Ordinary potatoes and sweet potatoes have a very similar consistency when raw, yet sweet potatoes roast in about half the time. What is the reason for this?

Alan Fellow, Edinburgh, UK

Why does the bottle of red wine vinegar I buy from my supermarket start off without a sediment and then, after a couple of months, acquire it?

Niall O'Sullivan, Liverpool, UK

If I freeze cakes with icing on them, when I defrost them the icing becomes soft and sticky. Why?

Paula Paine, Plymouth, Devon, UK

When I mix Ribena and Schweppes Diet Lemonade, the mixture seperates after a few seconds, with the Ribena on the bottom. But when I mix Ribena and Schweppes Original Lemonade this does not happen. Why is this?

This question arose during a conversation with my director regarding knife sharpening. I proposed that it should be possible to sharpen a knife with an object less hard than the knife itself, although it would take longer and the sharpening material would rapidly degrade. Thus in theory, you should be able to sharpen a knife with a block of parmesan, if you had enough time and enough parmesan. Suffice to say he disagrees. 

When I heat a tomato-based soup or pasta sauce either in the microwave or on the oven hob, the sauce splutters and spurts all over the place as it heats, usually leading to a clean up of the oven or microwave afterwards.

However sauces or soups that contain no tomato (e.g. chicken or beef soup, cheese sauce, gravy) don't produce this mess despite being cooked in exactly the same way.

What is it about tomato based products that cause this irritating culinary reaction?

Whilst the oven is on cooking what I fancy for my dinner I’ve always filled any empty shelves with food that could do with being baked and then frozen it for another day with the belief that I am saving energy by using up what would otherwise be wasted space in a hot oven, however, my question is this: am I in fact wasting energy by placing items upon an empty shelf that will now absorb energy to cook, energy that the oven must work to replace? Would it have been more efficient to leave the shelf empty so the oven needs only maintain the temperature?

The other day I cooked some egg pasta in salted water and left the pot on the stove. I was away for a couple of days afterward and was surprised on return at the extent of the bacteria that had grown in the slurry at the bottom of the pot. Can anyone identify the bacteria from the photo in particular the red blooms?

Dried, leftover cereal such as Frosties forms a cement which can take quite some scrubbing to remove, even with detergent. Why?

I have been informed by my family that the love of soup is genetic for the Scots. Myself, I can take of leave it, I am confused to whether it is a main or an app.

This weekend however I popped a bowl of soup into the microwave, and after 2 minutes when I reached in to get the bowl back out I found that the bowl was so hot that I an oven glove was required to retrieve it. The contents however were only lukewarm. I am ashamed to admit that I know very little about microwave ovens, it is therefore perhaps unsurprising that I am perplexed as to why the device would heat the ceramic bowl more than it would the contents. Emperical study would suggest that some ceramics get hotter than others. What's going on here then?