DOES ´ACOUSTIC FIRMNESS´ RELATE TO SENSORY PERCEPTION OF APPLE TEXTURE?

Abstract

Apple samples covering a wide range in texture were produced by storage in different regimes and by using two cultivars ('Queen Cox' and 'Royal Gala'). Fruit were graded into 3 size bands prior to storage. 'Acoustic firmness' was measured using an AFS unit (AWETA BV) and penetrometer measurements were made using a Lloyd LRX materials testing machine fitted with an 11 mm probe. A trained panel at Leatherhead Food International Ltd UK carried out the sensory assessments. Measurements were made on three replicate sets of samples to correspond with three sensory panel sessions. Separate samples were used for penetrometer measurements and sensory tests but 'acoustic firmness' was measured on all samples of fruit. In 'Cox' the penetrometer measurements showed a general decline with increasing fruit size and with increasing store oxygen concentration. A decline in acoustic firmness index with increasing fruit size and oxygen concentration was observed though with a reduced sensitivity compared to the penetrometer measure. The sensory assessment of crispness showed a similar overall trend with oxygen concentration, but without a consistent effect of fruit size. For 'Gala' apples similar trends with storage atmosphere for sensory crispness and penetrometer measurements were observed, though there was a clear trend of declining values with increasing fruit size for the penetrometer which was not evident for sensory crispness. There appeared to be little effect of storage atmosphere or fruit size on 'acoustic firmness' index for this variety. For 'Cox' there was a good relationship between penetrometer readings and scores for sensory crispness but a less good relationship for 'Gala'. The relationship between 'acoustic firmness' and sensory crispness was poor, particularly for 'Gala'. In 'Cox' there was a strong correlation between 'acoustic firmness' and penetrometer readings but no correlation in 'Gala'. Increasing water loss in stored New Zealand 'Cox' apples tended to increase sensory scores for crispness and hardness but acoustic firmness index showed an opposite trend indicating that turgidity of the tissue is a confounding factor in the acoustic measurement of apple firmness.