This is not a lie: I have a box of bicarb soda from pre-internet days. How long do things generally keep for, and what happens if I cook with them? PS. I always thought sifting dry ingredients was to catch weevils, and seeing as indoor kitchens and Tupperware have been in existence for a while now, it is one less thing to do. I mentioned it to a friend the other day and they are probably still rolling around on the floor in laughter. Have I been duped somewhere along the line?
Tupperware would certainly do the job with the weevils thing; however, if you do discover a weevil infestation, you can sift the suckers out. Even sans weevils, you should still sift (it aerates the flour). In short: yes, baking ingredients do go out of date, but, generally speaking, not for a very long time. Don’t worry too much about the best-before dates on your packets of flour and sugar—we’re talking years, not months nor decades. To prolong their lives, keep the ingredients in airtight containers in the freezer. They’re really not all that perishable, so unless they’ve been contaminated with mould or pests there’s little chance of getting sick from cooking with them—they just might not deliver the best consistency in a cake batter. As for your vintage bicarb soda: mix a quarter teaspoon with a splash of vinegar—if it bubbles immediately, it’s still good. Baking powder, on the other hand, does go off, especially in a humid environment, where it might not last more than a few months. To see if it still works, mix a teaspoon with half a cup of hot water and pray for bubbles.