Fats Fat plays various roles in food, affecting mouth-feel and the perception of temperature, as well as helping us to determine how big a bite of our sandwich we take. Jon Prinz explains all.

In this article

Fats and oil as a lubricant Thermal factors

Bread and butter Reducing fat

These days fat is a dietary demon, but it wasn't always so. Go back 100-200 years and many common foodstuffs were much fattier than their contemporary equivalents are. Pigs, for example, are now bred to be leaner because shoppers have demanded meat that is less fatty. 'Fat' is equated with 'bad' in many people's minds, while 'low-fat' foods are deemed better for health. At the same time, we also know that fatty and/or oily foods taste great - hence the great 'fat dilemma'.

The challenge for chefs and food scientists is to produce foods that are low in fat and that also taste good. It's not an easy task, because fat plays multiple roles in food - some related to physics, some related to chemistry and some linked to the psychology of food - which is why fat can't be dismissed out of hand as 'bad'.

Fats have multiple roles in determining how the food tastes: they affect mouth-feel because of their ability to lubricate; they affect the perception of temperature; the way fat is added to food during cooking can affect the taste; and the amount of fat in a mouthful of food also has a role in determining how big a bite we take.

Too little lubrication and food feels dry and or grainy, too much and it feels slimy. Somewhere in between lies perfection

It will come as no surprise to learn that fats and oils are lubricants - it's this ability to reduce friction between our tongue, cheeks and palate that gives them their characteristic mouth-feel. Too little lubrication and food feels dry and or grainy, too much and it feels slimy. Somewhere in between lies perfection. Many other materials - for example, gels - also reduce friction and can be used as partial fat replacements.

Smaller droplets of fat work better as lubricants than larger ones: when making mayonnaise, for example, the more the vegetable oil is mixed into the eggs, the smaller the oil droplets become and the smoother the mayonnaise. That's why one of the most important pieces of kitchen equipment is the mixing bowl. Find out how the amount of mixing affects the perceived fat content of food with this simple experiment:

It's been generally assumed that fat is detected by its effects on flavour, and by its lubrication of the oral mucosa (a mucous membrane that covers all structures inside the mouth, except the teeth). Recently scientists have proposed a third mechanism - its effect on the thermal conductivity of the food - as another detection system.

On a cold day metals feel cold, while wood and plastic feels warm, even though they're the same temperature, because of their relative ability to conduct heat (metal being a better conductor than wood or plastic). Foods also vary in their heat conductivity - even when served at the same temperature, high-fat foods feel warm and low-fat foods feel cold. Your lips and tip of the tongue are highly sensitive to temperature change. Try this simple experiment to find out how temperature can affect the perception of fat in foods:

The amount of fat in a mouthful of food also has a role in determining how big a bite we take. Even eating simple food, such as a slice of bread, involves a number of complex decisions. Where shall I make the first bite? How big a bite should I take? How many chews should I make? How much saliva should I produce?

After the first bite things get even more complicated because of the complex shape left behind - yet people manage to take the same amount of bread in each bite. If we replace the dry bread with buttered bread, we take bigger bites because we no longer have to produce as much saliva to wet the bread because the butter acts as a kind of 'raincoat'.

Find out more about the science behind bread making (with Professor Tony Blake)

Reducing fat in food is not as simple as just leaving the fat out or using some other food ingredient to substitute for the fat. We need to replace the lost aromas, control the taste, mimic the mouth-feel, and engineer how the food will be sensed once it's in the mouth.

It's also important to remember that the food that we actually swallow is far from being the same thing that was on the plate. It has been crushed and crunched by the teeth, soaked in saliva, some ingredients have dissolved and fats have melted. We need to take all these factors into account because no matter how beautifully it might be presented, the ultimate test of any food is what happens to it in the mouth.