Here鈥檚 something for all rich tea fans who like to dunk the biscuit in their tea. Try dunking it first in hot, black, unsugared tea. When you bite into the dunked portion, you will notice that although wet on the outside, it is just as hard on the inside as if you had never dunked it. Now add milk to the tea, stir, and dunk another biscuit into it. Amazingly, the dunked portion is moist and soft right through. So why the difference?
鈥� The secret is in the milk鈥檚 butterfat and how this interacts with the biscuit鈥檚 ingredients and structure to help the tea soak into it.
Rich tea biscuits contain both water-soluble ingredients, notably sugars (around 18 per cent by weight), and hydrophobic substances like vegetable oil (around 14 per cent). The bulk of the biscuit is wheat flour (around 45 per cent), itself predominantly insoluble but water-loving starch. The vegetable oil helps blend and bind the other ingredients into a workable dough, and acts as a shortening to make it crumbly.
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Other ingredients include glucose-fructose syrup, barley malt extract, raising agents and salt, though their proportions vary between manufacturers. The malt is what gives rich tea its distinctive flavour, while the syrup fine-tunes the sweetness. The raising agents release carbon dioxide that permeates the dough during baking, rendering the biscuit porous 鈥� and accordingly lighter.
Both black and milky teas are drawn into the pores by capillary action. However, because black tea is primarily water, its progress is hindered to some degree by the oil lining the pores, so it fails to reach the inside of the biscuit during a brief dunking. At the same time, the sugar and salt in the biscuit draw the tea out of the pores, reducing the amount of liquid that can seep deeper. The sugar also dissolves, softening the biscuit鈥檚 moist outside.
鈥淏lack tea is primarily water and its progress into a biscuit鈥檚 centre is hindered by oil in the biscuit鈥�
Milk, however, contains globules of liquid butterfat suspended in an aqueous solution of mainly lactose. The fat-loving butterfat globules enable the milk to seep more readily through the biscuit鈥檚 pores, allowing it to reach, and soften, the biscuit鈥檚 centre. Of course, if dunking lasted long enough, black tea would also soften the biscuit right through.
If you like your tea milky, yet are anxious not to lose your biscuit鈥檚 inner crunch, you could try replacing full-fat milk with skimmed, or perhaps semi-skimmed. But if that doesn鈥檛 do the trick, and you happen to like black coffee, then that might make a refreshing alternative.