Walnut quality and value maximized by harvest management
These studies show that early walnut harvest minimizes insect-damaged kernels and maximizes quality and resulting kernel value. Direct relationships exist between harvest date and nut quality. Differential in value within walnut growing districts is directly dependent on time elapsed between date of maturity and harvest. Temperatures during harvest may account for differences in value between districts. Once nuts are removed from the tree, picking should proceed as rapidly as possible to maintain optimum kernel color and value. Rate of loss in kernel quality is directly proportional to increasing air temperature. Kernel temperature of about 140°F appears to be the threshold for onset of kernel darkening, and is reached in the sun at air temperature above 90°F and in the shade at air temperatures of 104°F or greater. Once threshold kernel temperatures are attained, substantial loss in value can occur within nine hours. Threshold kernel temperatures are reached fastest when nuts are harvested during midday. Rapid pickup is essential at that time to maintain quality. Shaken nuts with hulls still intact, typical in harvest of early varieties, reach threshold kernel temperatures faster and remain at those temperatures longer than nuts without hulls. Sun-exposed nuts on the tree remain cooler than those exposed to similar condition on the ground. This suggests that growers should store nuts on the tree for short periods of time if delays in picking are anticipated.