Q: Recipes for homemade mayonnaise say you should use it within three days. I make my own and keep it as long as store-bought mayonnaise, sometimes for six weeks. What am I risking?
A: You are risking food-borne illness. Commercial mayonnaises are made with ingredients that have a specific level of acidity. That inhibits the growth of bacteria, keeping the mayonnaise safe.
Homemade mayonnaise doesn’t keep as long for several reasons. You probably used fresh lemon juice, which may be lower or higher in acidity than bottled lemon juice. There may not be enough to inhibit bacterial growth.
Next, you handled all the ingredients – the eggs when you cracked them, the lemon when you squeezed it, the mayonnaise when you moved it to a container. You could have introduced a slight amount of bacteria. Given enough time and exposure to higher temperatures, that slight amount will have time to grow into a lot.
Finally, you’re taking the mayonnaise in and out of the refrigerator without tracking how long it’s been at room temperature. The two-hour rule, the time that food can be between 40 and 140 degrees before bacteria begins to grow, is cumulative. Every time you take it out of the refrigerator while you use it, you’re adding to the amount of time it has been exposed, giving bacteria plenty of time to multiply.