My home-made ketchup needed sterilising and I realised I had a temperature-controlled water bath sitting on top of my kitchen cupboard! So this instructable explains how I used a Sous-Vide machine to sterilise some tomato ketchup.
I think you could use a slow-cooker set on the low setting quite successfully too, but you would need a good thermometer and to monitor the temperature carefully to make sure it got hot enough for the required time.
(Proper canning equipment is not common in the UK, so most people use either a big pan of water on the cooker - hard to control the temperature - or a pressure cooker - but only small containers will fit.)
Step 1: Fit Your Bottles Into the Water Bath
Get the empty bottles you are going to use, and work out how to fit them in the water bath. I had to tilt the bottles and wedge them into the rack.
Step 2: Fill and Pre-Heat the Water Bath
Fill the bath with hot water and pre-heat. My old preserving book said the ketchup needed to be in a water bath at 77 deg C for 30 minutes, so I pre-heated the Sosu-vide machine to 77 deg.
Step 3: Sterilise Your Preserve
Make sure the caps are on the bottles, but not too tightly. I put the ketchup in hot, straight from the pan, and sterilised it immediately - so the ketchup was pretty hot, therefore wasn't going to expand a lot and make the bottle explode!
I was using recycled commercial ketchup bottles (we eat a lot of ketchup!) so I checked that the cap seals looked OK. It would be better to use preserving bottles or jars with rubber sealing rings and clips, if you have them.
Put the bottles in the water bath, and add hot water if necessary so the level is above the level of the preserve in the bottles. Put the lid on and wait until the water bath reaches the correct temperature again (77 deg C), then time for 30 minutes.
Remove the bottles and tighten the caps immediately, while they are hot.
Store the ketchup in a cool place. It should keep for six months if your sterilising worked. Check when you open it that there is no fermentation or mould.