The load required to separate a red raspberry fruit from its receptacle, here termed the fruit retention strength (FRS), was measured on berries at increasing stages of ripeness. The FRS decreased from about 300 g for underripe berries to 25 g for overripe berries. The titratable acidity of the juice of these berries decreased from 392 ml to 107 ml of 0.1 N NaOH/100 ml of juice for underripe and overripe berries, respectively. The loge of the FRS was 107 ml correlated (r=0.92) with the titratable acidity of the fruit juice. The use of titratable fruit acidity to measure the efficiency of raspberry mechanical harvesters is discussed.