Use safe food-handling procedures. Start with clean kitchen equipment and surfaces, and use quality ingredients. Wash hands before handling ingredients and equipment, and at any time cleanliness is compromised. Limit airborne contaminants by keeping the starter loosely covered.

Flour is a raw agricultural product. Flour itself is not a ready-to-eat food and should always be cooked before consuming. Flour could become contaminated at any point along the food chain, especially at home when handling. Do not taste raw sourdough starter before baking. Instead, a bubbly appearance, tangy smell, batter-like consistency, expansion, and records of preparation steps should be used to determine when your starter is ready. The fermentation process will acidify the starter, which helps prevent pathogen growth. The baking step will kill any bacteria present.

Wild yeast is naturally on the flour and in the air. Yeast does not need to be intentionally captured from the air, nor does commercial yeast need to be added when making a sourdough starter. These wild yeasts are inactive, but under suitable conditions will become activated in the presence of water.

Contaminated starter should be discarded. Sourdough starter that shows any sign of mold (colored and/or fuzzy) should NOT be used, and the container should be thoroughly cleaned and rinsed before starting over. Sourdough starter may develop a liquid layer that smells of alcohol, and this is fine. The liquid is a by-product of the fermenting yeast and can either be poured off or stirred in. A sourdough starter that is kept in the refrigerator and not fed regularly may develop whitish clumps on the surface of the liquid layer that are safe yeasts, but not mold.

Monitor factors that influence microorganism growth:

Time: Creating a starter or rehydrating a dried starter will take several days of regular feeding. It will bubble and rise, and develop a pleasantly sour smell when ready to use.

Temperature: The fermenting microorganisms are more viable at temperatures that feel comfortable for you, a warm room temperature (around 70°F). Fermentation will slow at colder temperatures, and occur too rapidly or even stop when too hot for your own comfort.

Moisture: Water combined with the flour will provide the environment needed to cultivate the wild yeast and bacteria. Keep starter loosely covered to discourage mold development.

Acidity: Beneficial lactic acid bacteria (LAB) will produce lactic acid, which will increase the acidity, dropping the pH safely below 4.6. This rapid acidification of the sourdough starter will help limit growth of harmful microorganism, including mold.

Nutrients: Regularly spaced feeding intervals are necessary. Removal of some starter with each new addition of flour and water assists with nutrient access for optimal microbial growth. Flour type will also have an impact on the microbial development and final product.

Oxygen: Fermenting sourdough starters will produce carbon dioxide. The starter should be loosely covered in order to safely release the gas, but the culture does not require oxygen.