‘live’ helpful bacteria and will be labelled UHT, pasteurised or sterilised.
If it is fermented and NOT heat treated the live bacterial remain and the yogurt will be labelled ‘bio’ or ‘live’ yogurt. Bio yogurts are popular due to the health benefits they offer to people’s digestive and immune systems. They are known as probiotics.