Anomalous firmness changes in cool-stored kiwifruit

Abstract

Standard softening curves generated by destructive penetrometer measurements for fruit such as kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson cv. `Hayward') consistently decrease with time. However, kiwifruit softening curves that unexpectedly plateau and/or rise during medium to long periods of cool storage have been observed when using the impact force analysis technique to measure fruit firmness non-destructively. The anomaly is explained by experimentally showing that fruit impact force response is closely correlated with whole fruit stiffness, a fruit property quite different to the flesh rupture force measured by the penetrometer. Whole fruit stiffness is measured by parallel plate compression and can remain constant for long periods, or even increase slightly, whilst the flesh rupture force, however, continues to decrease. Mechanisms are suggested by which this could occur and the implications for non-destructive firmness measurement are discussed.