|The percent lost number is derived from the hop storage index (HSI) or is measured directly (9). These data are generally provided by hop brokers and researchers. If you know the HSI for a variety, you can calculate the percent lost by using the following formula (5):
where log is the base 10 logarithm.
The k value (or rate constant) is calculated according to the following formula (6):
k = (lnAo - lnAN)/180
where ln is the natural logarithm, AO is the original a-acid value, and AN is the new a-acid value after 180 days. Now you may ask, "How do I get the original a and new a values?" As it turns out, you don't need to know. If you know the percent lost you can calculate AO and can assume any arbitrary number for AN. Calculate AO (for any value of AN) using the following formula (7):
AO = (AN*100)/(100*%Lost)
The reason AN can be any arbitrary number is that all we are really calculating here is a ratio, based on percent lost. Just make sure that when you go back to the k calculation you use the same arbitrary number you used to calculate AO.
The temperature factor (TF) is based on the research that showed that the rate of deterioration is halved for every 15 °C (27°F) drop in temperature (1). This is an exponential curve, and I simply used Excel's curve-fit algorithms to fill in the data points, using 20 °C (68°F) as the point at which no adjustment is necessary (because this is the point at which the data are measured). What I have not done is to calculate a temperature factor for values above 20 °C (values >1) because the table was getting fairly large, and you really shouldn't be storing your hops anywhere near this temperature anyway.
The storage factor (SF) is based on interpretations of the data presented in the references. A factor of 1 means no adjustment, again correlating with the measurements of percent lost.