Tub Tim Krob — 'crisp rubies' in iced coconut milk

i. Origin & history

Tub tim krob is one of the best-loved Thai khanom (sweets), especially in the hot season, when it is sold from market stalls and street carts in bowls of crushed ice. It belongs to a large group of Thai iced desserts served in coconut milk or syrup — nam kathi and nam chuam — which also includes lod chong (pandan noodles in coconut milk, Thailand's version of cendol) and a whole range of jellies, beans and fruits over ice.

The name is poetic. Thapthim (ทับทิม) means both ruby and pomegranate in Thai, and the little red-coated cubes look like pomegranate seeds or polished gemstones. Krop means crisp, describing the fresh water chestnut inside. In Chinese-speaking communities in Thailand and Malaysia, the dessert is sometimes called by names meaning 'pomegranate seeds', for the same reason. The English spellings vary: tub tim krob, tab tim grob, thapthim krop.

The ingredients reflect Thailand's mix of influences. Water chestnuts are a Chinese ingredient, tapioca starch comes from cassava, a crop brought from the Americas, and coconut milk and jasmine-scented syrup are distinctly Thai. Like many street desserts it has no known inventor or date, but it is now a standard on Thai restaurant menus worldwide.

Its appeal is textural. Each 'ruby' gives first the soft, slippery chew of cooked tapioca, then the cool crunch of water chestnut, all in rich, slightly salty coconut milk that is chilled by ice. It is a dessert designed for heat.

ii. How the tapioca jacket forms

The magic of tub tim krob is the clear, chewy coating. The water chestnut cubes are first soaked in red-coloured water so they absorb the colour and stay moist on the surface. They are then tossed in tapioca starch, which sticks to the damp surface in a thin layer. When dropped into boiling water, the starch gelatinises almost instantly, turning from white powder into a translucent, glossy, chewy gel that clings to the cube — and the red colour shows through.

The coating must be thin and even. Tossing the cubes in starch, shaking off the excess, letting them rest a minute and then tossing again builds a good layer. Too thick and the jackets are gummy; too thin and they slip off.

The cubes are boiled in small batches until they float and turn translucent, usually 1–2 minutes, then plunged at once into iced water. The cold shock stops the cooking, firms the tapioca so it is chewy rather than slimy, and stops the pieces sticking together. Water chestnuts stay crisp even when boiled, which is why they are used rather than softer fruit.

The coconut milk is sweetened with a simple sugar syrup and seasoned with salt. Thai cooks often perfume the syrup with pandan or jasmine flowers. The salt is essential: it rounds the sweetness and brings out the coconut flavour.

iii. Ingredients

Makes 4 servings · scroll the side panel to adjust

• 200 g water chestnuts, fresh or canned, diced into 1 cm cubes
• 100 ml water, for colouring
• ½ tsp red food colouring, or 3 tbsp beetroot juice
• 100 g tapioca starch
• 100 g sugar (syrup)
• 125 ml water (syrup)
• 2 pandan leaves, knotted
• 400 ml coconut milk
• ½ tsp salt
• 300 g crushed ice
• 60 g ripe jackfruit, in strips (optional)

Fresh water chestnuts are crunchiest and sweetest; peel them with a small knife. Canned water chestnuts are convenient and work well — drain and rinse them. Tapioca starch is essential; cornflour does not give the same clear, chewy coating. Red food colouring is traditional; beetroot juice gives a natural colour but slightly earthy note. Pandan is optional but adds fragrance to the syrup.

iv. Method

1. Make the syrup: heat the sugar, 125 ml water and pandan in a small pan, stirring until the sugar dissolves. Simmer for 3 minutes. Cool and remove the pandan.
2. Mix the coconut milk with the salt and 4–5 tbsp of the syrup to taste. Chill.
3. Mix the food colouring with 100 ml water. Add the water chestnut cubes and soak for 15–20 minutes, until evenly red. Drain well, but do not dry.
4. Spread the tapioca starch in a bowl. Toss the cubes in it until coated, shake off the excess in a sieve, rest for 1 minute, then toss again.
5. Bring a large pan of water to a rolling boil and prepare a bowl of iced water. Shake off excess starch and drop the cubes into the boiling water in batches.
6. Boil for 1–2 minutes, until they float and the coating turns translucent. Lift out with a slotted spoon straight into the iced water.
7. Drain the rubies and toss them with a little of the remaining syrup to stop them sticking.
8. To serve, divide the rubies and jackfruit among bowls, add crushed ice and pour over the chilled coconut milk.

v. Tips & common mistakes

• Cut the chestnuts evenly. Uniform cubes cook evenly and look like gems.
• Coat twice. Two light coats of tapioca give a better jacket than one heavy one.
• Boil in small batches. Crowding drops the water temperature and makes the coating gummy.
• Shock in iced water. This makes the tapioca chewy and stops the rubies sticking together.
• Salt the coconut milk. Without it the dessert tastes flat and overly sweet.
• Serve soon after cooking. The tapioca hardens and turns opaque if left for hours.

vi. Variations

Modern versions use several colours — butterfly pea blue, pandan green and yellow — for a jewel-box effect. Jackfruit, young coconut strips and sago are popular additions. Some cooks swap water chestnuts for jicama (yam bean), which is similarly crisp.

For other iced desserts, see Malaysian cendol, Indonesian es teler and Filipino halo-halo. Within Thai sweets, compare butterfly pea flower pudding and khanom krok, and for another light coconut sweet, coconut jelly.

vii. Serving & storage

Tub tim krob is served immediately, in small bowls or glasses with plenty of crushed ice, as a dessert or afternoon refreshment on hot days. It is a natural finish to a spicy Thai meal.

Store the components separately. Cooked rubies keep in a little syrup in the fridge for up to a day, though they firm up and lose some translucency; the coconut milk keeps for 2 days. Do not freeze — the tapioca coating turns hard and opaque.

ix. More from Thai

• Butterfly Pea Pudding — blue coconut cups
• Khanom Krok — coconut-rice pancakes
• Sankaya — pumpkin custard
• Lemon Basil Sorbet — Thai-herb sorbet
• Cendol — pandan jelly in coconut milk
• Es Teler — Indonesian iced fruit cocktail