Yes, bacteria can grow in sugar water, but only under the right conditions. Diluted sugar water (think a light lemonade or a sugar syrup with less than about 50% sugar) has enough available moisture to support bacterial growth just like plain water does. Concentrated sugar solutions, on the other hand, can actually inhibit most bacteria by pulling water out of microbial cells through osmotic pressure. The key variable isn't whether sugar is present, it's how much sugar, how much free water remains, and what the storage conditions look like.
Can Bacteria Grow in Sugar Water? Food Safety Guide
Marcus Reeves
5 Jun 2026
Why sugar can (and can't) help bacteria grow
There's a common assumption that sugar feeds bacteria and automatically makes something more dangerous. That's partly true and partly misleading. Sugar is a carbon source, and bacteria that encounter a light sugar solution will happily use it as a nutrient. So in diluted sugar water, sugar can absolutely support bacterial growth by providing energy.
But flip to the other extreme and the story changes completely. Honey, for example, has a water activity of roughly 0.5 to 0.65, far below what most bacteria need to survive and grow. That low water activity comes from the extremely high sugar concentration, which creates a hyperosmotic environment. Bacteria placed in this environment lose water to osmosis and can't grow, and many can't survive at all. This is exactly why honey has been used as a preservative for thousands of years.
So sugar plays two roles: at low concentrations it's a nutrient that helps bacteria; at very high concentrations it's a barrier that stops them. Where the line falls depends on water activity, which is the actual measurement that matters here.
Sugar concentration and water activity: when bacterial growth is actually possible
Water activity (aw) is a measure of how much free, available water exists in a solution or food, expressed on a scale from 0 to 1. Pure water is 1.0. The more dissolved sugar, the lower the water activity. Most bacteria need aw above about 0.91 to grow. Some pathogens like Clostridium botulinum need aw of at least 0.93, and Bacillus cereus has a similar minimum around 0.93. Staphylococcus aureus is the hardiest common bacterial pathogen in this regard, capable of growth down to about 0.86 under aerobic conditions.
To put those numbers into real sugar concentrations: UC Davis measurements of sucrose solutions at 25°C show that a 28.6% sucrose solution has aw of about 0.969, a 44.4% solution drops to 0.941, 50% sucrose sits around 0.927, 60% sucrose reaches 0.900, and only at about 66–68% sucrose does aw approach the 0.85–0.86 range where even S. aureus growth is suppressed. That means a standard simple syrup (roughly 50% sugar by weight) still has water activity comfortably in the range where most bacteria can grow if conditions are otherwise right.
| Sucrose Concentration (% w/w) | Approximate Water Activity (aw) | Bacterial Growth Risk |
|---|---|---|
| 10–30% | 0.97–0.99 | High — most bacteria can grow freely |
| 44% | ~0.941 | High — most bacteria can still grow |
| 50% | ~0.927 | Moderate — most bacteria can grow; C. botulinum inhibited below 0.93 |
| 60% | ~0.900 | Lower — growth of common pathogens possible; S. aureus still viable |
| 67–68% | ~0.85–0.86 | Low — most bacterial pathogens inhibited; S. aureus at its limit |
| 75%+ (honey-like) | 0.50–0.65 | Very low — bacteria generally cannot grow |
The FDA uses 0.85 as its regulatory cutoff for low water activity in food preservation: products at or below aw 0.85 are considered controlled from a bacterial pathogen standpoint. Anything above that threshold needs additional safeguards. This is why light sugar syrups, toppings, and even some puddings can still harbor bacterial growth risks, the sugar content alone isn't high enough to drive aw down to a safe level.
Other conditions that decide whether bacteria actually grow
Water activity is the biggest lever, but bacteria don't just respond to one variable. Four other factors interact with sugar concentration to determine whether growth happens in a real-world sugar solution.
Temperature
Most foodborne pathogens grow fastest between 40°F and 140°F (4°C and 60°C), the classic danger zone. Leaving diluted sugar water at room temperature (around 68–77°F / 20–25°C) puts it squarely in the range where bacteria double rapidly. Refrigeration at 38–40°F (3–4°C) slows or stops growth for most common bacteria, though it's not a universal fix for every pathogen. Freezing stops growth entirely but doesn't kill bacteria.
pH
Most bacteria prefer a pH near neutral (6.5–7.5). Acidifying sugar water, for example by adding lemon juice to make lemonade, pushes pH down and creates an additional hurdle for bacterial growth. C. botulinum, in particular, is inhibited in acidic environments (pH below about 4.6). Research confirms that both aw and pH are key determinants for whether C. botulinum can grow and produce toxin, sugar concentration alone doesn't tell the full story.
Oxygen availability
Aerobic bacteria need oxygen and won't grow in sealed, oxygen-free environments. Anaerobes like C. botulinum, on the other hand, prefer the absence of oxygen, which is why it's a concern in sealed, canned, or vacuum-packed products rather than open containers. Because canned foods are often sealed and oxygen-limited, the bacteria that can grow there depend on conditions like water activity and temperature sealed, canned, or vacuum-packed products. If you're storing sugar water in an open container, aerobic spoilage bacteria and yeasts are the more likely concern. Seal it tightly without proper controls and anaerobes become relevant.
Time
Even when conditions are marginal for growth (aw just above a pathogen's minimum, temperature near the lower limit), bacteria can still multiply given enough time. A sugar-water drink left on the counter for two hours might be fine; the same drink left for 24 hours at room temperature is a different calculation. Time is the multiplier that turns a small contamination event into a food safety problem.
Survival vs. growth: what bacteria actually do in sugar solutions
There's an important distinction between bacteria surviving in sugar water and bacteria actively multiplying. In a moderately concentrated sugar solution (say, 60–66% sucrose), the water activity is low enough to suppress active growth for most bacteria, but it doesn't necessarily kill them. Pathogens can persist in a dormant or stressed state and revive if the solution gets diluted or warmed up.
In very high-sugar environments like undiluted honey (aw 0.50–0.65), the osmotic pressure is so extreme that most bacteria cannot survive for long. But even here, some organisms tolerate the stress better than others. Osmophilic yeasts, particularly Zygosaccharomyces rouxii, can grow slowly at aw as low as 0.61 and are responsible for fermentation in honey, jams, and concentrated syrups when the water content creeps up even slightly. These aren't bacteria, but they illustrate that no sugar solution is biologically inert.
For practical purposes: diluted sugar water (under about 50% sugar) should be treated like any perishable liquid. Concentrated syrups above roughly 67% sugar are genuinely resistant to most bacterial growth, but the protection is concentration-dependent and can be lost quickly if the syrup absorbs moisture, gets diluted, or is handled with contaminated tools.
It's worth noting that similar logic applies to other high-sugar or high-alcohol environments. The ability of microbes to grow in wine, for instance, depends on alcohol content and pH alongside residual sugar, sugar alone being present in grape juice tells you very little about whether wine is microbiologically stable.
Practical steps to prevent contamination and store sugar solutions safely
If you're dealing with a specific sugar-water scenario today, here's how to think through and manage the risk.
- Identify the concentration. If your sugar solution is less than about 50% sugar by weight (roughly 1 part sugar to 1 part water by volume), it has water activity well above 0.92 and can support bacterial growth. Treat it as perishable.
- Refrigerate diluted sugar solutions promptly. Anything below about 65–67% sugar (aw above ~0.86) should go in the refrigerator at 40°F (4°C) or below if you're storing it for more than a few hours. This applies to simple syrups, flavored sugar drinks, sugar-based sauces with water, and similar products.
- Use clean equipment. Bacterial contamination usually enters sugar water through unclean containers, spoons, or hands. Washing containers with hot, soapy water before use and avoiding double-dipping utensils reduces the initial bacterial load significantly.
- Cover open containers. Open sugar solutions at room temperature can pick up bacteria and yeasts from the environment and from insects (particularly relevant with honey and syrups). Keep containers covered when not in use.
- Consider acidification as a hurdle. If you're making a sugar-water product at home, adding lemon juice or citric acid to bring pH below 4.6 creates an additional layer of protection against bacterial growth, particularly against C. botulinum in sealed containers.
- Don't rely on sugar alone for preservation. Unless you're working with a measured aw at or below 0.85, don't assume sugar concentration makes a product safe. The transition from safe-if-refrigerated to shelf-stable requires a precisely high sugar concentration, and most kitchen preparations don't reach it without verification.
- Watch for signs of spoilage. Fermentation, bubbling, off odors, cloudiness, or mold growth in a sugar solution are signs of microbial activity. Discard the product and clean the container thoroughly.
- For shelf-stable sugar syrups used in food production, verify aw with an instrument rather than estimating from percent sugar. The relationship between concentration and aw is consistent at a lab level but can shift with temperature, other dissolved solids, and hygroscopic conditions.
The bottom line: sugar water is not inherently safe from bacteria. Bacteria can also grow on plastic surfaces when moisture and nutrients are present, even if the plastic itself is not a food source. Coffee is also a liquid food, so whether can bacteria grow in coffee depends on similar conditions like water activity and temperature sugar water is not inherently safe from bacteria. The protection sugar provides is real but concentration-dependent, and most everyday sugar solutions don't have nearly enough sugar to suppress bacterial growth on their own. Combine an accurate read on concentration with temperature control, good hygiene, and (where relevant) pH control, and you have a reliable, practical approach to keeping sugar-water products safe.
FAQ
If I made sugar water at home, can bacteria grow in it even when I don’t add anything else?
Yes, especially if the “sugar water” is light (roughly under 50% sugar by weight) and left warm. Even tiny contamination can start growth quickly, so it helps to treat it like other perishable drinks: keep it cold, and use it promptly after mixing.
What’s more dangerous, high-sugar syrup or diluted sugar water after it sits?
Dilution is the biggest risk shift. If you start with a concentrated syrup and later add water (or it mixes with meltwater, condensation, or leftover moisture), water activity rises and dormant microbes can resume growth. Always consider how the final mix will actually taste and measure, not just the recipe.
Does sealing sugar water stop bacterial growth completely?
Sealed containers can reduce oxygen for aerobic spoilage, but they do not automatically make the mixture safe. In sealed, low-oxygen environments, different organisms may become relevant, so you still need low water activity (high enough concentration) and safe temperature handling.
If I freeze sugar water, is it safe once thawed?
Freezing stops growth but doesn’t reliably sterilize. When you thaw, surviving bacteria can multiply again if the thawed sugar water ends up above safe temperatures. For safety, thaw in the refrigerator and don’t repeatedly warm and cool the same batch.
Can sugar water spoil if bacteria can’t grow?
Yes, yeasts can grow even when most bacteria are suppressed, particularly at intermediate water activity levels found in some syrups and jams. That can cause fermentation, bubbles, souring, or off flavors, so “no bacteria” does not mean “no spoilage.”
Does adding lemon juice make sugar water safe from all bacteria?
Lemon juice or other acidic ingredients can add an extra hurdle by lowering pH, but you cannot assume “lemon equals safe.” The mixture still depends on water activity, and some toxins or microbes have specific pH and temperature limits, so proper concentration and refrigeration still matter.
Is honey-based sugar water always safe because honey resists bacteria?
Honey is a special case because its water activity is typically too low for most bacteria, but it can change when honey is diluted, absorbs moisture, or is heated and then re-equilibrates. Use clean utensils and avoid introducing water to prevent water activity from rising.
How long can sugar water sit out before it becomes risky?
Temperature and time work together. A short period at room temperature may be low risk, but extended storage, especially around the 40°F to 140°F (4°C to 60°C) range, increases the chance that any contaminants will multiply to unsafe levels.
Can poor hygiene cause bacteria to grow in sugar water even if the recipe has enough sugar?
Yes. If you prepare sugar water with a spoon, container, or measuring cup that previously touched food or saliva, you can introduce microbes and start growth even if the sugar concentration would otherwise suppress bacteria.
If sugar water sat for a while, does reheating make it safer?
Some organisms can persist in stressed states at borderline conditions and then “bounce back” if the solution gets warmer or diluted. That means reheating or warming the drink later can be worse than keeping it consistently cold, since it gives surviving cells a better chance to multiply.
How can I tell if my sugar water is concentrated enough to inhibit bacteria?
Measure by concentration when possible, not just by “how sweet it seems.” Syrups can end up more dilute than expected due to incorrect ratios, humidity absorption, or mixing with ice, and that can move water activity into the range where growth is possible.
Citations
FDA notes that most foods have water activity above 0.95, which provides sufficient moisture to support growth of bacteria, yeasts, and mold.
Water Activity (aw) in Foods | FDA - https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
FDA uses 0.85 or less as a regulatory “low water activity” point: if water activity is controlled to 0.85 or less in the finished product, it is not subject to certain low-acid canned food regulations (21 CFR Parts 108, 113, and 114).
Water Activity (aw) in Foods | FDA - https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
FDA references that the minimum water activity for growth of Clostridium botulinum is approximately 0.93.
Water Activity (aw) in Foods | FDA - https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
FDA states that some low-acid canned foods may have water activity greater than 0.85 (examples listed include syrup, toppings, puddings, and some specialty sauces), showing that added sugar alone does not always guarantee aw ≤ 0.85.
Water Activity (aw) in Foods | FDA - https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
A Frontiers review states honey’s water activity typically ranges from 0.5 to 0.65, which is lower than amounts needed for most bacteria (~0.90), yeasts (~0.80), and molds (~0.70) to grow; it also notes aw < 0.61 inhibits osmophilic yeasts.
The role of honey in the ecology of the hive: Nutrition, detoxification, longevity, and protection against hive pathogens (Frontiers) - https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2022.954170/full
The review explains that honey’s high sugar concentration/low moisture creates hyperosmotic conditions that prevent growth of bacteria and yeast (osmotic/desiccation effect), while some yeasts can survive in honey if water activity is higher.
Honey: a reservoir for microorganisms and an inhibitory agent for microbes (review, PMC) - https://pmc.ncbi.nlm.nih.gov/articles/PMC2269714/
The review notes fermentation can occur if honey is diluted (water activity increases) and that the water activity of ripened honey is too low to support growth, while fermentation risk relates to honey water content.
Honey: a reservoir for microorganisms and an inhibitory agent for microbes (review, PMC) - https://pmc.ncbi.nlm.nih.gov/articles/PMC2269714/
This review states that many yeast/mold species are more osmotolerant and can grow at water activities around 0.80, while osmophilic molds/yeasts (e.g., Aspergillus spp., Zygosaccharomyces rouxii) can grow slowly around aw ~0.61.
Extreme Osmotolerance and Halotolerance in Food-Relevant Yeasts... (review, PMC) - https://pmc.ncbi.nlm.nih.gov/articles/PMC6333755/
A study on C. botulinum type G reported that the minimum aw at which growth and toxin formation occurred was 0.965 under the tested conditions.
Effect of water activity and pH on growth and toxin production by Clostridium botulinum type G (ASM journal) - https://journals.asm.org/doi/10.1128/aem.51.4.844-848.1986
An extension PDF table lists minimum water activity values for various organisms, including Bacillus cereus at 0.93.
Interpreting Water Activity Lab Results for (Virginia Tech extension PDF) - https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/FST/fst-485/FST-485.pdf
FDA guidance states that if aw is >0.85 but less than the aw that would allow growth of spores of public health significance (citing C. botulinum inhibited at aw < 0.93), firms need sufficient controls to ensure both water activity and thermal process are controlled.
Guide to Inspections of Low Acid Canned Food 7 | FDA - https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-guides/guide-inspections-low-acid-canned-food-7-0
A review notes honey water activity ranges about 0.562 to 0.62 and that undiluted honey inhibits bacterial growth due to osmotic pressure.
The antibacterial activities of honey (review, PMC) - https://pmc.ncbi.nlm.nih.gov/articles/PMC8071826/
UC Davis provides measured water activity values for sucrose solutions at 25°C, including: 50% sucrose (w/w) has aw ≈ 0.927; 60% sucrose has aw ≈ 0.900; 66.7% sucrose has aw ≈ 0.860; and 68%+ approaches ~0.85–0.86 range.
Water Activity of Sucrose and NaCl Solutions (UC Davis PDF) - https://ucfoodsafety.ucdavis.edu/sites/g/files/dgvnsk7366/files/inline-files/133655.pdf
UC Davis measured aw for sucrose solutions: 37.5% sucrose has aw ≈ 0.955 and 44.4% sucrose has aw ≈ 0.941 (at 25°C), showing that moderate household sugar levels still have relatively high aw.
Water Activity of Sucrose and NaCl Solutions (UC Davis PDF) - https://ucfoodsafety.ucdavis.edu/sites/g/files/dgvnsk7366/files/inline-files/133655.pdf
FDA states most foods with aw above ~0.95 support microbial growth, reinforcing that sucrose solutions with aw above ~0.91–0.95 are likely to allow bacterial growth if organisms are present.
Water Activity (aw) in Foods | FDA - https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
ASBE explains the ideal-solution concept for water activity: adding sugar reduces water activity (example given using 1 mole of sugar added to 1 L water yields an ideal aw ≈ 0.982), illustrating why dilution strongly increases aw.
Water Activity | American Society of Baking (ASBE) - https://asbe.org/article/water-activity/
A water-activity sorption study measured water activity in sucrose-containing systems at 25°C using a water activity instrument (Decagon Aqualab), supporting that aw must be measured/estimated for specific compositions rather than assumed from “% sugar.”
Water–solid interactions in amorphous maltodextrin-crystalline sucrose binary mixtures (PMC) - https://pmc.ncbi.nlm.nih.gov/articles/PMC4755289/
The Food Safety Institute summarizes general thresholds: most bacteria grow at aw above ~0.91 and provides a specific example that Clostridium botulinum has minimum aw about 0.93.
Water Activity: A Key Factor in Food Preservation (Food Safety Institute) - https://foodsafety.institute/food-fundamentals-chemistry/water-activity-food-preservation/
This source states that most bacteria require aw above about 0.91 to grow, while molds have a growth limit around 0.65 (and yeasts typically higher than that).
The Role of Water Activity in Food Spoilage (Food Safety Institute) - https://foodsafety.institute/food-fundamentals-chemistry/role-of-water-activity-in-food-spoilage/
FDA explicitly links aw control (e.g., ≤0.85) to preventing growth of microorganisms with public health significance in preservation contexts.
Water Activity (aw) in Foods | FDA - https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
IDC-Online (technical reference) states that growth of most bacteria is inhibited below about aw = 0.91 and gives an equivalent sucrose concentration context (noting ~57% w/w sucrose corresponds to aw ≈ 0.91 in their discussion).
Water activity (https://www.idc-online.com/technical_references/pdfs/chemical_engineering/Water_Activity.pdf) - https://www.idc-online.com/technical_references/pdfs/chemical_engineering/Water_Activity.pdf
A Staphylococcus aureus research PDF reports that S. aureus can grow at low water activity levels, including a low aw around 0.86 (with oxygen conditions noted as relevant to growth ranges).
Staphylococcus aureus water activity growth range (3tres3 research PDF) - https://www.3tres3.com/3tres3_common/art/3tres3/44217/fitxers/Staphylococcus_Aureus-Science_Research.pdf
The review highlights that osmophilic yeasts/molds such as Zygosaccharomyces rouxii can grow slowly at aw ~0.61, which explains spoilage/fermentation even in very sugary products.
Osmophilic molds/yeasts: growth at low aw (~0.61) (review, PMC) - https://pmc.ncbi.nlm.nih.gov/articles/PMC6333755/
The entry states Zygosaccharomyces rouxii can cause spoilage of high-sugar foods such as condensed milk, fruit juices, jam, pastries, and salad dressings.
Zygosaccharomyces rouxii (Wikipedia) - https://en.wikipedia.org/wiki/Zygosaccharomyces_rouxii
The entry describes Z. rouxii as osmotolerant (osmophilic) and capable of growth in high-sugar environments.
Zygosaccharomyces rouxii (Wikipedia) - https://en.wikipedia.org/wiki/Zygosaccharomyces_rouxii
A study/review notes Z. rouxii shows mechanisms to respond to extreme glucose stress and that very high sugar stress can inhibit growth in some yeasts (context for “tolerance vs inhibition”).
Genomic Insights Into Sugar Adaptation in an Extremophile Yeast Zygosaccharomyces rouxii (PMC) - https://pmc.ncbi.nlm.nih.gov/articles/PMC7026193/
An MDPI article states Z. rouxii is known for high osmotolerance and cites that it can tolerate low water activity (with statements about tolerance differences among Zygosaccharomyces species, including that Z. bailii needs higher aw).
Zygosaccharomyces rouxii control strategies and applications (MDPI, 2021) - https://www.mdpi.com/2311-5637/4/3/69/htm
A PubMed record on Z. rouxii reports experiments using growth media at water activity 0.93 supplemented with glucose and sucrose, illustrating that osmotolerant yeasts can still grow/recover at reduced aw depending on conditions and strain.
Zygosaccharomyces rouxii growth at low aw (~0.61) thresholds (PubMed) - https://pubmed.ncbi.nlm.nih.gov/1288843/
The same C. botulinum study demonstrates that both aw and pH are key determinants for growth/toxin formation (not just sugar concentration alone).
Effects of water activity and pH on C. botulinum (ASM journal) - https://journals.asm.org/doi/10.1128/aem.51.4.844-848.1986
FDA Chapter 13 states that controls using water activity ≤0.85 are used to prevent C. botulinum growth and toxin formation risk in certain contexts, and highlights aw/pH/other hurdles in that pathogen’s control.
FDA Fish and Fishery Products Hazards and Controls Guidance, Chapter 13 (PDF) - https://www.fda.gov/files/food/published/Fish-and-Fishery-Products-Hazards-and-Controls-Guidance-Chapter-13-Download.pdf
FDA/CFSAN BAM Chapter 17 states that refrigeration will not prevent growth and toxin formation by nonproteolytic C. botulinum strains (temperature control is not sufficient for all strains/hurdle sets).
FDA/CFSAN BAM Chapter 17 - Clostridium botulinum (EPA-hosted PDF) - https://www.epa.gov/sites/default/files/2015-07/documents/fda-bam-chap17.pdf
A shelf-life/specification guide states that decreasing temperature (refrigeration ~4–6°C) results in growth inhibition of most microorganisms and reduction of enzyme activity, though it does not imply complete prevention in all cases.
Refrigeration, growth inhibition (general microbial growth condition reference) - https://www.handary.com/shelflife-10084
The honey ecology review links honey water activity (0.5–0.65) to inhibition of most bacteria and yeasts/molds, with inhibition thresholds stated for osmophilic yeasts below ~0.61.
Frontiers review on honey water activity (honey fermentation inhibition) - https://pmc.ncbi.nlm.nih.gov/articles/PMC9359632/
A Virginia extension PDF includes a general statement that one pathogenic bacteria can grow at aw as low as 0.86, and suggests foods relying on water activity for safety should have aw of 0.85 or less.
Understanding the Water Activity of Your Food (Virginia Dept. of Agriculture and Consumer Services PDF) - https://www.vdacs.virginia.gov/pdf/wateractivity.pdf
An overview describes osmophiles as microorganisms adapted to low water activity/high osmotic pressure environments (common among yeasts responsible for spoilage/fermentation in high-sugar foods).
What do “osmophiles” do? (Osmophile overview, Wikipedia) - https://en.wikipedia.org/wiki/Osmophile
FDA uses aw as a central predictive parameter for microbial growth versus inhibition, reinforcing that “sugar water” is primarily about whether aw stays low enough.
Water Activity (aw) in Foods | FDA - https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
An FDA guidance document states that 0.85 is generally considered the safe cutoff level for bacterial pathogen growth (in the context of setting preventive controls).
Practical preservation threshold: low aw for bacterial pathogen growth (FDA guidance PDF snippet page) - https://www.fda.gov/downloads/AnimalVeterinary/GuidanceComplianceEnforcement/GuidanceforIndustry/UCM592870.pdf
UC Davis shows that at 30% sucrose (w/w) aw ≈ 0.888 (with their table including 28.6% at aw ≈ 0.969 and 37.5% at aw ≈ 0.955), demonstrating the steep aw dependence on concentration.
Water Activity table: sucrose solution aw at 25°C (UC Davis PDF) - https://ucfoodsafety.ucdavis.edu/sites/g/files/dgvnsk7366/files/inline-files/133655.pdf
UC Davis table includes 28.6% sucrose with aw ≈ 0.969 and 44.4% sucrose with aw ≈ 0.941, relevant to understanding why diluted sugar water can still support bacterial growth.
Water Activity of sucrose solutions at 25°C (UC Davis PDF) - https://ucfoodsafety.ucdavis.edu/sites/g/files/dgvnsk7366/files/inline-files/133655.pdf
FDA’s page notes water activity levels typical of most foods are >0.95 (supporting growth), which is consistent with many diluted sugar-water beverages having aw above bacterial growth-inhibition levels.
Water Activity (aw) in Foods | FDA - https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
The yeast/mold review explains that growth at low water activity is often extremely slow and that cells may survive but not multiply quickly under high osmotic stress—so “persistence” can occur even when active growth is inhibited.
Extreme Osmotolerance in yeasts (PMC) - https://pmc.ncbi.nlm.nih.gov/articles/PMC6333755/
The article states that the optimal temperature range for many osmophilic yeasts is ~77–86°F (25–30°C), implying room-temperature storage can favor osmophilic yeast fermentation when moisture/aw allows it.
Honey fermentation risk: osmophilic yeasts and conditions (Agriculture.Institute article) - https://agriculture.institute/hive-products-beekeeping-economics/understanding-honey-fermentation-prevention/
The same article states yeast activity in honey stops when temperature drops below ~11°C (52°F) (yeasts become dormant rather than dying), so warming can restore growth/fermentation if aw/moisture permits.
Honey fermentation prevention: temperature/dormancy concept (Agriculture.Institute article) - https://agriculture.institute/hive-products-beekeeping-economics/understanding-honey-fermentation-prevention/
The article claims fermentation becomes possible when honey water content is around 18–19%, connecting moisture content/aw increase with osmophilic yeast-driven fermentation.
Honey water content and fermentation threshold (Agriculture.Institute article) - https://agriculture.institute/hive-products-beekeeping-economics/understanding-honey-fermentation-prevention/
21 CFR defines water activity (a_w) as a measure of free moisture in a product (ratio of the vapor pressure of the substance to that of pure water at the same temperature).
21 CFR § 114.3 definitions (eCFR) - https://ecfr.io/Title-21/Section-114.3
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