I recently made pastry cream my usual way: whisking a mixture of sugar, eggs, milk and starch in a saucepan over a low heat until thick, before straining it into a large, clean metal bowl. I added a chunk of cold butter and, after it had melted, whisked it in, then left the mixture on the counter of my air-conditioned kitchen. I whisked the pastry cream several times until it was tepid, before transferring it to a smaller bowl, covering it with cling-film, then storing it in the fridge. A friend looked at all the dishes I'd used and asked why it was necessary to use so many. She thought I could have put the mixture from the saucepan straight into the smaller bowl, rather than using the large, interim container. I explained that I had been taught to cool down a hot mixture as quickly as possible, so it spends the shortest amount of time in the 'danger zone' of between four and 60 degrees Celsius. The first metal bowl facilitated cooling, because metal absorbs heat and the extra surface area of the larger bowl dissipated the heat faster. Adding cold butter also helped to cool the mixture. I learnt about the danger zone- the broad temperature range in which bacteria thrive- at culinary school. I was taught that food safety is vital- the last thing you want is to make your customers ill. Our instructors taught us to never put hot food in the fridge. Doing so would raise the temperature of the refrigerator, which might make food inside go off. Plus, when a large container of hot food is refrigerated, the surface starts to cool immediately but heat remains in the food underneath. The quickest way to make hot food cool enough to refrigerate is to use an ice water bath. Put a large quantity of ice cubes into a large bowl of cold water, then put the food (in a container) into the water bath. The water should come at least halfway up the side of the container holding the food, which should be stirred frequently to help dissipate the heat.