Bonfire night activity – Cinder Toffee (Hokey Pokey)
Equipment – kitchen scales, large saucepan, wooden spoon, sugar thermometer or digital cooking thermometer, accurate teaspoon, baking tray or cake tin lined with non-stick baking paper (AT LEAST 15cm x 20cm x 2cm), small bowl of cold water.
2tbsp water, 200g sugar, 100g golden syrup, 40g butter, 1 tsp bicarbonate of soda, ½ tsp vinegar (any type apart from balsamic because its flavour is too strong).
put the water, sugar, butter and golden syrup into the saucepan and warm gently, stirring until the sugar has dissolved. Heat more strongly – don’t stir now – until it just reaches 138⁰C (it might be labelled ‘small crack’ on the sugar thermometer). Be very careful now because a drop on your skin will burn you.
Double check the toffee is ready by putting a drop into the bowl of cold water. You should be able to pull the ball into firm threads. Heat it a little hotter if you want more ‘crunch’ and less ‘chew’.
Remove the pan from the heat and stir in the vinegar and bicarbonate of soda. Measure these accurately and don’t be tempted to put in any extra bicarbonate of soda because the taste of soap left behind is most unpleasant. The mixture will foam up most spectacularly!
Pour the foaming mixture into the baking tray and leave to cool and set. You can score the toffee into squares using a sharp knife when it is still soft (after about 20 minutes) and it will break neatly when completely cold.
Two chemical reactions are happening to produce the bubbles in cinder toffee.
The bicarbonate of soda and the vinegar react together to produce carbon dioxide gas. The reaction mixture is so hot when the soda and vinegar go in that the reaction happens rapidly. Bicarbonate of soda is sodium hydrogen carbonate, NaHCO3. The acid in vinegar which reacts with it is ethanoic acid, CH3COOH. Here is the chemical reaction:
sodium hydrogen carbonate + ethanoic acid → carbon dioxide + water + sodium ethanoate
NaHCO3(s) + CH3COOH (aq) → CO2 (g) + H2O (g or l) + CH3COONa(s or aq)
The second chemical reaction is thermal decomposition of sodium hydrogen carbonate. This happens above 50⁰C, and the hotter the toffee mixture is, the faster the decomposition happens.
sodium hydrogen carbonate → carbon dioxide + water + sodium carbonate
2NaHCO3(s) → 2CO2 (g) + H2O (g or l) + Na2CO3 (s)
The toffee cools quickly where it has been stretched thin by the bubbles. It then hardens and the bubbles of carbon dioxide are trapped within its structure.