Sir

I read with fascination Len Fisher's Commentary on the science and art of biscuit dunking (Nature 397, 469; 1999). Regrettably, I have not dunked for some years, as the outstanding biscuit for this enjoyment, the traditional English ginger nut, is rare in my country. Danish butter cookies are unsuitably crumbly. However, as a formerly avid dunker in England, I challenge his recommendation of near-horizontal (NH) dunking.

This technique allows dunking only if you abstain from drinking the liquid in your cup. In fact, the minimum dipping angle will exceed the recommended angle if you have navigated your cup through a crowded cafeteria. No: surely it is a question of simple timing combined with vertically-held (VH) dunking. The same formula applies, but the dunk time must be reduced to one quarter of that of an NH biscuit. To prove this, consider that the desired result is a biscuit stabilized by a dry layer, and that the biscuit is now soaked on both sides.

The desired thickness of remaining dry biscuit (whether lateral or central), however, is also related to the transfer time to the oral cavity, as capillary forces will continue to draw liquid from the supersaturated outer layers into the dry layer after withdrawal from the cup. Even a properly dunked VH biscuit, which is considerably more stable than an NH biscuit (there must be a formula for this), will produce an undesirable mush if the dunking formula ignores cup-to-mouth transfer time.

The problem of ‘asymmetrical’ glazed biscuits, which have an upper surface notoriously less permeable than the lower, needs further research. The physics gets really complicated with the abhorrent dunking practice observed in Latin countries — applying butter and jam to a croissant and then down into the coffee.