Stan and Odete, **the likely cause is yeasts or bacteria surviving in the ingredients and then multiplying in the bottled sauce.** They consume sugars in the chillies and produce gas. Washing bottles reduces contamination, but it does not preserve the sauce. Small batches may simply be eaten before this becomes noticeable. [University of Minnesota’s fermentation guidance](https://extension.umn.edu/food/preparing/cooking-at-home/food-preservation/fermentation)

**Garlic and chillies in oil also raise a botulism concern if the sauce is insufficiently acidified.** Adding lemon juice and vinegar does not, by itself, establish that the whole mixture is acidic enough. Discard the batches that are unexpectedly fermenting; do not taste them to check safety. Botulinum toxin cannot be detected by sight, smell or taste. [CDC guidance](https://www.cdc.gov/botulism/prevention/home-canned-foods.html)

For future batches:

- **To keep your present recipe:** treat it as perishable. Refrigerate immediately at **4°C or below**, use within **four days**, and freeze extra portions. This follows the conservative guidance for homemade garlic-in-oil mixtures. [National Center for Home Food Preservation](https://nchfp.uga.edu/how/freeze/vegetable/freezing-garlic-in-oil/)
- **For bottles stored at room temperature:** use a tested preservation recipe and follow its ingredient proportions, container sizes and heat-processing instructions exactly, including altitude adjustments for Johannesburg. A tested [Easy Hot Sauce recipe](https://nchfp.uga.edu/how/can/how-do-i-can-tomatoes/easy-hot-sauce/) is one option. To bottle your particular oil-containing recipe, have a food-preservation specialist validate its acidity and processing method.
- **Stop washing the chillies with household bleach.** Wash produce under running water. Bottle sanitation cannot replace a safe preservation process. [FDA produce guidance](https://www.fda.gov/media/77279/download?attachment=)

An equilibrium pH of **4.6 or below** is an important botulism control, but measuring that alone does not establish a safe shelf life or prevent all yeast growth; a validated heat process is also needed. [University of Minnesota’s canning guidance](https://extension.umn.edu/food/preparing/cooking-at-home/food-preservation/canning)