What the study found

Spicer, Fairhurst, Newton, and Morris studied milk kefir with magnetic resonance imaging. They first checked that changes in the MRI signal tracked traditional gas measurements, with correlation coefficients above 0.95.

They then compared larger grains with grains broken into pieces around 3 mm. Larger grains produced a signal change of about 0.03 arbitrary units per hour. The smaller pieces produced about 0.07 arbitrary units per hour, more than twice as fast.

The researchers linked this difference to grain shape and exposed surface area. Breaking a large grain creates more outer surface in contact with milk. That can give the bacteria and yeasts more direct access to the milk during fermentation.

The 5% ratio matters

The study used kefir grains at 5% relative to the milk. By weight, that is roughly 5 g of grains for every 100 g of milk. The number belongs to the study design. It helped the researchers compare grain sizes under a defined condition, but it is not a prescription for every home ferment.

Change the amount of grain and you change the amount of active culture working in the milk. More grains relative to milk often means a faster change in acidity and thickness. Fewer grains often means more time. Grain size changes exposed surface area, but total grain amount still matters.

That is why copying only the 3 mm detail may disappoint. A useful comparison keeps the grain-to-milk ratio, milk, temperature, and timing as consistent as possible.

What grain size did prove

1. Grain shape and size can affect fermentation speed because smaller pieces expose more surface to milk.
2. The effect can be large under controlled conditions. In this experiment, the measured rate was more than twice as high with pieces around 3 mm.
3. Surface area is a reasonable explanation, not a promise about every batch. Kefir grains are living communities, so their activity depends on more than physical size.

What the study did not prove

It did not show that 3 mm is ideal for all milk kefir, that smaller grains always taste better, or that they provide more probiotic benefits. It did not show that breaking grains makes a stressed culture healthy again, that grain size matters more than temperature or ratio, or that the result applies to water kefir. A faster ferment is not automatically a better ferment.

What actually matters in your jar

1. Grain-to-milk ratio

Keep the amount of grains and milk reasonably consistent. If kefir suddenly turns sour or separates much earlier than usual, the grains may have grown while the milk amount stayed the same. If fermentation is too fast, use more milk or fewer grains. If it is too slow, check the other variables before breaking the grains further.

2. Temperature

The study describes room-temperature fermentation around 20 to 25°C. That helps us understand the experiment, but it is not a fixed Taiwan rule. Kitchens in Taiwan vary across seasons, buildings, and locations. A warm summer counter can move a batch faster than an air-conditioned room. Treat temperature as a changing variable and use the jar as your guide.

3. Grain condition

Stable grains are usually pale and springy, with a cauliflower-like structure. A small grain is not automatically stronger, and a large grain is not automatically weak. After a change in milk, temperature, storage, or handling, watch several batches rather than judging one jar alone.

4. Time and observation

Kefir does not follow a perfect timer. Look for noticeable thickening and a fresh sour smell. Small bubbles can appear, but no bubbles alone does not mean the batch failed. If milk separates into curds and whey, fermentation has gone further. Strain sooner next time if you prefer a smoother drink.

5. Milk and handling

When you are trying to understand a change, use the same milk for a few batches. Switching milk, grain amount, room temperature, and fermentation time all at once makes the result hard to interpret. Handle grains gently. Test a portion rather than repeatedly crushing the entire culture.

Should you break large grains?

You can, especially if you want to compare a faster batch with your usual method. Gently pulling apart a large grain increases the surface exposed to milk, which is consistent with the study. But it is not mandatory. If your current batch is consistent, there is no research-based reason to change grain size simply because the pieces look large.

For a simple test, weigh the grains and milk, keep the same ratio in two jars, leave one portion intact, separate the other into smaller pieces, and keep the milk and location the same. Compare smell, thickness, separation, and time to reach your preferred finish. Change one variable at a time.

Troubleshooting

The kefir is fermenting too quickly

Check whether the grains have increased while the milk amount stayed the same. Also check whether the jar is warmer than usual. Try more milk, fewer grains, or a cooler location. Strain when the texture and acidity suit you.

The kefir is barely changing

Check temperature, milk condition, grain amount, and whether the grains were recently refrigerated or transported. Give the culture more time and keep the next few batches simple and consistent. Do not assume that breaking grains smaller will solve an inactive culture.

The kefir separates too soon

Separation usually means fermentation has gone beyond your preferred point. Use more milk, fewer grains, a cooler location, or a shorter fermentation next time. Stirring before straining can bring the texture back together, but it cannot reverse the acidity.

The grains are breaking apart

Some softening or separation can happen when grains are handled roughly or are adjusting to a new environment. Keep them in milk, avoid unnecessary rinsing, and make the next batches simple. If grains develop unusual color, fuzzy growth, a rotten smell, or another clear contamination sign, discard the grains and milk. Do not taste a visibly moldy or rotten batch.

Key points

Grain size does affect milk-kefir fermentation. The direct evidence here comes from one controlled study: at a defined 5% grain-to-milk proportion, pieces around 3 mm fermented faster than larger grains. That result explains why a jar may speed up after large grains are separated, but it is not a universal size rule. Keep the ratio in view, account for changing room temperature, observe the jar, and change one variable at a time.

Common questions

Sources

1. Spicer A, Fairhurst DJ, Newton MI, Morris RH. An evaluation of kefir grain size with magnetic resonance imaging to observe the fermentation of milk. Magnetic Resonance in Chemistry. 2019;57(9):730-737. doi: 10.1002/mrc.4853. PMID: 30801761.
2. Authors' accepted manuscript, Nottingham Trent University