Doubling or halving a recipe is not always a straightforward mathematical exercise, as the way food cooks can change with its volume, surface area and the size of the pan.
Simply multiplying every ingredient and extending the cooking time can produce an under-reduced stew, scorched meat, collapsed cake or poorly browned stir-fry. In some cases, preparing two separate batches is the safest option.
Why braises need a different approach
When a stew or braise is doubled, the quantity of food increases but the surface area of the cooking vessel may not. That means roughly the same amount of liquid evaporates during cooking, leaving the dish with only half the reduction it would normally receive.
Halving a recipe creates the opposite problem. With the same surface area but less liquid, the contents can dry out or the meat can scorch before the cooking time is complete.
Changing the size of the pot is the safest solution. If that is not possible, a doubled braise may need around one and a half times the original quantity of liquid, while the other ingredients are doubled. For a half-sized recipe, about 60 per cent of the original liquid may be more suitable than a direct half.
The issue is most significant when a braise is finished uncovered or reduced at the end. A covered pot loses less moisture, although a larger quantity of food will still generally need longer to cook, while a smaller batch may be ready sooner.
Scaling rice and other absorption-cooked ingredients
Rice absorbs a set amount of water, but recipes also include liquid to allow for steam escaping during cooking. That loss is influenced by the cooking surface and duration, rather than rising in direct proportion to the amount of rice.
A useful approach is to allow about 1¼ cups of water for each cup of rice, then add roughly another ¼ cup for evaporation, regardless of the size of the batch. Thus, a recipe using one cup of rice and 1½ cups of water would not necessarily require three cups of water when doubled; two cups of rice would use about 2¾ cups of water instead.
The same principle can apply to beans cooked by absorption.
Why bigger cakes can collapse
Baking presents a separate difficulty because heat enters a cake from the outside. If the batter is doubled in the same tin, the centre takes much longer to set while the raising agents continue creating air pockets.
Those pockets can merge and escape before the crumb has developed enough structure, leaving the cake to crumble or sink. Making two separate batches is therefore the most reliable option.
If the ingredients are scaled, the tin should be changed too: a larger tin for a larger mixture and a smaller one for a reduced batch. The aim is to keep the batter at approximately its original depth.
Weighing ingredients can also improve accuracy, particularly with flour, whose volume can vary depending on how it is packed. To halve an egg, it can be cracked into a bowl, whisked together and measured before half is used.
Visual and flavour cues remain important. Cookies may be ready when they reach the right golden colour, while a dish may already be salty enough before every ingredient has been doubled.
More food does not always require more oil
The oil used to sauté garlic, onions or other ingredients is intended primarily to coat the pan and conduct heat, so it should be scaled according to the cooking surface rather than automatically doubled with the food.
This is particularly important when browning. Adding twice the ingredients to one wok or frying pan can crowd it and cause the food to steam instead of developing colour.
The practical answer for a doubled stir-fry is to cook it in two batches. Crowding is less important when onions are being softened for a soup or braise, where the ingredients can be increased and only enough fat added to coat the larger quantity.
