FAQ: Preventing Food Spoilage
Food preservation is governed by biochemistry and thermodynamics. Here are answers to common practical and scientific questions regarding food storage safety.
No. Clostridium botulinum endospores and the neurotoxin they produce are colorless, odorless, and tasteless. Food can carry a lethal dose while appearing completely normal, without gas bulging or off-odors. For this reason, low-acid foods must always be processed under verified pressure canning schedules that reach 116°C–121°C (240°F–250°F).
While traditional open-pollinated heirloom tomatoes often had natural pH levels below 4.4, many modern hybrid cultivars feature higher pH levels ranging from 4.6 to 4.9. This puts them in the low-acid category where C. botulinum spores can germinate.
To safely can tomatoes in a boiling water bath, you must lower their pH below 4.6 by adding two tablespoons of bottled lemon juice or 1/2 teaspoon of USP-grade citric acid per quart jar.
Microbiologically, food kept continuously below -18°C (0°F) remains safe from bacterial growth indefinitely, because liquid water is unavailable for metabolic activity.
However, quality degrades over time. Enzymatic oxidation, lipid rancidity, and moisture loss through sublimation (freezer burn) will eventually make the food unpalatable in texture and flavor, even though it remains safe from microbial pathogens.
Total moisture content is the percentage of water by weight in a food. Water activity (aw) measures the energy state of that water—how freely it can participate in chemical reactions or support microbial growth.
For example, honey contains around 18% total water, but its high sugar concentration binds that water tightly, yielding an aw around 0.55—too low for bacteria to survive. Conversely, a food product with the same 18% moisture but no bound solutes could have a higher water activity and support mold growth.
Iron-based oxygen scavengers absorb ambient oxygen, creating an anaerobic (oxygen-free) atmosphere. If added to a container of food with water activity above 0.85 (such as moist baked goods or uncured meats) held at room temperature, this oxygen-free environment encourages the germination of C. botulinum endospores.
Oxygen absorbers should only be used with dry goods that have an aw below 0.60, such as dried beans, white rice, rolled oats, and thoroughly dehydrated vegetables.
A thin white film on the brine surface is typically "kahm yeast"—a pellicle formed by wild aerobic yeasts (such as Pichia species) when the ferment surface is exposed to oxygen.
While generally non-toxic, kahm yeast can metabolize the lactic acid in the brine, raising the pH and eventually allowing spoilage organisms to establish. To prevent it, use an airtight fermentation vessel with an airlock and ensure vegetables stay fully submerged under the brine.