Quality of fresh-cut apple (Malus domestica) as affected by the sharpness of the cutting tool

Fruit and vegetables represent an important source of nutritional components, such as vitamins, minerals and fibers. However, when subjected to minimal processing, a faster deterioration and a reduction in terms of nutritional value, as well as general appearance, can be observed. During fresh- cut process quality loss occurs mainly during cutting procedures, such as slicing, peeling and shredding. In fact, cutting tools are responsible of cellular membrane break, leading to a reduction in terms of chemical and physical characteristics, as well as a major exposure to microorganisms contamination. Mechanical damage is correlated both to the morphology of the cut tissue and the sharpness of the cutting tool. Many studies have been conducted regarding the response of fresh-cut products to mechanical damage, especially for fresh-cut apples, unlike the characteristics of cutting blade. This thesis aims to set an effortless method to evaluate and compare blade sharpness and explore the effects of cutting blades, according to various Degrees of Sharpness (DoS), on fresh-cut apples. A methodology was developed for the objective assessment of cutting damage on fresh-cut produce based on DoS, defined as the required force exerted by the instrument to cut a reference body. To this aim, 3 kitchen knives (A, B and C) were used with 8 DoS values (from 1, sharpest to 8, bluntest) to cut silicon plugs as reference bodies with cross-section diameters of 6 and 8-mm. For the 6-mm plugs and knife A, the force varied from 16.2±0.5 N (DoS=1) to 93.6±7.0 N (DoS=8), for knife B from 17.9±0.4 (DoS=1) to 80.1±6.1 N (DoS=8), and for knife C from 17.4±0.9 (DoS=1) to 155.3±2.0 N (DoS=8). As for the 8-mm plugs forces ranged for knife A, B and C from 26.0±1.1 N (DoS=1) to 105.6±7.6 N (DoS=8), from 29.0±1.3 (DoS=1) to 104.6±6.5 N (DoS=8) and from 26.1±0.7 (DoS=1) to 195.9±11.7 N (DoS=8), respectively. Cutting force mean values for all knives and DoS depicted statistically significant differences in relation to standard deviation values. Following, kitchen knives at 3 different DoS corresponding to 30, 90, and 140 N of cutting forces, labeled as DoS1, DoS2 and DoS3 respectively, were used to cut fresh apples and quality of the product was evaluated during storage in terms of changes in the visual appearance (sensorial), CIELAB values and chemical response. The results confirmed that color changes in terms of browning were higher as the DoS decreased with data significantly fitting an exponential relation, pointing out that the blade sharpness is a key factor involved in the success of the cutting operation. On the base of the results achieved, a methodology has been developed in order to define an objective method to be used as essay to evaluate cutting damage on fresh tissue. The methodological approach involved the use of 3 kitchen knives (A, B and C) each possessing 4 DoS values from the sharpest to the bluntest following an order starting at 30 N to 90 N, 140 N, and 190 N respectively, and were addressed to cut in total of 96 apples into 288 slices for the physical quality assessment of color, visual acceptance score, electrolytic leakage which represents the severity of cell damage, and nutritional quality evaluation of total soluble solid content (SSC), total acidity (TA), pH and total phenol during 14 days storage. A two-way ANOVA was performed to assess the effect of DoS on the quality of apples during storage and the results showed that there was no any significant correlation between DoS and nutritional quality, however color change in terms of browning index and a* increased significantly as the DoS decreased with the value at 39.42, 2.68 in 30 N and 41.41, 3.11 in 190 N respectively, whereas for the visual acceptance score, the best was obtained at 3.98 for DoS 30 N allowing the shelf-life of the fresh cut apple up to 14 days while the worst at 2.95 for DoS 190 N with the shelf-life reaching only up to 5 days. The impact of mechanical damage on the physical and chemical properties of ‘Golden Delicious’ apples was investigated and the results were further validated using the X-ray micro CT. The apples were cut into slices using kitchen knife maintaining four degrees of sharpness (DoS), termed as DoS1 (sharp blade) DoS2, DoS3 and DoS4 (blunt blade). The analysis of the color parameters over a 15 days storage period indicated that apples sliced with DoS1 depicted higher L* value of 80.81 in accordance with non-remarkable changes in the a* value over the storage period. In case of the apple slices cut with blunt knife (DoS4), the L* value significantly reduced to 78.08 after the 15-day storage period together with an increase in the redness values (a*) from 1.22 to 3.32 on the 15th storage day. X-ray micro-CT revealed that while in case of DoS1 slices have a porosity of about 13% close to the cut surface, for the DoS4 samples this parameter drops to 11% for the same sample portion, confirming how a sharp blade induces a minor wounding stress. To sum up, cutting blade sharpness is an important factor affecting the marketability of fresh-cut apples. Data presented underline how the use of a sharp cutting tools significantly reduce physical symptoms, particularly browning, supported by the results obtained from X-ray micro-CT. This could be a hopeful base for further research to better understand the threshold above which, both for physical and microstructural point of view, wound stress becomes harmful for fresh-cut products.