Boil Once: The Fuel Cost of Safe Water for Formula, and How Long a Flask Keeps Water Inside the Safe Band

Powdered infant formula is not sterile, and established guidance is to reconstitute it with water at no less than 70 °C so that heat inactivates Cronobacter sakazakii and Salmonella that the powder may carry. Where fuel is scarce, heating water for every feed is a real daily cost, and storing boiled water in a vacuum flask is the obvious alternative — provided the stored water is still inside the safe band when it is used. This paper couples two computations. First, with adopted thermal-inactivation constants (Cronobacter sakazakii: decimal reduction time 2.5 min at 60 °C, z-value 5.5 °C; Salmonella enterica: 1.8 min at 60 °C, z-value 6.0 °C), a 6 log10 reduction takes 0.228 min at 70 °C but 15.000 min at 60 °C and 986.900 min at 50 °C, so temperature at the moment of mixing, not holding time, is the binding constraint. Second, heating water costs 0.0837 kg of wood per litre on the stated stove at an ambient temperature of 25 °C, and the boil-every-feed pattern (8 feeds, a minimum practical kettle volume of 0.30 litres per feed, 2.4 litres boiled per day) uses 0.2009 kg of wood per day against 0.1005 kg per day for boiling the day's 1.2 litres in flask-sized batches — a saving of 0.1005 kg per day, 50.0%, or 36.7 kg per year. A Newton-cooling model of household vacuum flasks filled at 95 °C then gives the flask window, the hours the water stays at or above 70 °C: a 0.5 litre flask lasts 4.84 h at an ambient temperature of 15 °C, 5.71 h at 25 °C, and 6.96 h at 35 °C, and so cannot cover an 8 h night at any ambient temperature modelled; a 1.0 litre flask lasts 7.92 h, 9.34 h, and 11.40 h, failing the night only in the cold room; a 1.8 litre flask lasts 11.20 h, 13.21 h, and 16.12 h and covers the night everywhere modelled. A half-emptied 1.0 litre flask falls to a window of 4.67 h at 25 °C, so the flask must be filled to the brim and the last feeds of a stretch are the first to leave the safe band. The supported protocol — one heating per flask fill, powder added fresh at each feed, water below the band never used for a young infant's feed — halves the fuel cost of safe preparation, and its limits are printed with it.