ask the chef

I often buy tonic soups from a soup-delivery service. It arrives hot in a Thermos flask and I put it immediately in the fridge to cool down so I can drink it later. The last time I did this the soup turned sour. What caused this and is it unsafe to drink?

If you want to cool the soup, you should pour it into a larger container and let it cool uncovered. The long, narrow design of the Thermos flask and its insulated sides all serve to keep the soup hot because there's little surface area to let the heat and steam disperse. You need to cool the soup as quickly as possible. Foods have a 'danger zone' from four to 60 degrees Celsius (40 to 140 degrees Fahrenheit) in which bacteria thrive (this doesn't apply to dry-storage products). By pouring the soup into a larger container, you give it a bigger surface area, allowing it to cool more quickly. Stir occasionally so the hot soup at the bottom of the container has a chance to come to the surface.

Safety is essential with all foods. If you're cooking something and want to cool it to reheat and serve later, put it in a container with as much surface area as possible - wide and shallow instead of long and narrow. If the room is warm, use an ice-water bath; put the container of food into a larger container filled with ice and water. This lets the food cool on the sides and bottom as well as the surface. Once it has reached room temperature (or close to it), it can be covered and refrigerated.

That your soup smelled sour was a good indication it was bad, but food that has gone off can smell and taste fine. This is why it's important to handle food safely at all times. Food service professionals have a saying: 'When in doubt, throw it out.' It's advice we should all follow.

I made your recipe for chocolate sauce and it was really astringent. Shouldn't it have contained some kind of sweetener? What kind of chocolate did you use?

The sauce recipe I gave contained bittersweet chocolate and cream. I suspect you used bitter chocolate instead. This contains no sugar at all (it is sometimes labelled 'unsweetened chocolate') and its taste, as the name implies, is bitter. Bittersweet chocolate contains a small amount of sugar, although the amount varies from brand to brand. Taste several to find the ones you like; I almost always use Valrhona, which I buy in one-kilo blocks (the best price seems to be at Great in Pacific Place, where - last time I looked - it costs $149 a kilo). I also like Lindt, Tobler and Callebaut, although these seem to be available in Hong Kong only in 100 gram bars.