Guide · cold brew

Cold brew: ratio, steep time and safe storage

Cold brew trades heat for time: coarse coffee steeps in room-temperature or fridge-cold water for hours, then is filtered. Make it as a concentrate to dilute later, or at drinking strength.

Updated 2026-10-14 · sources listed at the end

What cold brew is

In the encyclopedic definition, coarse-ground coffee is soaked in water, usually at room temperature, for 12 to 24 hours. The grounds are then filtered out through a paper coffee filter, a fine metal sieve such as a French press, or felt. The result is usually a concentrate that is diluted with water or milk and served chilled or over ice. Japan's slow-drip "Kyoto-style" method is a separate variant.

At a glance

Long contact calls for coarse grounds, as with a French press — see the grind size chart.

Concentrate vs. ready-to-drink

The difference is how much coffee steeps per gram of water; published research uses both.

A concentrate saves fridge space and lets each drinker choose a dilution; a ready-to-drink batch needs no math. Because the grounds hold back some water, a 1:5 concentrate diluted with an equal weight of water ends up a little stronger than a direct 1:10 brew, though the two are in the same range. To scale either ratio to your jar, use the ratio calculator.

How long to steep

Long steeps are traditional, but UC Davis Coffee Center research that tracked dissolved solids (TDS) over time suggests the flavor settles sooner.

UC Davis summarized the work this way: a shorter extraction, "like eight hours instead of the commonly used 24 hours," could give similar sensory qualities. Brew time was the least important of the three factors tested; roast level mattered most, then temperature.

Step by step

1. Clean everything first. Wash your hands, the jar, stirring spoon and filter holder. There is no boiling step (see storage).
2. Weigh and grind coarse. For a concentrate, 1:5 (for example 100 g coffee to 500 g water). For ready-to-drink, 1:10 (for example 35 g to 350 g).
3. Add water and stir. The UC Davis brews were stirred for 15 seconds.
4. Cover and steep. At room temperature, start checking after 5–6 hours. In the fridge, allow 10–12 hours. Up to 24 hours is common.
5. Filter. Pour through paper, or mesh then paper, into a clean container.
6. Dilute a concentrate with water or milk; an equal weight matches the UC Davis 2× dilution. Adjust to taste.
7. Refrigerate the filtered coffee in a covered container right away.

Taste, acidity and caffeine

• Taste. In UC Davis descriptive panels at a matched 2% TDS, cold brews were "statistically significantly more floral and less bitter, less sour, and less rubbery" than chilled hot brews.
• Acidity. At equal strength, cold brews were only about 0.1–0.2 pH units higher (less acidic) than chilled hot brews. Averages ran about 4.9 (light roast) to 5.5 (dark). In a separate study, cold brew pH ranged from 4.96 to 5.13, and every cold brew had lower titratable acidity than its hot-brewed counterpart. The Coffee Science Foundation's summary: cold brew is "less sour" to tasters, "but not necessarily less acidic chemically."
• Caffeine. The encyclopedic summary reports that a 24-hour, low-temperature brew has more caffeine per equal volume than a 6-minute brew at 98°C.

Food safety and storage

There is no official home shelf life for cold brew in the sources below, and this page does not give one. Here is what they do say:

• It is a low-acid drink. NC State Extension defines low-acid foods as those with a final pH above 4.6 (and water activity above 0.85). The pH values reported for cold brew, roughly 4.9 to 5.5, sit above that line. That is why packaged ready-to-drink cold brew falls under FDA low-acid canned food rules. In June 2024 the FDA posted a recall of all of one company's canned coffee products because its process "could lead to the growth and production" of botulinum toxin and had not been filed with FDA.
• There is no boiling step. University of Georgia food-safety researchers added Bacillus cereus, Listeria monocytogenes, E. coli and Salmonella to cold brew. None of the bacteria grew, but they survived for 9 to 12 days, and B. cereus for more than 30 days. So brew contaminated at the start, through ingredients, equipment or hands, can still make someone sick.
• Refrigerate it. The UGA article says cold brew "is safest stored in the refrigerator" and is unlikely to spoil if kept there "for a limited time." The FDA advises keeping a refrigerator at 40°F or below and checking it with an appliance thermometer. NDSU Extension says to read it when the compressor has just turned on, when the fridge is warmest.
• Commercial studies are not home rules. A National Coffee Association-commissioned challenge study found no pathogen growth in commercial concentrate or UHT-processed cold brew held 11 days at 85°F (29°C). The FDA Food Code had no specific cold brew rule at the time of that 2023 report, and these results do not set a shelf life for a home-made jar.

Checklist

• Hands, jar and filter washed before brewing
• Coffee weighed and ground coarse: 1:5 concentrate or 1:10 ready-to-drink
• Steep timed: about 5–6 h at room temperature or 10–12 h in the fridge, up to 24 h by taste
• Filtered into a clean, covered container
• Labelled with the brew date
• Fridge at 40°F or below, checked with a thermometer
• Concentrate diluted about 1:1 with water or milk, to taste
• When in doubt, throw it out (FDA's food-safety rule of thumb)

Lab tip: filter one jar at 8 hours and one at 24, dilute both to the same strength and taste side by side — the UC Davis experiment in miniature.