Why does the milk curdle in a oven baked rice pudding?

Rice contains spores of *Bacillus subtilis* and *Bacillus megaterium*. These spores survive in uncooked rice and resist the heat of the oven. Once the rice pudding is cooked, these spores germinate within 15 minutes to 3 hours. Germinating spores can quickly increase in numbers, reaching 10 million per milliliter within 12 hours. Because the oven is closed and the temperature is constantly between 185°F and 212°F (*B. subtilis* and *B. megatherium* optimum temperatures are 185°F and 176°F, respectively), the rice pudding is an ideal medium for the growth of these bacteria.

The bacteria can produce protein-digesting enzymes known as proteases, which break down protein molecules into smaller fragments, including peptides and amino acids. These peptides and amino acids can form large chains of interconnected proteins, causing the milk to thicken and curdle. The proteases produced by *Bacillus* can remain enzymatically active at a pH as low as 5.0, so the bacteria are not deterred by the slightly acidic nature of the rice pudding.

The curdling can also be accelerated if the rice pudding contains high level of calcium. Calcium can aid the action of proteases or cause heat-induced reactions with other protein components in the milk.

Preventing Curdling

To prevent the milk from curdling, you can:

- Use ultra-high temperature (UHT) milk, which is heat-treated to a high temperature for a short time, killing most of the bacteria, including *Bacillus* spores.

- Add a small amount of salt to the rice pudding. Salt can help to inhibit the growth of *Bacillus* spores.

- Cook the rice pudding at a lower temperature( less than 180°F). This will slow the growth and spore germination of *Bacillus*.

- Add the milk to the rice pudding just before serving. This will reduce the amount of time the milk is exposed to the heat and thus reduce the chances of curdling.