How Long Does Simple Syrup Last, and How to Explain It to Someone Else

A 1:1 simple syrup made by weight and refrigerated in a clean sealed bottle holds roughly one month; a 2:1 rich syrup holds roughly six months. Those figures describe syrup you weighed. Measure the same two recipes in US cups and you get 45.8% and 62.8% sucrose by weight instead of 50.0% and 66.7%, because a cup of granulated sugar weighs 200 g in USDA FoodData Central while a cup of water weighs 236.6 g. Water activity tracks that sugar concentration: about 0.94 and 0.88 for the cup versions, against the 0.927 and 0.860 published for the weighed ones. Only the weighed 2:1 lands under 0.88, the value below which the FDA Food Code 2022 stops classifying an unpackaged food as requiring temperature control, and not one of the four reaches the Code's flat 0.85 exclusion. Refrigerate all of them, write the bottling date on the bottle, and pour out any syrup that turns cloudy.

Start by naming what you actually made

I write step cards for boxed dinners, and the instruction I am never allowed to give is "you'll know." I cannot see your pan or your scale. So when a card says one part sugar to one part water, my question is a boring one: measured how?

The answer changes the food. A cup-measured 1:1 is 200 g of sugar in 436.6 g of syrup, or 45.8% sucrose by weight. Weigh the same recipe at 100 g and 100 g and you get 50.0%. The gap looks trivial written down. It is 4.2 points of sucrose, and it moves the wrong way: the version most Americans make is the more dilute one.

Rich syrup separates further. Two cups of sugar to one cup of water is 400 g in 636.6 g, or 62.8%. Two parts to one by weight is 66.7%. Four recipes, all called simple or rich syrup, spread across nearly 21 points of sucrose.

I learned this the expensive way. In 2017 I shipped a card reading "1 cup sugar, 1 cup water" for a soaking syrup the test kitchen had developed at 500 g and 500 g. No proofreader catches that. It cost a reprint and six weeks of tickets complaining that the cake soak ran off the plate, and I read every one.

Why a sugar ratio alone cannot give you a shelf life

Sugar preserves by holding water. The number that matters is how much water is still chemically available once the sugar has taken its share, and that is water activity, written aw, on a scale where pure water is 1.00. Microbes read aw. They do not read your recipe.

A reference table titled "Water Activity of Sucrose and NaCl Solutions," distributed by the UC Davis food safety program from Principles of Food Science: Part II, page 250, lists sucrose solutions against aw, and two of its rows land exactly on the bar-world ratios. At 100 g sucrose per 100 g water — a weighed 1:1, 50.0% — aw is 0.927. At 200 g per 100 g — a weighed 2:1, 66.7% — aw is 0.860.

Be honest about what that table is. It is calculated, from the formula a = 1 / (1 + 0.27n), where n is moles of sucrose per 100 g of water, and it is not a set of meter readings on your bottle. Run the cup-measured versions through the same formula and 45.8% gives about 0.94, 62.8% about 0.88. Both sit between published rows, so treat them as estimates.

I used to end syrup cards with a date. Until roughly 2019 mine said "keeps one month refrigerated," full stop, and I defended it as the concrete cue I demand everywhere else. I stopped when the logs showed two people following one card and producing syrups four points of sucrose apart. A date is only a cue if everyone arrives at the same food. Now the card says weigh it, then date it.

Where the FDA's line falls, and what it does not mean

The FDA Food Code 2022 defines a Time/Temperature Control for Safety food at §1-201.10(B), and its exclusion list includes any food with a water activity of 0.85 or less. Beneath that flat cutoff, the definition points to two interaction tables reproduced in FDA's Job Aid: Time and Temperature Control for Safety Foods. Table B governs food not heat-treated, or heat-treated and not packaged — a boiled syrup poured into a home bottle, since that bottle is not hermetically sealed and commercially sterilized.

Below aw 0.88, a food is non-TCS at any pH, because 0.88 is the floor for Staphylococcus aureus. Between 0.88 and 0.90, it stays non-TCS until pH climbs above 5.0. Above 0.92 with pH over 5.0, a product assessment is required, meaning the food is treated as TCS until a challenge study says otherwise.

Plain syrup runs near neutral, roughly pH 6.5 to 7, since sucrose contributes essentially no acid and the water sets the value. Drop the four syrups into that column and the weighed 2:1, at 0.860, is the only one reading non-TCS. The cup-measured 2:1 at about 0.88 sits on the boundary of the next row up. Both weighed and cup 1:1 syrups land in "product assessment required." Same two recipe names, opposite sides of a regulatory line, decided by whether someone used a scale.

Non-TCS means the food does not need temperature control to stop pathogen growth. It does not mean the food will not spoil. Sugar-tolerant organisms live far below these numbers: Zygosaccharomyces rouxii, the yeast that ruins syrups and fruit concentrates, has been recorded growing at aw 0.620, and the review "Is there a common water-activity limit for the three domains of life?" in The ISME Journal puts the reliably reproduced floor for any eukaryotic microbe at 0.640 once failed replications are accounted for. Even the conservative figure sits well under 0.860. A rich syrup FDA would not call TCS will still grow mould, given months.

I have never held a refractometer or a water-activity meter to a syrup. Every number above came from a published table, not from my bench. What I can vouch for is the failure mode, from three seasons of complaint logs: syrup problems arrive as cloudiness and thin texture, and they arrive earlier for people who measured in cups.

A fruit or herb syrup is a different food

Readers give every sweet syrup one storage rule, and produce breaks that rule in two directions at once.

| Syrup | Sucrose by weight | aw | Typical pH | FDA Table B result | |---|---|---|---|---| | 1:1, US cups | 45.8% | ~0.94 | 6.5–7.0 | Product assessment required | | 1:1, weighed | 50.0% | 0.927 | 6.5–7.0 | Product assessment required | | 2:1, US cups | 62.8% | ~0.88 | 6.5–7.0 | Boundary; assessment likely | | 2:1, weighed | 66.7% | 0.860 | 6.5–7.0 | Non-TCS | | Mint or basil, 1:1 base | below 45.8% | above 0.94 | ~6–7 | Product assessment required | | Strawberry, 1:1 base | below 45.8% | above 0.94 | 3.0–3.5 | Non-TCS, on acid alone |

Strawberries run pH 3.0 to 3.5 from citric and malic acid, so a strawberry syrup drops into Table B's pH-under-4.2 column and reads non-TCS whatever its water activity. On the bacterial axis it is the safest syrup listed. On the quality axis it is among the worst, because the fruit brought water that dilutes your sucrose, plus sugars, nitrogen, and a resident population of acid-loving yeasts and moulds perfectly happy at pH 3.2. Cider vinegar is proof that low pH is no barrier to fermentation.

A mint or basil syrup gets the second half of that trade and none of the first. Herbs add water and microbes without adding acid, so the syrup stays near neutral and its water activity rises. Herb syrups are the shortest-lived thing here, and I treat a week in the refrigerator as the ceiling.

For the pH of your specific infusion, use a calibrated meter. A recipe title cannot tell you and neither can your tongue, since sweetness masks acid.

What a splash of vodka actually does to the number

Start with the common home batch: 1 cup sugar plus 1 cup water, 436.6 g at 45.8% sucrose. At that concentration the density is about 1.21 g/mL, so the batch is roughly 362 mL. Add one tablespoon, 14.8 mL, of 80-proof vodka. That is 5.9 mL of ethanol in 377 mL of finished syrup: 1.6% ABV. Use a full fluid ounce and you reach 3.0%.

Kalathenos and Russell, in "Ethanol as a food preservative" (Food Preservatives, 2nd edition, 2003), report that bacterial growth begins to be inhibited around 8–11% v/v, and that solutions under 30% v/v are rarely biocidal. To bring that 362 mL batch to 15% ABV you would need about 217 mL of 80-proof vodka — a bit over 7 fluid ounces, nearly a cup of spirit into a cup and a half of syrup. At that point you have made a cordial and should label it as one.

The strongest case against me is partly correct. Ethanol dissolves in the water phase, not in the sucrose, so quoting ABV across the whole volume understates the concentration where microbes live. I grant that. It does not rescue the tablespoon: 14.8 mL of 80-proof vodka is 4.7 g of ethanol against roughly 237 g of water in the batch, about 1.9% by weight of the aqueous phase, still under a quarter of the lower inhibition figure.

The other half of the objection is empirical. In the only public side-by-side most bartenders have seen — Camper English's test at Alcademics, published 3 August 2009 — refrigerated 1:1 syrup clouded at about one month, 1:1 with one tablespoon of vodka at about three months, 2:1 at about six months, and 2:1 with vodka went past six. My reading is that a tablespoon of neutral spirit buys real time against light surface contamination without converting a syrup into a preserved food. English called the test "pretty non-scientific" himself, the endpoint was visible cloudiness rather than a plate count, and there was one bottle per condition. The post also never records whether those ratios were weight or volume. The most-quoted shelf-life numbers in American bartending are missing the single variable this article is about.

Bottles, sanitation, and reading your own syrup

Nothing above matters if the syrup is contaminated on the way into the bottle. Yeast and mould arrive on a wet spoon, a rinsed-but-not-sanitized bottle, or a cap left face-down on the counter. Hot-filling — pouring at near-boiling and capping immediately — treats the headspace and the closure. A cooled syrup poured into an air-dried bottle does not.

A procedure you can hand to somebody else:

Weigh the sugar and the water. Record the grams. Every later number depends on this one.
Compute sucrose by weight. Sugar mass divided by total mass, times 100.
Look up the water activity for that percentage in a sucrose aw table, and note whether it is above or below 0.88.
Sanitize the bottle and cap, then fill the syrup hot and cap it at once.
Label the bottle with the date, the ratio, and the word "weight" or "cups."
Refrigerate, including rich syrup, including syrup with vodka in it.
Inspect before every use for cloudiness, a film, bubbles, or ropiness, and pour out the bottle at the first sign.

Cloudiness in a syrup that poured clear is the earliest signal and the one worth acting on. Bubbling is fermentation. Syrup is sugar and water; nothing in it is worth risking a drink for.

So when someone asks how long their syrup keeps, ask what they measured with before answering. Until that is settled there are four syrups on the table, spread across nearly 21 points of sucrose, and exactly one sits under 0.88.

Frequently asked questions

How do I know if simple syrup has gone bad?

Cloudiness in a syrup that poured clear is the first and most reliable sign. Also check for floating specks, a surface film, stringy texture, bubbles or fizzing, and any sour or solvent-like smell. Any one of these means discarding the whole bottle rather than skimming it.

Can bacteria grow in simple syrup?

Pathogenic bacteria struggle in syrup, because Staphylococcus aureus stops at water activity 0.88 and a weighed 2:1 syrup sits at 0.860. Yeasts and moulds are the real spoilers. Zygosaccharomyces rouxii has been recorded growing at water activity 0.620, far below any syrup you will make at home.

Is it okay to use expired simple syrup?

If it is clear, smells clean, and shows no film, bubbles, or specks, syrup past its date is usually fine, since the date estimated quality rather than guaranteeing safety. Judge the bottle, not the calendar. Any cloudiness, fizz, or off smell means throwing it out untasted.

Can syrup that expired two years ago be eaten?

Do not drink it. Two years is long enough for sugar-tolerant yeasts and moulds to grow even at low water activity, and Xeromyces bisporus germinates near its limit only after roughly 120 days of incubation. A clear-looking bottle at that age has had ample time to spoil. Discard it.

How long does 2:1 simple syrup last?

Refrigerated in a sanitized sealed bottle, roughly six months when weighed at 200 g sugar to 100 g water: 66.7% sucrose, water activity 0.860. Measured in cups it is 62.8% sucrose with higher water activity, so shorten that expectation. The six-month figure comes from one informal test.

How long does homemade syrup last unrefrigerated?

Assume days, not weeks, and treat any unrefrigerated batch as a quality gamble. No standard home ratio reaches the FDA Food Code's 0.85 water activity exclusion, so none is shelf-stable by that measure. Room temperature also speeds yeast and mould growth. Refrigerate every syrup, including rich syrup.