Thompson identifies tub tim grob in *Thai Food* as a Bangkok street and restaurant dessert — a preparation of city refinement rather than country tradition, its careful colour and texture the product of an urban aesthetic sensibility.
Water chestnuts coated in red-dyed tapioca flour, briefly cooked to a chewy, slightly crunchy exterior, served in sweetened coconut milk with crushed ice — the dessert that most vividly expresses the Thai kitchen's relationship with texture as flavour. Tub tim means ruby (from the deep red colour); grob means crunchy-crisp. The preparation's excellence is in the contrast between the ice's cold, the coconut milk's sweetness, and the coated water chestnut's unique chewy-crunchy texture — the tapioca coating translucent and yielding on the outside, the water chestnut crisp within.
**Ingredient precision:** - Water chestnuts: fresh water chestnuts, peeled and diced into 1cm cubes. Canned water chestnuts have approximately 60% of the textural character of fresh — their cell walls are softened during canning. Fresh water chestnuts are available at Chinese and Asian grocery stores. - Red food colouring: the traditional colour is deep red from artificial colouring (Thompson does not suggest a natural substitute — the colour is the visual signature of the preparation). - Tapioca flour (not tapioca pearls — the flour): the water chestnut pieces are coated in tapioca flour and cooked briefly in boiling water. The tapioca flour gelatinises in the hot water, forming a translucent, slightly chewy coating around each piece. - Coconut milk: sweetened with palm sugar and a pinch of salt. Chilled. - Crushed ice: essential — the ice's temperature prevents the coconut milk from becoming cloying. **The preparation:** 1. Toss the diced water chestnuts with a few drops of red food colouring. Allow to absorb for 10 minutes. 2. Coat in tapioca flour — toss to coat completely, shaking off the excess. 3. Drop into boiling water. Stir to prevent sticking. 4. Cook for 90 seconds — the tapioca coating turns from opaque white to translucent and slightly chewy. 5. Remove with a slotted spoon. Rinse under cold water (stops the cooking and removes surface starch). 6. Sweet coconut milk: combine coconut milk, palm sugar, salt. Warm to dissolve the sugar. Cool completely. Chill. 7. Service: crushed ice in a bowl, coconut milk poured over, coated water chestnuts floated in. Decisive moment: The 90-second cook and the immediate cold-water rinse. Under-cooked tapioca coating (60 seconds): the coating is not fully gelatinised and is powdery rather than translucent and chewy. Over-cooked (2 minutes): the coating begins to dissolve and lose its structural integrity. 90 seconds and immediate cold rinse: the tapioca sets in its translucent, chewy form.
*Thai Food* (2002); *Thai Street Food* (2010)