Make sure that you read the important update at the end!
It's summer here, and it's been really hot, often 35-40ºC. We've got tomatoes coming on and the freezer has filled up. The last thing I want to do during this time of year is heat up the house with more cooking inside, and I've been thinking for some time that it make sense to take advantage of the hot weather.
I've been looking at how to make use of recycled materials to build a solar cooker. You can find out about the basic principles here. A point to note about my approach is that I am lazy and apply the proverb 'don't do what you don't have to' - seeing what I can get away with before trying to perfect the process. Or, if you prefer, Principle 7: 'Design from patterns to details'.
My first attempt (last summer) was to use a small satellite dish to reflect light to heat water in a billy (a parabolic cooker for fast heating). I had a dish, and I had some white paint. What I learnt from this experiment is that you need to use a more reflective surface than just white paint and a larger reflective area to bring water to the boil. Still, it did heat the water, but not a lot.
|Using a satellite dish to reflect light to heat water|
First I found some cement sheet to use as a flat base. I used some left over insulated reflective foil (permifloor 500 left over from the house build) on top of the sheet which I sat a tyre on top of. I then filled the tyre with some wool, to help insulate the 'box', and then made up a large cylinder from the foil to fit inside the tyre. Finally I placed a large sheet of glass on top.
|Tyre placed on flat base with reflective insulated foil (permifloor) then filled with wool for insulation.|
I half filled a small pot with water for my first test and placed a BBQ thermometer in the chamber to observe the temperature. The pot was aluminium (a good conductor), painted black (to absorb more heat) and it was sitting on corks (no thermal bridging). On a clear 35º day the temperature inside reached 80-90ºC, not quite boiling point. The addition of a highly reflective surface, like a mirror, to reflect more light in would certainly help. I was also thinking of using a sheet of double glazed glass to help retain heat inside as ways to improve the design - but didn't have one on hand.
I remembered an article that I read in Grass Roots magazine (i think) about pasteurising using a solar cooker, and realised that 80-90º would be a perfect temperature range to do this with the current design. It's best not to boil food in the pasteurising process. We had been freezing tomato puree from our home grown harvest in order to collect enough to justify using our Vacola kit. With the Vacola process the jars would sit inside a vessel of water which is heated to just below boiling point for about an hour, where by the jars are removed and allowed to cool. This kills bacteria inside and provides a vacuum seal which means that you can store the food (in a cool dark space) for an extended period of time - even years. I decide to take a punt and try using the tyre cooker instead.
After defrosting the tomatoes I cleaned our jars and lids and filled them with the puree (not quite to the top). I fitted the lids (not too tight) and placed them in the tyre at around midday. The internal temperature rose slowly through the day to 70-80ºC, after about 3 hours I removed the jars. I left them overnight to cool and was wrapped to discover that all had vacuum sealed by morning.
The beauty of the process is that you can preserve on the go, no need to freeze until you collect enough - one jar at a time is fine - provided that it's a hot sunny day. It also means that you don't have to heat your house or use water or electricity unnecessarily.
24th Jan 2013 IMPORTANT UPDATE:
While I thought that the pasteurising went perfectly, it didn't. Yesterday I was a bit upset to discover that several jars had began fermenting. Kunie poured out the content, recooked it and bottled it using the tried and tested Vacola method. A shame because it will now taste NQR (not quite right) - (Further update: we used the tomato on pizza that night and it was really yummy). I really should have tested with just one jar - I was overconfident.
I'm still keen to experiment further with this idea - perhaps using a mirror to reflect more light in, and / or using a double glazed piece of glass. Leaving the jars in for longer should help too.
|Jars of tomato with lids screwed on placed within along with a thermostat to monitor temperature. A sheet of glass sits on top of the tyre.|