The constituent profile of pure neem oil remains relatively stable as long as the oil remains solidified and out of direct sunlight. When subjected to repeated "thawing" to a liquefied state at temps above 60°F, neem oil will deposit a thick black viscous layer called stearin, at the bottom of the drums and containers. That's not necessarily a loss per se. Fatty acids are especially vital in the holistic context, and neem contains a wide array beyond stearin.
A bit more is known about azadiractin content. Cold pressing of the mature seed kernel of the neem tree results in an oil with an azadiractin content as high as 3000 to 3500 ppm. This group of constituents is what gives neem its striking impact on insect development. Studies indicate that long time storage at a temperature above 30°C (86°F) results in the reduction of azadirachtin content below 1800 ppm. Extrapolate from there.
The upshot? It's reasonable to store neem for a year or two, perhaps even three, if a bulk purchase saves money in the long run. Ideally that larger container gets "thawed" but once, at which time the oil is poured into batch size containers. (If you need a quart of neem oil each time you spray, that then is the batch size for your orchard). Liquefy each batch gradually by bringing it out of cool storage the day before it's needed, and if it's still gunky in the morning, stick the jug in a pot of warm water for an hour or two. Don't use very hot water as this will reduce the effectiveness of the oil.
Finally, Peter, as to increasing the concentration to account for age ... absolutely not. Any oil proves phytotoxic at higher concentrations than the recommended 0.5% for foliar application.
Lost Nation OrchardZone 4b in New Hampshire
Edited 1 time(s). Last edit at 03/05/2014 12:30AM by Michael Phillips.