El proceso del rechazo

Highly significant effects of coating concentration in suppressing rco"

dj1dt and

dhldt were detected by ANOVA. However, with the univariate analysis of variance it is

not possible to identify how fruit treated with different coating concentrations compare

with respect to all attributes considered together, or how these attributes may be

interrelated. CDA simultaneously examines differences between coating treatments,

and indicates the relative contribution of each ripening attribute for treatment

discrimination (Cruz-Castillo et al., 1 994). The CDA results confirmed observations

made by Amarante et al. (1 998b) that respiration presents less variability within

treatment groups relative to between treatment groups than firmness and skin colour.

The previous authors observed a larger spread of values for dj1dt and dhldt at a given

level of internal 02 partial pressure (Pb,) than for rco,. CDA finds linear combinations of

the original attributes that best separate the means of the treatment groups relative to

within-group variation (Cruz-Castillo et al., 1 994). On this basis, respiration would be

expected to have more power in discriminating coating treatments than firmness and

skin colour. This was confirmed by the DRC values of each attribute shown in Table

3.3. This raises an important issue about the quality of coated pears: fruit treated with a

single coating concentration would have variable quality in terms of fIrmness and

colour change. This is especially important for fruit treated with intermediate levels of

coating concentrations, which generate more variable permeance to O2 (Amarante et

al., 1 998a) resulting in variable Pb, (Amarante et al., 1 998b; Banks et al., 1 997). This

may result in variable ripening behaviour, especially for change in skin colour (Fig.

3 . 1 6), which is strongly suppressed by any reduction of Ph, (Amarante et al., 1998b).

For 'Bose' low coating concentrations (up to 1 0-20% concentration) only slightly

suppressed ripening; high coating concentrations (2 40%) had a more substantial effect

(Table 3.3). The skin of 'Bose' contains lignifIed epidermal cells. The permeance to

gases of the epidermis is very low and most of the gas exchange occurs through the

large and shallow lenticels, which have a rough internal surface comprised of small

stone cells (Amarante et al., 1 998b). Low coating concentrations can not effectively

block these large lenticels of the skin, resulting in small modifIcation of permeance to

gases (Amarante et al., 1 998a) and small modification of fruit internal atmosphere

(Amarante et al., 1 998b), resulting in small suppression of fruit ripening. For the other

three cultivars, coating concentrations as low as 5-10% were enough to signifIcantly

delay ripening (Table 3 .3). These cultivars have a smooth skin with smali lenticeis and

small increases in coating concentration can substantially block pores in the skin,

reducing skin permeance to gases (Amarante et al., 1 998a) and greatly modifying the

fruit internal atmosphere (Amarante et al., 1 998b), resulting in substantial suppression

of fruit ripening.

The potential to improve skin gloss by coating was shown to be dependent on

coating concentration (Fig. 3 .8) or, in another words, on the total amount of coating

wax left on the skin. Similar results were reported by Johnston and Banks (1998) for

avocados coated with a polyethylene wax. However, these benefits also depended

strongly on skin characteristics of the commodity. Glenn et al. (1 990) reported that

fruit finish in apples coated with a wax was highly dependent on the extent of fruit

cracking on the skin. Fruit cracks were not always completely filled by the wax,

Chapter 3

Postharvest Physiology of Coated Pears: 1 33

Figure

3.16 Variability in skin colour of ' Cornice ' pears treated with 20% coating concentration, after 1 5 days shelf life at 20°C and 60-70% RH.

resulting in lower light reflectance and poor finish quality of coated fruit. This seems to be the case for pears as well, and shows that cultivars with low natural gloss may benefit less from increasing coating concentration in terms of improved skin finish. This was especially the case for ' Bosc ' , where the presence of stone cells on the skin impaired scope for coating to improve skin finish. Enhancement of skin gloss seems also to be dependent on coating formulation. Hagenmaier and Baker ( 1 994) have shown that citrus fruit treated with shellac and resin-based coatings had higher gloss than fruit treated with polyethylene and carnauba waxes. However, fruit treated with shellac and resin presented a larger decrease in gloss during shelf life as a result of

higher water loss than fruit treated with polyethylene and carnauba wax, resulting in no

significant difference in gloss between these coating treatments at the end of the shelf life period. Drake and Nelson ( 1 990) also reported no evident difference in skin finish

among apples treated with shellac, carnauba or resin-based waxes after long tenn cold

storage. Since increasing coating concentration reduces skin penneance to water and,

consequently, fruit water loss (Amarante et al., 1998a), it is possible that improving the

character of cover of the skin by increasing coating concentration may help retain the

gloss by reducing shrivel.

All work with optimisation of surface coatings should include assessment of final

product quality, especially for sensory attributes and the incidence of physiological

disorders, since these will affect fruit acceptability by the consumer. The sensory

analysis in the current work shows that, with the exception of 'Bosc' pears that become

anaerobic when treated with undiluted coating fonnulation (Amarante et al., 1 998b),

increasing coating concentration did not induce off-flavours. Instead, by delaying

senescence, increasing coating concentration can reduce the accumulation of

senescence-related volatiles that occurs during fruit ageing, as well as improving

flavour and finnness retention (Fig. 3.9).

'Bartlett' and 'Bosc' had very high susceptibility to internal disorders associated

with high CO2 and/or low O2 levels when treated with high coating concentrations

before cold storage (Figs. 3 . l 0A, 3 . l 0C, and 3 . 1 1). The high susceptibility of these

cultivars to internal disorders may be the result of high internal CO2 partial pressure

(Peo)

at very low Po, for fruit of both cultivars when treated with high coating

concentrations, as observed by Amarante et al. ( l998b). In the case of 'Bartlett' held at

20° C, Peo, was - 1 0 kPa as PO, approached 0 kPa presumably as a result of high

respiration rate (- 1 00 nmol'kg-l 's-l for fruit treated with coating full strength; Fig.

3 .4). 'Bosc' began to fennent when fruit were treated with coating concentrations

;;:: 40% and PO, dropped below about 2 kPa, and Peo, reached up to 25 kPa as PO,

approached 0 kPa (Amarante et al., 1 998b). Neither cultivar developed the disorder if

treated after removal from the cold storage, and left at 20°C for the same period (Figs.

3 . 1 0B and 3 . l 0D). Although modifications of internal atmosphere would have been

less severe at the storage temperature of about O°C, the effects of coating may have

been exacerbated by the enhanced solubility of CO2 at low temperature. The low

Chapter 3

Postharvest Physiology of Coated Pears: 1 35

temperature may also have impaired the metabolic repair of the internal gas stress. Amarante et al. ( 1 998b) reported for 'Cornice' and 'Packham's' a Pto, below 1 0 kPa when

P6,

was close to 0 kPa for fruit held at 20°C, as a result of reduced respiration rates of both cultivars (below 80 nmol·kg-1 ·s-1 for fruit treated with coating full strength; Fig. 3 .4). These characteristics, coupled with a lower susceptibility to high CO2 and/or low O2 levels may have played a role in the absence of the disorder in 'Cornice' and 'Packham's'.

Data on physiological disorders presented in Fig. 3 . 1 0 were based on assessment of only 1 5 fruit per treatment, and should therefore be considered as indicative only. Experiments with a larger volume of fruit should be carried out to confmn the results obtained with such small samples of fruit.

By delaying ripening and senescence of 'Cornice' pears during long term storage, increasing coating concentration reduced the incidence of senescent scald and senescent breakdown, especially the former disorder. Senescent breakdown develops during long term cold storage (Meheriuk et al., 1 994). However, since the modification of internal atmosphere by coatings is modest at low temperature (Amarante et al., 1 998b), the beneficial effect of increasing coating concentration in reducing senescent breakdown during long term cold storage was very small. Senescent scald usually develops after long storage periods but mainly after removal from cold storage (Meheriuk et al., 1 994). At ambient temperatures increasing coating concentration substantially reduces

P6,

(Amarante et al., 1 998b); this could suppress the browning reaction brought about by polyphenol oxidase (Amarante et aI., 1 998c) and reduce senescence scald during shelf life after long term cold storage. Increasing coating concentration also reduces water loss (Amarante et al., 1 998a) that could have helped reduce the incidence of senescence scald by preserving skin integrity.

For 'Cornice' and 'Packham's', as well as for 'Bartlett' not subjected to low temperatures, high concentrations of coatings should be preferred to achieve more substantial effects in reducing water loss, delaying ripening and reducing the incidence of senescence physiological disorders (scald and breakdown), with low risk of

developing off-flavours. Coating concentrations between 20-40% should be avoided for 'Packham's' destined for fresh market as they can cause skin blotchiness. High coating concentrations can substantially delay ripening during cold storage, but the fruit may fai l to change in colour during shelf life, while still being able to soften. This is the main drawback of high coating treatments for fruit destined for fresh market. However, these treatments have a high potential for fruit destined for the processing industry, since skin colour is not of paramount importance in such fruit and they can still soften. This could reduce storage costs and permit long term preservation of quality for fiuit destined for processing.

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