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Henry Shaw, UC Master Food Preserver Online Program Volunteer

Part 1: When is a tablespoon not a tablespoon?

Common salt, sodium chloride (NaCl), is the most widely used condiment worldwide. It’s also the only rock we normally eat. (“Halite” is the geological term for rock salt.) Today, you can buy a pound of salt for a dollar or so, but a few centuries ago, salt was literally worth its weight in gold (extended history down below). 

In food preservation recipes, salt can play two different roles. Often, it’s simply a flavoring agent, in which case the amount of salt in a recipe can safely be adjusted to suit one’s personal preferences. For some techniques, though, the quantity of salt used in a recipe or process is critical to the safety and quality of the final product. Prime examples of when the salt content is important are recipes for lacto-fermented products like fermented pickles, sauerkraut, and kimchi. In the lacto-fermentation process, beneficial lactic-acid bacteria (primarily Lactobacillus species), convert the sugars in food to lactic acid, which lowers the pH and creates a characteristic sour flavor. The resulting acidity of the final product is important for the long-term preservation of the product and for suppressing the growth of Clostridium botulinum (the bacterium that causes botulism).

How do we encourage the growth of desirable bacteria and suppress the growth of undesirable bacteria and molds during fermentation? If you’ve read this far, then you might guess that salt is involved, and it absolutely is! Adding salt to your ferment is how we “rig the game” to help lactic-acid bacteria win the microbial race against the growth of undesirable microorganisms during fermentation. At salt concentrations between 2% – 5% by weight, lactic acid bacteria grow more quickly than other microbes and have a competitive advantage. At lower concentrations, undesirable bacteria can survive/thrive, possibly out-competing lactic-acid bacteria and spoiling your ferment. Too much salt will suppress the growth of lactic-acid bacteria, leaving your vegetables unpickled. Furthermore, salt-tolerant yeasts can grow more quickly, consuming lactic acid and making the product less acidic and more hospitable to spoilage microbes.

Unfortunately, most fermentation recipes specify the amount of salt to use in terms of volume (i.e., X tablespoons of salt per quart of brine or pounds of produce). The problem with this is that there are many different types of salt, and the size and shape of individual particles in these salts can vary dramatically. These microscopic differences in the texture of the salt change the “packing density” of the salt particles, causing the weight (mass) of salt per tablespoon to also vary dramatically. Ideally, the quantity of salt and all other recipe ingredients would be given in terms of weight, which would eliminate the uncertainty in how much salt to use. If a recipe uses volume to specify the amount of salt it should also specify the type of salt to use. If it does not, use a salt designed for canning and pickling. If a specific type of salt is specified, be sure to follow the recipe and use that type of salt.

My wife and I like to cook so we have collected many different salts in our pantry. For fun, I compared the weight of a tablespoon of the eight different salts we have on hand. I also took microphotographs of these salts to illustrate the large differences in particle size and shape in this sample of the “salt universe”. I used a 2-tablespoon measure to scoop each salt from its container and leveled the measure with the back of a knife. The weight of each salt was measured with a calibrated kitchen scale that’s accurate to ±0.02 g. The resulting weights were divided by two to get the weight per tablespoon shown in Figure 1 and tabulated in Table 1, graphed in Figure 2.  

I was surprised by the magnitude of the variation in bulk densities of the salts I measured. Morton Canning & Pickling Salt was the densest, weighing in at 18.9 grams/Tbs. while Diamond Crystal Kosher Salt Flakes was the “fluffiest” and least dense, weighing in at 8.2 grams/Tbs. That’s a factor of 2.3 difference; one would need to use 2.3 tablespoons of the Diamond Crystal Flake salt to equal the mass of 1 tablespoon of the canning salt! If I were following a standard recipe that says to use 3 Tbs. of canning salt per 5 pounds of cabbage in a sauerkraut recipe (aiming at adding 2.25 – 2.50% salt) and I used 3 Tbs. of the Diamond Crystal Flake salt instead of the canning salt, the mixture would not have nearly enough salt to ferment properly and it would probably end up a rotten mess. I would have needed to add 6.9 Tbs. (almost ½ cup) of the flake salt to have added enough!

The microphotographs in Figure 1 tell the story; the canning salt consists of very uniform, ~0.5 mm rounded cubes while the two “flake” salts (Penzey’s and Diamond Crystal) have larger, highly irregular particles. The uniform and smooth-sided particles of canning salt can pack more closely. In contrast, the irregular, jagged flakes of the flake salts prevent the close packing of the individual salt particles and leads to a less-dense salt.  Our usual “table salt”, the Diamond Crystal Granulated Salt, had a density about 10% less than that of the canning salt, so it would be a reasonable substitute for canning salt.  Its particles are more irregularly shaped than the canning salt and they have more variation in particle size, but their rounded shapes allow them to pack at a relatively high density. The average particle size does not seem to be the primary factor determining the bulk density of salt; the shape of the particles is more important. Flaky, irregular particles pack more loosely leading to lower bulk densities.

Figure 1. Macro- and microphotographs of various commercially available salts. The white scale bar in the microphotographs is 3 mm. long. The weight of salt in a tablespoon of salt is given below each salt type and in Table 1.

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Table 1. Characteristics of various commercial salts 

Figure 2. Bar graph showing the bulk density (grams of salt per tablespoon) of the salts listed in Table 1.

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Image credits: Henry Shaw, 2025.

Our salts include two “specialty” items that one would not normally think of using in a ferment: the Maldon Salt and the Fleur de Sel. Both are “finishing” salts, intended to be sprinkled on foods just before they are served or as a top-garnish on baked goods or chocolates. They are both characterized by very large, thin particles, with the Maldon particles having a characteristic “hopper growth” shape (think of a hollow, stepped, Aztec pyramid). As one might expect from the shape of the particles, both these salts came in at the lower end of the range of bulk densities.

Stay tuned for Part 2 of this series of articles on salt, in which I’ll discuss the chemistry of various salts as it related to food preservation… Can I use iodized salt in fermentation? What are “anti-caking agents” and why do I care? What’s the difference between sea salt and regular salt? Why is “Himalayan pink salt” pink and can I use that in preserving?

[Disclaimer: The mention of specific products in this article does not imply endorsement or opposition of these products by the University of California. Product names are only given for informational purposes.]

Extended history...

Some salt is a necessary part of our diet to maintain our electrolyte balance, nerve function, and muscle function. Perhaps more relevant to this article, salt has been used for centuries in food preservation and was highly valued for that purpose. Salt deposits are rarely seen exposed at the surface of the Earth; they rapidly dissolve in rainwater and get washed away. Communities located far from the ocean typically had no local source of salt and relied on trade via long caravans or river routes like the famous Trans-Saharan salt trade or China's salt roads. These inland societies traded valuable goods like furs, grain, or gold (sometimes on one-for-one basis by weight), in exchange for salt. Today, we produce hundreds of millions of tons of salt globally per year by evaporation of seawater and other natural brines, and hard-rock or solution mining of large, underground halite deposits. 

Travis Alexander, UC Master Food Preserver Online Program Coordinator

In our funding agreement with the California Department of Food and Agriculture, we committed to conducting periodic evaluations (6-month, 12-month, 18-month) of our monthly newsletter. We would greatly appreciate it if each of you could take the time to fill out the 6-month evaluation using the link below. As an extra incentive, one lucky respondent will be chosen at random on May 30 to receive a food preservation booklet collection (up to a $50 value). Thank you in advance for your time. 

Click Here for Evaluation

Kirsten Hansen, UC Master Food Preserver Online Program Volunteer

County of residence: Out of State

My name is Kirsten Hansen. I joined the inaugural class of the University of California Master Food Preserver (UC MFP) Online Delivery program in December 2023, graduated in the summer of 2024, and have been volunteering since then. I wanted to participate in a UC MFP program for a long time, but never lived in a county that offered it. So when I learned about the Master Food Preserver Online program, I jumped at the opportunity!

I’ve been water bath canning since I was very young. My parents had a large garden, and we canned lots of tomato sauce every year, plus some jam. We also froze a lot of produce. Once I got a little older, I joined 4-H and began making more jams and pickles to show in our annual 4-H fair. I still can a lot of tomatoes; I hauled home 20 pounds of tomatoes from the farmers market on my bicycle last summer and processed 40 pounds in a weekend! I kept canning while living in numerous suboptimal rental apartments, and now have an atmospheric steam canner and dehydrator, in addition to my hot water canning set up. I hope to add a pressure canner and chest freezer once I have a little more space!

and limited storage space. I do my boiling water canning in a large stock pot and most of my canning equipment fits in a 1’x1’ cube organizer. I’m passionate about demonstrating that you don’t need a lot of money, space, or special equipment to preserve food safely and deliciously.

The most valuable thing I’ve learned during my time as a UC Master Food Preserver is the why of food preservation. It’s reassuring to understand the mechanics of foodborne illnesses so that I can undertake the proper safety methods when preserving, including using research-based best practices and recipes from reputable sources. I find the science behind food preservation fascinating and it’s satisfying to have answers to why things sometimes go wrong, like jam failing to gel. I also have a lot more confidence in not just my own food preservation but being able to teach others.

Food preservation really is for everyone, and the UC MFP Online Delivery program is available to help the public preserve food safely now and relish it later. 

Henry Shaw, UC Master Food Preserver Online Program Volunteer

“Water activity” or “the activity of water” (typically abbreviated as “aw”) is a critical parameter in food science that influences microbial growth and chemical reactions (including enzymatic processes) in food products. Unlike moisture content, which measures the total amount of water in a product, water activity quantifies the availability of water for microbial and chemical processes.

Formally, the activity of water in a food sample is defined as the ratio of the vapor pressure of water in that sample to the vapor pressure of distilled water under identical conditions (U.S. Food & Drug Administration, 1984). It can take on values ranging from 0 (completely dry) to 1.0 (pure water). This formal definition can seem like a lot of hard-to-understand scientific gibberish, but there’s a concept that most of us are familiar with that can help us understand the definition. The water activity of a food sample is simply the relative humidity (RH) in the air in a sealed jar that contains the food in question after that food has had time to equilibrate with the air.  Relative humidity is usually reported as a percentage, but if we express the percentage as a decimal fraction (i.e., divide the percent relative humidity by 100) we get the water activity: aw = RHjar/100.

At 100% relative humidity (aw = 1) it’s raining; the air contains as much water vapor as it can hold at that temperature. At lower atmospheric relative humidities, “wet” things (i.e., those with an aw greater than RH/100) will dry out. If we seal a jar with a food product and let it equilibrate, the food will determine the relative humidity in the air in that jar by either evaporating water from the product or absorbing water vapor from the original air in the headspace. The final relative humidity in the headspace, divided by 100, is the activity of water in that product.

Microbial growth is highly dependent on water activity and the goal in food preservation is to lower the aw to a value at which pathogens and food-spoilage organisms cannot thrive. Most bacteria require a water activity above 0.91 to grow. Most yeasts can only survive at water activities above 0.88, and molds can survive at even lower water activities, with a limit of about aw ~ 0.65. By reducing the water activity in a product, we can inhibit the growth of these spoilage organisms and pathogens, thereby extending the product’s shelf life and enhancing food safety. For reference, the horizontal dashed lines in Figure 1 show the water activity levels below which various types of food spoilage organisms can no longer grow.

In addition to affecting microbiological activity, the activity of water influences chemical reactions such as lipid (fat) oxidation and Maillard browning. These reactions can lead to undesirable changes in flavor, color, and nutritional quality. By lowering water activity, we can slow these reactions and maintain product quality and prolong storage life.

Food preservation methods like dehydration, freezing, and the addition of humectants (things that “bind” with water and reduce its availability, e.g., salt or sugar) are commonly used to reduce water activity. Dehydration is an effective means for reducing the activity of water. This process physically removes water to lower the aw. Dried fruits, jerky, and powdered milk are preserved in this way and typically have water activities less than 0.75, which is below the threshold needed for most microbial growth (see Fig. 1). Note that when we “condition” dehydrated foods prior to long-term storage (i.e., let them sit in a closed jar for a week or so, shaking the jar daily), what we are doing is letting the dried pieces of food equilibrate so that the aw becomes the same in each piece.  If we have different types of food in that jar (e.g., a mixture of dehydrated vegetables for use in later soup making), the activity of water in each piece will be the same after conditioning, but the water content of each type of vegetable may well be different due to the differing chemical compositions of the different vegetables.

Freezing immobilizes water molecules (and slows both the metabolic activity of bacteria and the rate of chemical reactions responsible for food degradation). Humectants bind free water to reduce its chemical availability. For instance, as shown in Figure 1, a 13 wt. % solution of salt (NaCl) has a water activity of ~0.91, which is low enough to suppress the growth of most “ordinary” bacteria. In contrast, one needs a 55 wt. % solution of sugar to reach the same aw. On a weight basis, therefore, salt is much more effective at reducing aw in a water solution than sugar.

All these techniques lower the activity of water in preserved foods to prevent spoilage and ensure that food remains safe for consumption after storage.

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Figure 1. The activity of water in salt and sugar solutions as a function of solute concentration. The two curves show how increasing salt or sugar concentrations lower the activity of water in the solution. Both curves stop at the solubility limit of the solute at room temperature. Note that the weight percent of the solution on the horizontal axis is the mass (weight) of the sugar or salt divided by mass of the water plus the mass of the sugar or salt in the solution. Dashed horizontal lines indicate the growth limits of various food-degradation organisms; at an aw lower than a given line, an organism will not grow. (Data from Gekas et al., 1998; FDA, 2012; Bressan and Mathlouthi, 1993; Fundamentals of Consumer Food Safety and Preservation, 2018)

References:

Bressan, C. and M. Mathlouthi, 1993, Thermodynamic activity of water and sucrose and the stability of crystalline sugar. AVH Association 1st Symposium, Reims, France.

Food and Drug Administration, 2012, Bad Bug Book, Foodborne Pathogenic Microorganisms and Natural Toxins. Second Edition. Appendix 3. Factors that Affect Microbial Growth in Food, p 261.

Gekas, V., C. Gonazlez, A Sereno, A Chiralt, and P. Foto, 1998, Mass transfer properties of osmotic solutions. 1 Water activity and osmotic pressures. Intl. J. of Food Properties, I(2) 95-112.

U.S. Food and Drug Administration, 1984, Water Activity (aw) in Foods. Inspection Technical Guides No. 39, https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods.

Washington State University and University of California, 2018, Fundamentals of Consumer Food Safety and Preservation, Master Handbook, p 1-10.