Why a Potato Loses Weight Sitting Still in Storage — and How to Calculate It

Water accounts for 78 to 88 percent of a raw potato's weight, the bulletin notes, citing tuber composition data compiled by Leonel et al. (2017) — starch, protein, ash, crude fiber, and fat make up the rest. That single fact explains why water loss, not decay or sprouting, is described as the primary driver of storage weight loss, occurring mainly through transpiration and, to a lesser degree, respiration, according to Czerko et al. (2023) as cited in the bulletin. Burton (1989), also cited by the authors, found that 97.6 percent of that water escapes through the tuber's skin, with the remaining 2.4 percent exiting through lenticels alongside the carbon dioxide produced by respiration. Both pathways are shaped by biological factors — surface-to-mass ratio, bruising, suberin and wax coverage, maturity — and environmental ones, namely storage temperature, humidity, and air velocity.

Temperature, not humidity alone, sets the ceiling on how much water a tuber holds onto. The bulletin explains that the air inside a potato's intercellular spaces sits at close to 100 percent relative humidity, so a tuber's internal vapor pressure is essentially the saturation vapor pressure for whatever temperature the tuber itself has reached. Using saturation-pressure values sourced to Díaz-Pérez (2019), the authors work through an example at 48.2°F: with surrounding air at 95 percent RH, the vapor pressure deficit driving water loss out of the tuber is 0.058 kPa — but drop that same air to 90 percent RH and the deficit doubles to 0.115 kPa, with no change in temperature at all. The bulletin puts the theoretical break-even point at 97.8 percent RH, assuming air and tuber temperatures match exactly. Even there, Xanthopoulos et al. (2017) are cited noting that respiration keeps producing some water loss even when the vapor pressure deficit reaches zero — meaning true zero-loss storage isn't actually achievable.

Storage managers have a blunt option and a precise one for tracking shrinkage. The simple version places mesh bags of tubers around a storage space and compares initial and final weight — in the bulletin's worked example, 30 lb dropping to 27 lb works out to a 10 percent loss. That number is comprehensive but crude: it lumps together sprouting, decay, and pure water loss with no way to separate the causes. The more targeted option is the ASAE EP475.3 formula, which isolates loss driven specifically by vapor pressure deficit and sprout percentage. Run through the bulletin's own example — nonsprouting potatoes, two weeks into storage, 59°F, 95 percent RH — the formula yields an estimated 0.45 percent weight loss per week. Critically, the authors flag their own model's limits directly: for immature potatoes or tubers with significant harvest damage, actual weight loss can run twice as high as the formula predicts, and they describe such cases as inherently difficult to estimate with confidence.

Heat generated by the tubers' own respiration creates a temperature gradient from the bottom of a storage pile to the top, since cold air enters at the base and warms as it rises through the crop. The bulletin calls this gradient delta T and recommends keeping it between 0.5°F and 2.0°F, since a wider gradient means warmer, drier air moving through the pile and pulling more moisture off tuber surfaces as it passes. But the authors are explicit that no single formula or delta T target generalizes across every operation: weight loss depends on cultivar, maturity, handling, disease pressure, stress, harvest-time weather, growing location, storage temperature and humidity, sprouting, and postharvest treatment — a list long enough that the bulletin treats its own equations as estimation tools rather than guarantees.

The bulletin closes with a set of practical recommendations built directly from the physics it lays out:

• Harvest physically mature potatoes, since immature tubers show higher weight loss.
• Harvest at cooler tuber temperatures to minimize the initial vapor pressure deficit.
• Minimize mechanical injury during harvest.
• Cure potatoes as quickly as possible whenever curing is recommended.
• Maintain a delta T of 0.5°F to 2.0°F across the pile.
• Provide adequate ventilation to cool tubers, supply oxygen, and remove carbon dioxide while holding proper humidity.
• Apply sprout suppressants to limit sprout-driven water loss.