Acidity, spoilage, and the batches these documents tell you to throw away

Every number on this page comes from a regulatory text or a university extension publication, and each one is named where it appears. The reason is simple: fermentation and pickling are food preservation, and a wrong figure can make someone ill. Where the sources publish nothing, this page says so and stops.

Where 4.6 comes from and what it governs

The threshold of 4.6 is regulatory, not advisory. It is fixed by the FDA in the Acidified Foods regulation, 21 CFR 114, which defines acid foods as "foods that have a natural pH of 4.6 or below" and low-acid foods as "any foods, other than alcoholic beverages, with a finished equilibrium pH greater than 4.6 and a water activity (a_w) greater than 0.85." The same regulation places cucumbers, cabbage, cauliflower, peppers, and other vegetables, singly or in any combination, among the foods that become "acidified foods" when acid is added, and requires that "a finished equilibrium pH value of 4.6 or lower is achieved within the time designated in the scheduled process" (21 CFR 114.80(a)(1)).

What 4.6 governs in practice is the boundary between what may be processed in a boiling-water canner and what requires pressure treatment. The microbiological basis is printed in Appendix 4, Table A-1, of the FDA's Fish and Fishery Products Hazards and Controls Guidance, fourth edition, June 2022: 4.6 is the minimum pH for growth of C. botulinum type A and proteolytic types B and F. That table describes fishery products and is not a home vegetable recipe. Colorado State's botulism material states it plainly: "growth is inhibited at an acidic pH of 4.6 or lower," and "a pH near 7 or neutral favors the growth of C. botulinum." Oregon State's PNW 355 gives the practical version: "When the acidity of a canned food is high, harmful bacteria such as Clostridium botulinum cannot grow. That is why pickling (adding acid) prevents spoilage."

Where 4.0 comes from, and what it actually governs

The figure of 4.0 does not govern the safety of the product in the regulation. It governs the method of measurement. Under 21 CFR 114.80(a)(2), "If the finished equilibrium pH of the food is above 4.0, the measurement of the finished equilibrium pH shall be by a potentiometric method," while if the finished equilibrium pH is 4.0 or below, the measurement of acidity of the final product may be made by any suitable method. The same split appears at 21 CFR 114.90(b), which allows a colorimetric method "in lieu of the potentiometric method if the pH is 4.0 or lower."

Separately, 4.0 is the published minimum pH for growth of pathogenic E. coli and of S. aureus, in Table A-1 of the FDA appendix. A product at pH 4.0 or below therefore sits under the published minimum of those two pathogens in addition to that of C. botulinum. None of this adds up to a recipe instruction. No accepted source prescribes 4.0 as a target for home fermentation, and this page does not either.

The published list of what not to do

The prohibition on changing salt levels is the most repeated instruction in these sources. "Do not attempt to make sauerkraut or fermented pickles by cutting back on the salt required," writes the national guide. Penn State adds that the exact ratio "controls pathogen growth" and that "changing the proportions could result in an unsafe product." On the specific question of salt substitutes: "Use of reduced-sodium salt in fermented pickle recipes is not recommended," and PNW 355 carries the caution, "Do not use reduced-sodium salt in brined pickles or sauerkraut," because "these products need a specific amount of sodium to control bacterial growth and give a firm texture." Penn State notes one permitted reduction: rinsing at serving, never before canning, since cutting salt before canning lowers acid content and therefore safety.

• "Do not alter vinegar, food, or water proportions in a recipe" (the national guide; Colorado State). "It is not safe to reduce the amount of vinegar in recipes" (PNW 355).
• "Do not use garbage bags or trash liners" as containers (the national guide); garbage cans and non-food-grade containers are likewise excluded (PNW 355).
• "Do not use zinc, copper, brass, galvanized metal or iron utensils," which may react with acid or salt (PNW 355); the same exclusion covers aluminum and lead-glazed crocks (Penn State; Clemson).
• Vegetables must stay "1 to 2 inches under brine while fermenting," with the container covered by a clean, heavy bath towel against insects and molds (the national guide; PNW 355; Clemson).
• "Select tender vegetables without blemishes or mold" (PNW 355 safety checklist), and trim cucumber blossom ends, which "may contain enzymes that soften the pickles" (PNW 355).

Published causes of failure include "Insufficient amount of brine" and "Salt brine too weak during fermentation" (the national guide). If brass, copper, or zinc utensils were used, "do not use pickles" (the national guide). On heat treatment, PNW 355 orders: "Fermented pickles must not be processed until they have a sour taste," and marks it "extremely important to wait until fermented pickles taste tart before processing." Low-temperature pasteurization carries its own caution, "Use only when recipe indicates," and "Temperatures higher than 185°F may cause unnecessary softening of pickles" (the national guide).

Lime and alum, two additives with different standing

Food-grade lime may firm pickles, but "excess lime absorbed by the cucumbers must be removed to make safe pickles": rinse, then soak in fresh water for one hour, and repeat twice more (the national guide; PNW 355). PNW 355 adds the caution, "Do not use lime sold at garden centers or lumberyards." Alum is a divergence of use, not of safety, and it is not arbitrated here. Penn State prints that it "may be used safely to make fermented cucumbers crispier" but is unnecessary with good ingredients; PNW 355 excludes it from its publication and states it does not improve the firmness of quick pickles.

Garlic in oil, the anaerobic hazard on the counter

Oil combined with a low-acid food such as raw garlic creates conditions Clemson describes as "low acid, low oxygen, moist," held at temperatures between 38°F and 118°F. Unacidified infusions must be refrigerated and "used within two or three days," and unacidified oil left more than two hours at room temperature should be discarded (Clemson). The published acidification is a soak in a 3 percent citric acid solution for 24 hours, because "this is the amount of time required for the acid to fully penetrate the ingredients and bring the acidity beyond the growth limit of the botulism bacteria" (Clemson). Colorado State lists "chopped garlic in oil" explicitly among foods in which anaerobic conditions favorable to C. botulinum have developed.

Signs of a batch to throw away, and the published disposal procedure

During fermentation: "If the pickles become soft, slimy, or develop a disagreeable odor, discard them" (the national guide; PNW 355; Clemson; Penn State). Colorado State is firmer still: "Soft, slippery, or slimy pickles are spoiled and should be destroyed." Proper sauerkraut must show "firm texture, have brine that is not cloudy, and be free of any sign of mold or yeast growth" (Penn State). White or pink surface scum on sauerkraut "can be removed and discarded" (Colorado State). When opening a jar, PNW 355 instructs checking the vacuum seal, looking for cloudy liquid, rising bubbles, unnatural color, spurting liquid, mold, and off odors, with the caution, "If jars unseal during storage, do not consume."

The disposal procedure is published, not improvised. Any product not made from a tested recipe "should be treated as having botulism" (PNW 355). A still-sealed jar is labeled "Danger: Poison. Do not eat.," placed in a closed heavy bag, and discarded. An unsealed, opened, or leaking jar is detoxified first: cover the laid-down jars with one inch of water, bring to a boil, and boil 30 minutes, then cool and discard food and lids; disinfect contaminated surfaces with a bleach solution of 1 part unscented 5 to 6 percent sodium hypochlorite to 5 parts water, left for 30 minutes; soak metal utensils 30 minutes in 1 teaspoon of bleach per quart of water; wear gloves throughout (PNW 355). Colorado State concurs: "Boil suspect foods for 30 minutes before disposing to ensure that any toxin present will be destroyed."

What this page does not say: none of these regulatory and extension sources publishes a target pH, or even an achieved pH, for a home-fermented sauerkraut or pickle. That number was searched for and not found. The 4.6 figure is a regulatory boundary, the 4.0 figure is a measurement-method rule that doubles as a growth minimum for two pathogens, and neither is offered anywhere as a domestic fermentation endpoint.

Sources of the figures on this page

• Office of the Federal Register and the Food and Drug Administration, Title 21, Part 114, Acidified Foods, sections 114.3, 114.80 and 114.90, source 44 FR 16235, March 16, 1979; section 114.80 amended 80 FR 56144, September 17, 2015; CFR edition 2024. The regulatory definitions, the 4.6 equilibrium pH that separates an acidified food from a low acid one, and the 4.0 measurement rule.
• U.S. Food and Drug Administration, Fish and Fishery Products Hazards and Controls Guidance, chapter 13 and appendix 4, fourth edition, June 2022. The limiting conditions for pathogen growth: minimum pH, minimum water activity, maximum water phase salt and minimum temperature, organism by organism.
• National Center for Home Food Preservation, University of Georgia, Causes and Possible Solutions for Problems with Fermented Pickles, adapted from So Easy to Preserve, sixth edition, 2014. The published table of defects and their causes, and the instruction to discard.
• Colorado State University Extension, Botulism, by M. Bunning, C. Davis and P. Kendall, published December 2005, reviewed August 2025. The 4.6 threshold restated for the home kitchen, minimum water activities, and chopped garlic in oil.
• Penn State Extension, Botulism: A Deadly Concern, updated March 22, 2024. That the spores of Clostridium botulinum cannot grow in a high acid environment, and the requirement to follow a tested recipe.
• Colorado State University Extension, Making Pickles, published October 1999, reviewed October 2011. Non iodised salt, 5 percent vinegar, the prohibition on altering proportions, and the instruction to destroy soft or slimy pickles.
• Clemson Cooperative Extension, Home and Garden Information Center, Herb Infused Oils, HGIC 3471, originally published June 5, 2017. The conditions under which garlic in oil becomes a risk, and the published acidification and refrigeration limits.
• National Center for Home Food Preservation, University of Georgia, General Information on Pickling, adapted from the USDA Complete Guide to Home Canning, revised 2015, reviewed February 2018. The four classes of pickled product, the 5 percent vinegar requirement, and the prohibition on altering proportions.