Protein content of plant tissues is usually estimated by multiplying total
nitrogen by a conversion factor of 6.25. This technique assumes that all ni
trogen originates from protein. When applied to fruit pulp, it overestimate
s protein content because pulp typically contains free amino acids and many
nitrogenous secondary metabolites. At issue is the extent of error and, co
nsequently, what the conversion factor between nitrogen and protein should
be. We calculated a conversion factor based on pulp samples from 18 species
collected in the southeastern United Slates. We also report a new and simp
le method of estimating protein and free amino acids in fruit pulp. Because
previous studies have found high variation in protein and secondary metabo
lite content among fruit species, use of a single conversion factor for all
species will generate error. In an attempt to reduce such error, we calcul
ated protein contents and conversion factors separately for two common frui
t types: lipid-rich/carbohydrate-poor and lipid-poor/carbohydrate-rich. We
found no difference between these types of fruit and hence combined results
in calculating an average conversion factor of 5.64, Use of an accurate co
nversion Factor is important in estimating protein consumption by wild anim
als and in formulating diets of captive animals. It can also reveal whether
loss of body mass in captive animals on fruit diets is due to insufficient
protein consumption, secondary metabolite toxicity, or an imbalance of ami
no acids.