The Coin on the Ice
An ice storm cuts the power for 31 hours and the parish food bank's chest freezer holds 230 kg of donated food. The assistant's battery logger gives a cooling curve, a time constant of 63 hours and a prediction that nothing has thawed; the fridge beside it, at the hall's 9 degrees, is a different answer. One bin is filled and one is not, and the reason is written on a card.
The power went at ten past two on the morning of the fourteenth of January, in the middle of an ice storm that put a centimetre of glaze on every wire between Strathcorrie and the substation at Aberfeldy, and it did not come back for thirty-one and a half hours.
Dermot Rennie found out at seven, when he let himself into Saint Fillan's hall to put on the urn for the Wednesday volunteers and the urn did nothing. He had been a grocer in Pitlochry for forty years and had run the parish food bank since he retired, and he stood in the dark kitchen with his hand on the lid of the big chest freezer, which was cold, and the upright fridge beside it, which was cold, and he did what every grocer in Scotland does. He opened both to look.
The freezer lid was up for about forty seconds. He shut it, and then he stood there and did the sums he had been taught in 1981.
"Tally," he said. He did not much like the thing, which had been installed by the diocese's volunteer office, but it ran on a battery and it was the only voice in the building. "What's the temperature?"
"The freezer is at minus sixteen and a half at the top, where my logger sits," said Tally. "It was minus eighteen and a quarter when the power went. The fridge is at six and rising. The hall itself is at nine. I have been logging since ten past two on the little coin-cell units, so I can tell you what has happened and not only what it is now."
"How long will it hold?"
"I would rather give you a curve than a number. May I?"
Here is the idea, and it is simpler than it sounds.
A freezer with the power off is a heavy box of frozen food leaking heat in through its walls. The food is the store of cold. The walls are the leak. How fast the leak runs depends on how far the inside is from the room, and as the inside warms, the gap closes, so the leak slows. The temperature does not climb in a straight line. It climbs fast at first and then more slowly, always heading for the temperature of the room, like a cup of tea going cold.
That shape has a single number in it, the time constant. It is the heat the food can soak up for each degree of warming, divided by the heat the walls let in for each degree of gap. Big thermal mass makes it longer. Poor insulation makes it shorter. After one time constant the gap to the room has closed by about sixty-three per cent. After two, by about eighty-six.
Tally had two of the numbers in hand. The food bank's stock book said the chest freezer held about 230 kilograms of donated food, which is mostly water, and frozen food takes up about 1.9 kilojoules for every kilogram for every degree, a round figure good to perhaps fifteen per cent. That made the load worth about 437 kilojoules per degree, ignoring the freezer's own walls and the packaging. It did not have the third number, the leak. The leak it could get from the logger, by waiting.
By Wednesday afternoon it had been logging for twelve hours. By ten at night, twenty. The numbers at the top of the freezer, thirty-minute readings, ran like this:
Hour 0, minus 18.2. Hour 12, minus 13.3. Hour 24, minus 9.5.
"Fitting those to a room at nine degrees," said Tally, on the Wednesday evening, to Dermot and to Sister Agnes Lennie and to the Wednesday volunteers, who had come anyway, in coats, "gives a time constant of about sixty-three hours, with an error of perhaps five or six. That means the walls let in about two watts for each degree of gap, which is believable for a chest freezer of that age."
"What's it mean for the food?"
"It means I can predict where it is going, not only report where it has been. The temperature at hour thirty-one and a half should be about minus seven and a half. If the power returns by then I expect the freezer to read minus seven and a half at the top. The food will reach minus two, where it starts to soften and ice starts to melt, at about hour fifty-seven. It will reach zero at about sixty-nine. That is a prediction, and I will tell you at the hour whether it was right."
Sister Agnes said, "And if the prediction is wrong?"
"Then my first guess is that the walls leak more than I think, which would shorten all of it, and I would see the next reading drift low against the curve and say so. I can't tell you the curve is right. I can tell you what would show it wrong."
Dermot said, rather loudly, that the rule on the diocese's food-safety card was that a freezer without power for four hours was to be emptied. He had it in his hand. It was laminated.
"The card is for the fridge, and it is right for the fridge," said Tally. "It is also written to be safe with nobody measuring anything, which is a reasonable thing to ask of a card. A full freezer with the lid down is commonly said to hold for about two days and a half-full one for about one, and those figures are also for people who have no logger. You have a logger. May I read what it says, and then you decide?"
The fridge was a different story and Tally did not soften it.
It was a domestic upright, packed lightly: milk, butter, about thirty cooked meals in foil trays that the Tuesday cooks had made from donated mince and potatoes, soft cheese, opened tins decanted into tubs, six dozen eggs. It had no thermal mass to speak of and a door seal that Dermot had been meaning to replace. The logger inside it had read six degrees at seven in the morning. By noon it read nine, which was the hall's own temperature, and from noon onward it moved with the hall, a little under ten at the warmest hour and just under nine in the evening.
"The chilled limit is eight degrees," said Tally. "That is the legal ceiling in the shop, with a short allowance when something is out being served. The fridge has been above eight for nearly twenty-eight hours. Above eight for that long is the condition the card is written for."
"The milk smells all right," said Dermot.
"Most of the things that would make you ill don't have a smell. I am not saying that to frighten you. I am saying that nothing in this fridge that was cooked, soft, opened or made from meat can be kept on the strength of how it smells. Butter, hard cheese, eggs in their shells, and anything sealed and shelf-stable that happened to be in there: those are fine, and are not going in the bin. The thirty meals are going."
Nobody said anything.
"They were for Saturday," said Sister Agnes, at last.
"I know," said Tally. "I'm sorry. I would sooner tell you this now than on the Sunday."
Dermot lifted the foil trays himself, one at a time, into the black sacks. Nobody helped him and nobody offered. The Tuesday cooks had done them in November on a Tuesday after Mass, and he said the names of the people who had signed the donation tickets, quietly, the way you do when you carry a coffin.
The power came back at 09:40 on the fifteenth, with a clunk from the cupboard where the meter lived and the freezer's compressor coming on with a shudder that made everyone in the kitchen look up.
"Hour thirty-one and a half," said Tally. "The top of the freezer reads minus seven and one tenth. I predicted minus seven and a half. The error is under half a degree, which is inside what I said I could do, and I am glad of it, because it is the only way I have of knowing I wasn't fooling myself."
Dermot lifted the lid and put his hand flat on a bag of frozen peas. They were hard. He lifted three bags and tried a block of mince, then the tub of ice cream somebody had donated in December that nobody had had the heart to give out, and the ice cream was soft at the edges and hard at the middle.
"The logger sits at the top, in the air, where it is warmest," said Tally. "The middle of the pile will be colder than the number I gave you. I would trust the pile to be at least as cold as the logger says, and that is on the safe side. It is not the same as saying every single item is fine. If you find any pack with the wrapping bloated, any liquid pooled at the bottom of a tub that has refrozen into a block, or anything that has plainly gone soft and is not frozen in the middle, that is a separate decision, one item at a time. The ice cream will not be good ice cream. That is quality, and it is not safety."
"The card says if there are ice crystals it can go back in."
"It does, and that is the card's second page. I would keep to it."
So they kept the freezer. They went through it that morning, all of it, with the volunteers in a line passing things out to a table and back in: 230 kilograms, which Dermot reckoned at about 600 meals at the 380 grams the food bank gave to a person. They threw away eleven items. Seven were wrapping that had split; three were mince that had been bought on its sell-by date and had no business surviving a thirty-one-hour power cut; one was the ice cream, which Dermot ate, standing up, from the tub, and said was not bad.
The Saturday distribution went ahead. There were twenty-six households. The thirty meals were not there, and the soup that Sister Agnes made on the Friday, on the hall's gas ring, went further than anyone had a right to expect.
What remained was the thing Tally had been trying to say all week and had not found a way to say in the middle of it.
On the Sunday evening, after Mass, it asked Dermot whether it could change something about the freezer, and Dermot, who had by then decided it was not the worst thing the diocese had ever installed, said yes.
"There is a trick for people with no logger," said Tally, "and I should have set it up in November, when I first put the units in. It is a cup of water frozen solid in the freezer with a coin laid on top of the ice. If you come home after a power cut and the coin is still on top, the freezer never got above freezing for long. If the coin has sunk to the bottom or into the middle, the ice melted and froze again, and everything above the coin's depth was warm enough to matter."
"A coin," said Dermot.
"A ten-pence piece. It costs nothing, it needs no battery, and it cannot be argued with. I would like to put one in the freezer and one in each of the other freezers the parish lends out, with a card on the lid that says what it means and what to do. The card would say that a sunk coin means throw it out, and that a coin on top means check by hand, and in neither case rely on a smell."
Dermot thought about this. He said it would be a good deal cheaper than a logger.
"It is not as good as a logger," said Tally. "A logger gave us a curve and a prediction. The coin gives only yes or no. But yes or no, in a cup that cannot be lost or misread, is what a woman on her own in a farmhouse in Glen Lyon will actually use."
On the Monday Dermot froze the cup. On the Tuesday he laid a ten-pence piece on the ice, took a photograph for the file, and wrote on the laminated card in biro, under the diocese's four-hour rule, Fridge: four hours. Freezer: look at the coin..
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This story was written by an AI and is fiction. Strathcorrie, Saint Fillan's hall, Dermot Rennie, Sister Agnes Lennie and the assistant Tally are invented; Aberfeldy and Pitlochry are real places, and no real food bank, parish or person is described. The numbers are estimates and are rounded. A closed freezer with the power off warms toward the room temperature roughly exponentially, T(t) = Troom − (Troom − T0)·exp(−t/τ), with τ the thermal mass divided by the heat leak. With 230 kg at an assumed 1.9 kJ per kg per degree (1.7 to 2.1 is typical for frozen food; the freezer's own walls and the packaging are ignored), 437 kJ per degree and τ = 63 hours give a leak of about 1.9 W per degree, in the plausible range for a chest freezer. Three logged readings (−18.2, −13.3 and −9.5 at hours 0, 12 and 24, in a 9 degree room) fit that curve; it then predicts about −7.1 to −7.5 at hour 31.5, −2 at about 57 hours and 0 at about 69. Freezing and thawing absorb latent heat, so the curve is optimistic near zero and the story treats it as a warm-up estimate only in the sub-zero range; a half-empty freezer has less thermal mass and warms faster; and a logger in the air at the top is the warm side of the load. The "about two days full, about one day half-full" figure and the four-hour rule for a refrigerator are widely published rules of thumb that assume nobody is measuring; the 8 degree limit for chilled food is the UK legal ceiling in retail, and the home advice is to keep a fridge at 5 or below. The coin-on-ice cup is a widely recommended home check. Which items to keep or discard, and when, depends on the food and on your own national food-safety authority's guidance; this is not food-safety advice, and a food bank should follow its regulator's rules.