Understanding Sauce Souring
What Makes Sauce "Turn"?
While canned tomato products are commercially sterile in the can, microbes unknowingly introduced into a sauce at the kitchen level can create "off" flavors and premature spoilage. This technical information explains the sources of "sauce aging" microbes within the kitchen (e.g., untreated seasonings) to help you keep your tomato sauce at its flavorful best!
What Makes Sauce Sour?
Although infrequent, having your tomato-based sauce go "sour" or spoil can be an annoying problem. What makes a tomato-based sauce go sour? Unless the can was "bulged" before opening, acid-forming "microbes" that cause a "sour sauce" almost never come from the canned tomato products but rather from external microbe sources, including ingredients (e.g., seasonings) added by the operator making the sauce, and/or on-site sources. Naturally sweet tomato products that taste better may be lower in acidity and more susceptible to external microbes souring your sauce.
Microbes Are Everywhere
Microbes (yeasts, molds and bacteria) are everywhere. In fact, the growth of certain microbes is encouraged in the manufacture of many foods (e.g., cheese, wine, bread, salami, pepperoni, etc.) to add beneficial flavors in fermentation. However, this is not the case for tomato products which are heat sterilized during the canning process.
Canned Tomatoes Are Sterilized
Hermetically sealed (canned) tomato products are shelf-stable because internal microbes are killed by heat sterilization in processing. (In the absence of commercial sterility, live microbes in the product would quickly cause the can to swell, or to burst at the seams, or create a sour aroma easily noticed upon opening the can.) So, if the can isn't bulged or burst, and if the tomato product doesn't smell or taste "sour" when the can is first opened, the tomato product is commercially sterile upon leaving the can. However, if you are adding seasonings to your tomato sauce, you should know some facts: many seasonings (including dehydrated) carry very high levels of microbes (See chart.)
Why Do Untreated Seasonings Carry Microbes?
The group of dried plants commonly called "seasonings" includes leaves (e.g., oregano, basil, parsley), berries (white and black pepper), and root bulbs (onion, garlic, etc.). Many such seasonings are grown and harvested in third world countries, often under primitive conditions, where microbe contamination can be caused by soil, processing water, and/or unprotected exposure to the elements, animals and insects.
For example, seasonings like black pepper are generally harvested by hand and then spread directly on the ground or on mats to dry in the open air and sun, before being sold on a bulk price basis in commodity markets. Furthermore, seasonal crop shortages can also lead to increased imports of lesser quality seasonings from less desirable sources, like China, where conditions are particularly primitive.
Some Seasonings Aren't Microbe Treated
While processors of "fully prepared" tomato sauces use seasonings that are treated for microbes, for reasons of cost, as much as 25% of seasonings commercially sold to the restaurant trade are not treated. So even though a restaurant follows good sanitary practices, the operator may unknowingly be introducing microbes into his/her "scratch sauce" via added ingredients. This added microbe load from seasonings can be a greater problem when the tomato product is sweet-tasting (moderate acidity) rather than tart-tasting (high acidity).
"On-Site" Yeasts Can Also Be a Source
For Restaurateurs making dough (for pizza/bread) from scratch in the same kitchen where sauces are being made, the presence of the otherwise beneficial yeasts (airborne or on surfaces of utensils) can also increase the probability of sauce "fermentation" if the sauces aren't properly handled.