Peter Barham - Department of Physics, University of Bristol and Copenhagen University, Department of Food Science Kitchen Chemistry

Salt – why use it? Most people, when they cook, add salt to nearly everything. Almost all recipes call for a “pinch of salt”. But is it really necessary?

Cooking green vegetables Text books tell us the “correct” way to cook green vegetables is: Get a large pan of water boiling vigorously Add salt Throw in the vegetables (making sure the water does not come off the boil) Cook for two or three minutes Remove the vegetables and store over ice until needed Just before serving reheat by immersing for a minute in boiling water – drain and serve.

Cooking green vegetables Does any of this make sense? Boiling vigorously – water boils at 100°C Add salt – Why? – Flavour Hardly any salt “sticks” to the vegetables any remaining salt washes off on quenching Temperature Adding salt increases the boiling point of water by about 0.2°C Colour Bi-valent ions such as calcium can affect colour Sodium ions do not.

Taste sensations We each have five different types of taste buds distributed around the tongue and the sides of the mouth. Salt Sweet Bitter Sour Umami

Testing your taste Where do you taste the different sensations? Those of you I have given a piece of chewing gum please start chewing it now. Keep on chewing – do not take it out - we’ll come back to it later! Where do you taste the different sensations? Prepare dilute solutions of each of the five tastes Sweet - sugar, sour – lemon juice or vinegar, bitter – tonic water , salt - salt and umami - MSG. Take a cotton bud and dip it in one of the solutions – then wipe it on your partner’s tongue until moving around until they can taste it. (Repeat with other tastes). Remember which parts of your tongue are most sensitive to each sensation.

Testing your taste How sensitive are you to the different sensations? Now offer three solutions to your partner to taste – two the same tastant and one of pure water (or two water and one of the diluted tastant) – ask them which is the odd one out and why. Keep on testing, reducing the concentration of the tastant sample until they can’t identify it any more. Then repeat this time increasing the concentration of the tastant until they can identify it.

Perception of Flavour Flavour is the combination of taste in the mouth (from the taste buds) and aroma in the nose. Our noses are very much more sensitive than our tongues – we have around 400 different types of aroma sensor compared to just 5 types of taste sensors. Different people have different combinations of sensors so we all have different perceptions of a particular flavour.

Perception of Flavour Now what about your chewing gum. The flavour has long since gone. Now take a swig of the sugar water and see what happens.

Palate Fatigue & Cauliflower Risotto

Perception of Flavour We use all our senses to interpret flavour Colour of food affects apparent flavour – try offering a wine expert three red and three white wines to taste – they will use different sets of words to describe the flavour fo the red and white wines – even if the “red” wines are just the white wines with added food colour. What we hear affects how we eat – try recording yourself eating a crunchy food and then play the sound back while trying to eat something soft!

Cooking vegetables When you heat green vegetables the cellulose walls between the cells start to break down and soften the vegetables – at the same time nutrients and flavours can leak out.

Cooking vegetables – keeping the flavour Some flavour molecules are very soluble in water, while others dissolve easily in oil. So if we want to preserve as much flavour as possible in our vegetables we must cook them in a medium where the minimum amount of flavour can escape.

Cooking potatoes – “mash” Potatoes are made up of lots of starch granules. The amylose molecules in the starch granules are highly crystalline as grown. Importantly in these crystals all the amylose molecules point in the same direction. On heating above about 60°C these crystals melt. Once the crystals have melted the starch molecules form gels with the surrounding water. A single potato starch granule can swell up to make a gel of more than 99% water content. These highly swollen gels (not usually this swollen) give cooked potatoes their soft texture – and allow us to make flavoured mashed potatoes

Cooking potatoes – “staling” If you cool down a potato the starch will manage to re-crystallize But in the new crystals the amylose chains are packed up and down and not parallel as happens in the uncooked potato. This requires more space, so in this form the amylose co-crystallizes with some water molecules. This leads to the “rubbery” texture of cold potatoes. The same process causes bread to go “stale”

Cooking potatoes – “crispness” Often we want to make crisp potatoes To get the outside crisp we have to turn the starch gel layers on the outside of the potato into a glass. To do this we must remove most of the water without letting the starch recrystallize. So we remove the water at a temperature well above the melting point of the crystals (45°C) This is most easily done above the boiling point of the water (which is greater than 100°C) by immersing in hot oil. But if water diffuses back into the nice crisp glassy layer the glass will transform into a rubber state and the potatoes will become “soggy”

Cooking potatoes – “optimising crispness” A working method First cook the potatoes in boiling water for long enough that the outer layers absorb water and start to break up Next remove as much water as possible from these highly porous, high surface area, outer layers Then heat in oil to above 100°C (say 110°C) to form the crisp, glassy outer layer. Finally increase the temperature to around 180°C to brown the potatoes (and help prevent them going soggy later)