Ambientes motivacionales positivos

Splitting can happen during the harvest, storage and packing processes as produce is exposed to changes in environmental conditions and mechanical stresses. The principle post-harvest changes to the crop which are thought to effect susceptibility to splitting as a result of mechanical damage are changes in tissue water status and temperature.

36 Previous research into the causes of harvest splitting has focused on these two areas. These have been summarised in Table 1-7 and Table 1-8.

1.3.4.1 Tissue water status

Table 1-7 Summary of the effects of tissue water status on harvest splitting in different crops as reported in published scientific papers

Factor Crop Effect on splitting Reference

Increased turgor Carrot Increased residual stress and strain

Kokkoras (1995)

Decreased turgor by partial lifting

Carrot Decrease Gracie (2004)

Increased turgor Potato Increase Konstankiewicz and Zdunek (2000) Bajema et al. (1998) Increased water

potential and turgor

Carrot Decrease in failure force

McGarry (1993)

Increased turgor Several (review paper) Reduction in resistance to damage Galindo et al. (2004) Increased water potential Carrot and radish

Increase in cut force Herppich et al. (2004)

Postharvest tissue water status is an important factor affecting splitting susceptibility. It is thought to affect tissue mechanics and splitting susceptibility through turgor pressure. At high turgidity plant cell walls are believed to already be stretched and as a consequence are more easily ruptured (Kokkoras 1995).

An indication of water status having an effect on splitting comes from cherry varietal tests for splitting. These are performed by placing cherries into water and recording the percentage which split. Although this test does not fully replicate in situ splitting, correlations have been observed with results from the field (Measham 2011). Evidence of

37 a reduction in turgor reducing splitting susceptibility is provided by Gracie (2004). Gracie (2004) found a reduction in turgor pressure caused by partially-lifting carrots reduced splitting susceptibility. The carrots were partially-lifted to sever the fibrous root system then left in the soil over night before harvesting the following morning. These carrots with reduced turgor had a greatly diminished splitting susceptibility and could not be induced to split using a penetrometer. Further evidence to support turgor pressure affecting splitting susceptibility is provided by the investigation by Konstankiewicz and Zdunek (2001). In this investigation the turgor pressure of potato tuber tissue was manipulated by immersion in solutions of mannitol at different concentrations. They found the compressive strength of the tissue samples decreased with increasing turgor pressure suggesting potato tubers are less susceptible to splitting when they are more turgid (Konstankiewicz & Zdunek 2001). However, the use of mannitol has been criticised as it can directly influence tissue mechanical properties, it has been suggested by Bajema et al. (1998) that the effects of water relations should be investigated using fresh samples at different turgor levels achieved by dehydration in air (Bajema et al. 1998). Despite this, the results obtained by Konstankiewicz and Zdunek (2001) are similar to those of Bajema et al. (1998) who found potatoes with lower turgor, as a result of dehydration in air, had higher compressive strength than more turgid potatoes. McGarry (1993) also found failure force was negatively correlated with both water potential and turgor pressure. McGarry (1993) measured the failure strain of phloem parenchyma tissue by driving a wedge into blocks of tissue with a texture analyser. However, turgor pressure was only negatively correlated with failure strain within cultivar. The turgor pressure of the splitting susceptible cultivar used in the experiment was lower than the split resistant cultivar suggesting differences in splitting susceptibility can only be partially attributed to turgor pressure. In their review paper into factors affecting the postharvest quality of vegetables, Galindo et al. (2004) concluded that although higher turgor had been shown to correlate with lower resistance to damage it did not explain all the variation. Galindo et al. (2004) considered other important factors in determining splitting susceptibility may be the strength of superficial tissues, cell packing, adhesion and cell wall composition (Galindo et al. 2004).

38 Further confusion into the effects of tissue water potential on tissue strength comes from Herppich et al. (2004), who showed water potential was positively correlated with cutting force in carrots and radishes. They acknowledged their results contradict those of previous researchers and suggested the discrepancy may be due to differences in the mode of failure induced by the different testing methods (Herppich et al. 2004). Herppich

et al. (2004) used cutting force, whereas McGarry (1993) used a wedge to induce fractures. The effects of turgor are thought to depend on the type of splitting which is occurring. When the mode of failure is plasmoptysis higher turgor pressure has been shown to reduce tissue strength but if splitting occurs as a result of cell debonding the opposite is true (Lin & Pitt 1986). In support of this McGarry (1993) found failure force of phloem parenchyma occurred due to cell rupture (plasmoptysis), as would be expected from an increase in splitting susceptibility at higher turgor pressure. The mode of failure was not recorded by Herppich et al. (2004).

1.3.4.2 Temperature

Table 1-8 Summary of the effects of temperature on harvest splitting in different crops as reported in published scientific papers

Factor Crop Effect Reference

Decreased temperature

Several (review paper)

Increased firmness Bourne (1982)

Decreased temperature

Potato Increased splitting Bajema et al. (1998)

Decreased temperature

Carrot Increased stress Kokkoras (1995)

Decreased temperature

Carrot Decreased cut force

Herppich et al.

(2002) Decreased

temperature

Radish No effect Herppich et al.

39 There is evidence within the literature that temperature affects splitting susceptibility. In a review of the effects of temperature on a range of fruits and vegetables, Bourne (1982) showed for the majority of crops tested, increased temperature was associated with decreasing firmness, measured as failure force with a texture analyser. This relationship was represented by an approximately linear relationship. Bajema et al. (1998) also found a decrease in compressive failure strain and tissue toughness with increasing temperature in potatoes. In this investigation the effects of turgor were also investigated and a similar pattern was observed. The similarities between the effects of temperature and turgor led the investigators to conclude that the same mechanism must explain both the effects of temperature and turgor. Kokkoras (1995) also found an effect of temperature on tissue stress within carrots but found no effect of temperature on tissue strain. Tissue strain was established by cutting discs of carrot tissue and measuring the resulting deformation. The value for stress was then calculated from this. Stress was calculated by multiplying the values of strain by the modulus of elasticity. The modulus of elasticity was measured by tensile tests with a texture analyser. By affecting the modulus of elasticity, temperature affects the stress but not the strain within the carrot tissue. Kokkoras (1995) concluded low temperatures cause an increase in cellular turgidity by causing differences in contraction between the vacuole content, which is predominantly water, and the cytoplasm and cell wall. It is thought the cytoplasm and cell wall may contract to a greater extent than the vacuole at low temperatures causing an increase in turgor pressure. Therefore, low temperatures increase damageability because at low temperatures tissue becomes more turgid (Kokkoras 1995).

Contradictory results were found by Herppich et al. (2002), who found the force required to cut a carrot was negatively correlated with temperature with the highest forces being required cut the carrots at 5°C. When testing the force required for cutting radishes no relationship was found between temperature and cutting force. In the same investigation Herppich et al. (2002) investigated the effects of water status on cutting force and found a relationship for both carrot and radish. This led Herppich et al. (2002) to conclude differences in the force required to cut carrot at different temperatures must not be turgor

40 specific but must be tissue or species specific. It was speculated that the differences may be due to the ability of carrots to undergo cold acclimation, changing their cell wall properties and tissue stiffness within hours of transfer to cold conditions. They concluded the differences in cutting force at different temperatures were due to changes in the cell walls of carrots (Herppich et al. 2002). The differing results of Herppich et al. (2002) may have been due to the method of texture analysis. Herppich et al. (2002) used cut force whereas Bajema et al. (1998) used compression failure force and Kokkoras (1995) used tissue deformation. This highlights the need to design experiments so they measure the correct type of tissue failure for splitting susceptibility.