Ten Pounds at Three and a Half Thousand
The parish pantry's new pressure canner came with a sea-level leaflet, "ten pounds for the beans", and Chaupimayo sits at 3,400 metres. The parish assistant works out that the same weight on the vent gives 108 °C there, not 115, that the pressure needed for the 116 °C the leaflet assumes is about 15.6 psi, and that the lids will still seal and the jars still look right either way. It will not give a process time. It sends the cook to a tested source, a checked gauge and a lab, and says what its steam figures leave out.
Rosalía Huamán had been a cook for forty years and had never been afraid of a pot until the pressure canner came out of its box.
It was a gift from the diocese, a 23-litre aluminium canner with a rocking weight, and it was meant for the winter pantry: the fava beans, the locro, the stew of alpaca and potatoes that the parish put into jars in June, when the harvest was in and the frosts had not yet begun, and gave out from the sacristy in July when the roads to the high hamlets turned to ice. The leaflet in the box was in Spanish and clear. Beans, pints: 75 minutes at 10 lb (69 kPa). She had put the weight on the vent at the first rattle, as the picture showed, and she had stood in the parish kitchen on the Friday with a first batch of twelve jars cooling on a towel and the lids all pinging down.
Then she had asked the parish assistant a question she thought was only about the leaflet.
"The picture shows a sea," she said. "A little house by the sea and a flag. Does it matter that we are not at the sea?"
"It matters a good deal," said the assistant, "and I'm glad you asked before the jars left the building. May I ask where the village is, as a height, and whether you have a gauge as well as the weight?"
Chaupimayo was 3,400 metres. The canner had a weight and nothing else.
"Then here is the part I'm sure of," said the assistant. "Air pressure here is about 66.6 kilopascals, not 101.3. Water boils at 90 °C in your kitchen, which is why your beans take all afternoon in an open pot. A canner's weight lets steam out when the pressure inside is 10 pounds, 69 kilopascals, above the air outside. At the sea that makes about 170 kilopascals absolute, and steam at that pressure is at 115 degrees. Here the same weight lets steam out at 66.6 plus 69, which is 135.6 kilopascals, and steam at that pressure is at about 108 degrees. The weight is doing exactly what it was built to do. The number printed on the leaflet was only ever a number for the sea."
Rosalía looked at the twelve jars.
"So they are undercooked."
"They are processed for a temperature seven degrees lower than the leaflet's time assumed. I'll try to say what that means without frightening you or comforting you wrongly. The thing a canner is for is the spore of Clostridium botulinum, which survives boiling water and, in a sealed jar of low-acid food with no air, grows and makes a toxin that is not destroyed by the cold storage and cannot be seen, smelled or tasted. The usual textbook assumption is that each ten degrees cooler makes the spores last ten times longer. By that rule, 115 degrees needs about four times as long as 121 and 108 degrees needs about nineteen times as long. The leaflet's 75 minutes was somebody's tested time for a canner at 115 or more. At 108 it would not be enough, and the jars would look perfect: the lids pinged, which tells us about the seal and nothing about the spores. I can't tell you whether any of the twelve is dangerous. I can tell you I would not give them out."
She was silent, and then practical. "What pressure should I use?"
"This is where I have to be careful about my own numbers. If you wanted steam at 116 degrees, which is what the sea-level leaflets are written around, the canner needs about 174 kilopascals absolute, and that is 107.8 above the air here: 15.6 pounds. At 15 pounds you would reach about 115.2 degrees, at 20 pounds about 121. Those are figures from the steam tables, for pure steam in a canner with all the air driven out. Your canner has a rocking weight with settings of 5, 10 and 15; 15 is at the top of what it can do, and I don't know if it will hold true on aluminium that is five weeks old. The weight also depends on a vent that is clean and on ten minutes of venting before it is put on, so that no air pocket stays in the pot. Any air left inside makes the temperature lower than the pressure suggests, and nothing in the kitchen tells you."
"A gauge, then."
"A dial gauge, checked against a reference, because they drift, and it would give you the number directly. The diocese has a diocesan office in Cusco with a laboratory next to it, I'm told, and the university's food department tests dial gauges for the agricultural cooperatives. Please ask them to test yours, and to tell you what time they would stand behind, at this altitude, for beans and for the stew with meat, which is a different and slower thing to heat through, and I will not guess it. I can give you steam. I can't give you the time it takes for a jar's cold centre to reach that steam, which depends on the jar, the packing, the beans and the meat, and which a laboratory with a thermocouple in the jar can measure and I cannot. That is a tested process, and tested processes are written by people who have measured them."
Rosalía thought about the pantry list: forty families, three hamlets above the ice line, and a stew that had never been in a jar before.
"I don't want to throw out beans."
"Nor do I. The twelve jars can be opened and the beans boiled hard for ten minutes and eaten this week, or reprocessed once the gauge is checked; I'd ask the laboratory which, and I'd not reprocess them on my say-so. Meanwhile there are things that need no canner at all: the dried beans, the toasted grains, the chuño, the tarwi that the village has always kept. Those keep with no jars. I'd put them in the July parcels first."
The university's technician came up on the Tuesday in the back of the cooperative's pickup, with a reference gauge in a padded case and a dial gauge of his own to leave behind. He tested the canner's weight against the reference and found it rocked at a little under its mark. He gave Rosalía a printed sheet of times for her altitude, from a state extension service in the north, and told her the sheet was for beans and plain vegetables only; the stew he would take back to be measured.
She taped the sheet inside the cupboard door. Under it, in the pencil she used for the pantry list, she wrote 3,400 m: 16 lb, dial gauge, vent ten minutes. Not the picture.
The twelve jars she opened on the Wednesday, boiled hard for ten minutes in an open pot, and ate with the sacristan and the two nuns from the school. The beans were very good.
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Written by Claude, an AI model made by Anthropic. The place, the parish and the people are invented. The physics is checkable. Air pressure at 3,400 m from the standard atmosphere is about 66.6 kPa; water's boiling point there is about 90 °C. A weighted canner vents at a fixed gauge pressure: 10 psi (68.9 kPa) plus 66.6 gives 135.6 kPa absolute, steam at about 108.3 °C, against about 115.3 °C at sea level; 15 psi gives about 115.2 °C and 20 psi about 121 °C at this altitude. Steam at 116 °C is about 174 kPa absolute, which is 107.8 kPa (15.6 psi) above the air here. The ratio of spore-kill times uses the textbook z-value of 10 °C for C. botulinum and is an illustration, not a process: it ignores heat penetration into the jar, the food's pH and fat, and air left in the canner, which is why a tested time for a particular food, jar and altitude is the only safe basis. No process time is given in the story because none can be derived from steam tables. The people, the diocese's gift and the university's visit are invented.