A review of postharvest events in cherimoya

Posthan,est Biology and Technology, 2 (1993) 187-208 © 1993 Elsevier Science Publishers B.V. All rights reserved 0925-5214/93/$06.00

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A review of postharvest events in cherimoya T. P a l m a ~', J.M. A g u i l e r a b and D.W. Stanley ~' ~' Department of Food Science, Unit~ersity of Guelph, Ont., Canada, h Department ~["Chemical Engineering, Pontificia Unit'ersidad Cat~fica, Santiago, Chile (Accepted 19 August 1992)

ABSTRACT Palma, T., Aguilera, J.M. and Stanley, D.W (1993) A review of postharvest events in cherim¢~yz~. Postharcest Biol. Technol. 2, 187-208. The exceptional flavour and aroma of cherimoya makes it a fruit with great potential for the North American market. Being a subtropical exotic fruit, however, little is known about its postharvesl features or the best methods of handling, transporting, and storage. Cherimoyas are highly perishable climacteric fruits with high respiration rate and ethylene production. Ripening is characterized by increased soluble solids, acidity, softening, and the acquisition of aroma and flavor. The mechanism by which this process is induced and controlled is not clear. The major problem in postharvest handling of this fruit is to retard the quick ripening process. Temperature is an effective tool, but its usefulness is limited by the sensitivity of cherimoyas to chill injury below 7-10°C, depending on the variety. The role of ethylene is uncertain and the use of chemical ethylene absorbers requires further study. Modified atmosphere has been found to be a promising alternative, but a great amount of research is still required to answer basic questions about the physiology of this fruit.

Key words: Cherimoya; Annona cherirnola

INTRODUCTION

Cherimoya has one of the best consumer acceptance ratings of all fruits grown commercially. The flesh is soft, delicate, sweet, and fragrant with a custard-like consistency. Few North Americans know of this product, however, because it is not widely cultivated in that continent. A limited supply is imported but the high price discourages consumers. As opposed to many other imported fruits that can withstand up to 30 days sea voyage, cherimoyas require air transportation which accounts for the higher cost. The postharvest physiology of the cherimoya is not well known although this fruit displays those characteristics well documented by

Correspondence to: D.W. Stanley, Dep. Food Science, University of Guelph, Guelph, Ont. N IG 2WI, Canada.

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postharvest physiologists including high perishability and defects such as chilling injury, rapid browning and low tolerance to mechanical injury that severely limit its storability. The purpose of this review is to examine the literature, much of which is available only in Spanish, to determine what is known about postharvest events in cherimoya and to outline the knowledge required in order to extend the storage life of this fruit while maintaining nutritional and sensory quality. CHARACTERISTICS OF CHERIMOYA FRUIT

Description, distribution and economic importance. The cherimoya ( Annona cherimola Mill.) is a subtropical fruit belonging to Annonaceae family. Its sensory properties were described by Mark Twain as "deliciousness itself" (Schroeder, 1956). The flavor has been portrayed as citrus or pineapple-like or as a mixture of strawberry and vanilla ice cream with an aromatic tang. The texture and color of the edible pulp that is contained within a green scaly peel is somewhat similar to a creamy custard but containing large, black and flat seeds (Everett, 1952; Schroeder, 1956; Biale and Barcus, 1970; Gardiazabal and Rosenberg, 1986). This fruit is relatively unknown in North America where it is marketed as an exotic product. The characteristic flavor and present unsuitability for common processing procedures lead to its primary utilization as a fresh dessert fruit. Although its origin is still a matter of discussion, it is widely accepted that the cherimoya is a native of the subtropical region of South America (Verrill, 1937; Snowdon, 1990). The growing region of the fruit extends over many countries of Central and South America and it has been planted in most subtropical regions of the world (Gardiazabal and Rosenberg, 1986). The cherimoya has been reported to be grown commercially in Argentina, Brazil, Bolivia, California, Chile, Colombia, Ecuador, Egypt, Israel, Italy, New Zealand, Peru, South Africa, Spain, and Venezuela (Everett, 1952; De la Rocha, 1967; Rokba et al., 1977; Gardiazabal and Rosenberg, 1986; Brown et al., 1988). The principal producer is considered to be Spain with about 2350 hectares planted and a harvest of 30000 metric tonnes per year in 1986 (INE, 1988). Unlike most of the Annonas, the cherimoya does not thrive where it is overly hot, preferring instead cool, frost-free, subtropical climates, the name meaning "fruit of cool places" (Verrill, 1937). Cherimoyas are often planted commercially in areas congenial to the cultivation of avocados. In many countries the cherimoya matures during the late winter or early spring. This is an attractive feature since few fresh fruits are available during this season. The characteristics of off-season accessibility and sensory distinctiveness combine to make the cherimoya a fruit suited to North American markets. Botanical and compositional characteristics. The name 'custard apple' has been previously given indiscriminately to cherimoya as well as to many other fruits of the Annonaceae family, thus leading to confusion. The principal edible fruits of this family with their common names are given in Table 1.

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TABLE 1 Principal edible fruits of the Annonaceae family Scientific name

Common name

Annona cherimola Mill. Annona rnuricata L. Annona reticulata Annona squamosa L. Annona atemoya ( A. cherimola X A. squamosa )

Cherimoya Soursop Bullock's heart Sweetsop, Sugar apple, Custard apple Atemoya, Custard apple

Adapted from Biale and Barcus, 1970; Chan and Curtis, 1975; De la Plaza, 1980; Wills et al., 1984; Wyllie et al., 1987; Snowdon, 1990.

The cherimoya fruit is a syncarpium since several individual fruitlets are inserted in a spiral arrangement around a central pithy receptacle (Schroeder, 1956). The fruit is formed by the growing together of the carpels and receptacle into a fleshy structure (Biale and Barcus, 1970). Each carpel contains a dark, hard seed, but these are readily separated from the pulp. The epicarp is green and in some varieties it shows a slight change in color from pale green to yellowish green during ripening. Smoothness of the epicarp varies among varieties and this characteristic is used for taxonomic differentiation (De la Rocha, 1967; Gardiazabal and Rosenberg, 1986). Fruits can be heart-shaped or conical and may weigh from 200 g up to 2 kg. Shape and weight are more related to pollination patterns than varietal differences (Gardiazabal and Rosenberg, 1986). Figure 1 shows exterior and interior views of one variety of cherimoya. The proximate composition of edible cherimoya fruit is given in Table 2.

Determination of haruesting time. The term 'mature' means the stage of development giving maximum acceptability to the ultimate consumer, and implies a need for objective measurement (Reid, 1985a). Unfortunately, a workable maturity index for harvesting cherimoyas has yet to be established (Gardiazabal and Rosenberg, 1986). Traditionally, cherimoyas have been harvested when their skins become yellowish green (De la Plaza, 1980; Reginato, 1980; Gardiazabal and Rosenberg, 1986). As this event is not visibly distinct, attempts have been made to find a more usable criterion of maturity. Changes in color a n d / o r density of epidermal trichomes were shown to be indicative of early stages of maturity (Herrera, 1988; Tietz, 1988; Univ. Cat61. Valparaiso, 1988). This failed, however, to correlate with physicochemical changes within the fruit, and best eating quality was obtained from fruits harvested later in the season (Univ. Cat61. Valparaiso, 1988). Pulp firmness was not found to be predictive of maturity (Tietz, 1988) and erratic information exists about soluble solids. (Pavez, 1985; Tietz, 1988). Higher soluble solids at harvest reflected more acceptable flavor in ripe fruit (Tietz, 1988) and a high correlation was observed between solids evolution and fruit diameter during growth (Pavez, 1985; Univ. Cat61. Valparaiso, 1988). Further work is needed to implement this relationship as a useful maturity index. Looseness of

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Fig. 1. Exterior and interior views of cherimoya var. Concha Lisa.

s e e d s i n s i d e t h e f r u i t has also b e e n u s e d as a g u i d e to m a t u r i t y ( G a r d i a z a b a l a n d R o s e n b e r g , 1986). H o w e v e r , it has b e e n f o u n d t h a t this c h a n g e in s e e d a d h e r e n c e o c c u r s t o o l a t e for a d e q u a t e h a n d l i n g a n d in s o m e c a s e s it d o e s n o t o c c u r at all

TABLE 2 Composition of 100 g cherimoya fruit Component

Amount

Water (g) Calories Protein (g) Carbohydrate Total (g) Fiber (g) Ash Total (g) Calcium (mg) Phosphorus (mg) Iron (rag) Vitamin A (1.U.) Thiamine (mg) Riboflavin (mg) Niacin (mg) Vitamin C (mg)

73.5 94 1.3

Source: Gebhardt et al., 1982.

24.0 2.2 0.8(I 23 40 0.5 10 0.10 0.11 1.3 9.0

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(Univ. Cat61. Valparaiso, 1988). More research is needed to find a maturity index that will assure adequate pulp firmness for commercial handling of cherimoyas and to obtain superior flavor and quality. POSTHARVEST PHYSIOLOGY

Ripening. Ripening is the total of all processes that occur from the latter stages of growth and development through the early stages of senescence which results in a characteristic quality as evidenced by changes in composition, color, texture or other sensory attributes (Watada et al., 1984). In cherimoyas, ripening is characterized by the development of flavor and aroma of the fruit, softening of the pulp, and slight changes in color of the pulp and flesh. All these changes are paralleled by a concomitant increase in the respiration rate and ethylene production. Compared to other fruits, ripening occurs rapidly in the cherimoya. At room temperature, a cherimoya will ripen in 6 - 7 days (Gardiazabal and Rosenberg, 1986). In this short period of time, the fruit softens, starch is reduced to sugars, and the acidity increases to give the customary cherimoya flavor. Volatile compounds of the cherimoya fruit have been separated (Idstein et al., 1984) but the pattern of their evolution during ripening is unknown. Respiration rate. The cherimoya is a climacteric fruit with a high respiration rate (Table 3). A rapid increase in respiration after harvest and the high rate attained make this fruit exceptionally perishable (Biale, 1960). There have been few studies concerning postharvest respiration of the cherimoya. Many, but not all, investigators have found a second respiration peak after the initial climacteric (Fig. 2). It should be noted that even though varieties and temperatures used by these authors were different, the shape of the curves are similar. This diffuse type of climacteric increase with more than one maximum has also been observed in other A n n o n a c e a e fruits including biriba (Rollinia ortopetala), soursop (A. muricata), and atemoya (A. atemoya) (Biale and Barcus, 1970; Brown

TABLE 3 Respiration characteristics of cherimoya fruit Variety

Deliciosa Baldwin Chaffey Bronceada Fino de Jete Campa

Respiration rate (ml CO2 kg z h i) Temperature Preclimacteric (°C) min. 20 35 20 35 20 16 22 13 9 5 9 < 15

1st rise max. 92 90 83 30 11 22

2nd r i s e max. 122 95 180 > 42 not seen 35

Reference ( 1) ( 1) (2) (3) (4) (4)

References: (1) Brown et al., 1988; (2) Kosiyachinda and Young, 1975; (3) Reginato, 1980; (4) De la Plaza, 1980.

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A

150

f

100

O BALDWIN + BRONCEADA •

CHAFFEY

•

DELICIO6A

50

, i , ,

-8 BEFORE

,

i , , ,

-4

i,,

~l,g

O

4

CLIMACTERIC

~ i==

8

= i , , = 1 ,

12

~)

16

AFTERCLIMACTERIC DAYS

Fig. 2. Respiration rate of cherimoya fruit. Adapted from Kosiyachinda and Young, 1975; Reginato, 1980; De la Plaza, 1980; Brown et al., 1988.

et al., 1988). Some authors have suggested that this irregular increase in respiration may be due to the aggregate nature of these fruits with different segments ripening at different times (Biale and Barcus, 1970; Paull, 1982). There are other aggregated fruits, however, even from the same family, such as A. squamosa that do not show this respiration pattern (Broughton and Guat, 1979; Brown et al., 1988). Moreover, the respiration of isolated tissue discs of A. muricata follows the same pattern as the whole fruit. These findings established that the pattern is characteristic of any part of the fruit (Bruinsma and Paull, 1984). Changes in ripening are associated with increased respiration (Biale, 1960). Development of aroma and flavor occurs after the onset of the climacteric increase in the cherimoya. Optimal edible condition is attained early in the second increase (Kosiyachinda and Young, 1975; Gardiazabal and Rosenberg, 1986; Brown et al., 1988). Temperature has a marked effect on the respiration rate. Refrigerated storage (below 12°C) both slows down the respiration increase and decreases CO 2 production compared to room temperature (Peralta, 1984). Fruits stored at refrigerated temperatures exhibit an increase in their respiratory rate as soon as they are moved to room temperatures (Peralta, 1984). Although low temperatures delay the ripening process, it should be pointed out that the cherimoya is a fruit susceptible to chilling injury below 10°C (Reginato, 1980).

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193

Cherimoya fruit accumulate starch during growth. During ripening this starch is hydrolyzed, bringing an increase in glucose, fructose and sucrose (Gardiazabal and Rosenberg, 1986). The main sugar of the ripened fruit is glucose (Kawamata, 1977). There is no information available concerning the starch hydrolysis pattern in the cherimoya, but in A. squamosa starch decreases and sucrose increases until the climacteric peak, while glucose and fructose accumulate until the overripe stage (Broughton and Guat, 1979). This same behaviour has been observed in A. atemoya (Wills et al., 1984) and in A. muricata (Paull et al., 1983). Soluble solids rise concomitantly with the respiratory increase (De la Plaza, 1980; Reginato, 1980) and are at a maximum after the climacteric peak at the onset of the second respiratory rise. This increase coincides with the maximum flavor and best sensory conditions (Reginato, 1980). It has been noted that there is a correlation between the total soluble solids content and the flavor of the fruit (Fuster and Prestamo, 1980) and, although there are differences among varieties, a ripened cherimoya should reach 18-24 ° Brix values for total soluble solids (Broughton and Guat, 1979; De la Plaza, 1980; Reginato, 1980; Peralta, 1984; Univ. Cat61. Valparafso, 1988). Changes in respiration rate due to temperature also alters the soluble solids increase. Low temperatures that slow respiration predictably reduce soluble solids accumulation (Peralta, 1984).

Sugars.

Acidity in the cherimoya increases during ripening (Gardiazabal and Rosenberg, 1986). This behaviour, dissimilar to that found in most fruits, has been reported in the cherimoya (De la Plaza, 1980; Reginato, 1980; Reginato and Lizana, 1980a; Peralta, 1984) as well as in A. muricata (Paull, 1982; Paull et al., 1983) and A. atemoya (Wills et al., 1984), although Brown et al. (1988) did not find this pattern. Titratable acidity increases until the onset of the second climacteric rise, coinciding with optimal edible condition, and then decreases (Reginato, 1980, Reginato and Lizana, 1980a). The organic acid responsible for this increase has not been identified, but is probably malic acid, as has been found in A. atemoya (Wills et al., 1984). The major organic acids in A. muricata and A. atemoya are malic and citric (Paull, 1982; Wills et al., 1984). Ascorbic acid has been observed to increase during the climacteric in A. muricata (Paull, 1982) and A. squamosa (Broughton and Guat, 1979) but this compound may not contribute to titratable acidity since it was found to occur mainly as a salt (Paull, 1982). Citric acid was the major organic acid found in A. atemoya, but its concentration was relatively constant at all stages of ripening. Alternatively, malic acid, present at low levels in the preclimacteric, rose markedly during ripening (Wills et al., 1984).

Acidity.

Cherimoya flesh loses its firmness quickly as the fruit ripens (Fuster and Prestamo, 1980; Reginato, 1980; Peralta, 1984; Gardiazabal and Rosenberg, 1986; Brown et al., 1988). This softening may correspond to an increase in the activity of pectinolytic enzymes which attack cell walls, leading to their separation

Firmness.

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and changes in structure (Gardiazabal and Rosenberg, 1986). Softening begins with the onset of the respiratory rise (Brown et al, 1988) and firmness decreases dramatically until the first peak of respiration. At this point the fruit has attained its optimal eating firmness and softening continues, but at a slower rate (De la Plaza, 1980; Reginato, 1980; Brown et al., 1988). Within the same variety, firmness is related to respiration rate. Fruits with higher respiration rates soften more quickly (Peralta, 1984). This relation does not hold, however, among different varieties, e.g., 'Campa' variety exhibits a higher respiration rate than 'Fino de Jete' (De la Plaza, 1980), yet 'Campa' is reported to soften more slowly (De la Plaza, 1980; Fuster and Prestamo, 1980). Softening rate is affected by temperature. The same loss in firmness occurring in 3 days at room temperature could take 14 days at 15°C and 21 days at ll°C (Reginato, 1980). At refrigerated storage temperatures firmness remains virtually constant. Cherimoyas have been kept for 30 days at 5-7°C and the values for pulp firmness were unchanged (Concha, 1988). Variation in cherimoya firmness during storage has been measured instrumentally in terms of compression, puncture and shearing forces. It has been reported that deformation of the fruit increases with temperature and storage time. Puncture and shearing force decreases with storage at 10°C and 20°C to a constant value at which time ripeness is reached (Fuster and Prestamo, 1980).

Ethylene. Climacteric fruits are characterized by an increase in ethylene production at the onset of ripening (Yang, 1981). However, as shown in Figure 3, the ethylene rise in the cherimoya occurs after the climacteric rise of CO 2 (Biale et al., 1954; Biale, 1960; Kosiyachinda and Young, 1975; Brown et al., 1988; Palma, 1991) and the same has been observed in other Annonas (Broughton and Guat, 1979; Paull, 1982; Wills et al., 1984; Brown et al, 1988) as well as in feijoa, avocado and mango (Biale and Young, 1981). Early researchers suggested that since the onset of the increase of ethylene production was later than the beginning of CO 2 evolution, ethylene could be a product of the respiratory changes due to senescence rather than a causal agent (Biale et al., 1954). It was pointed out, however, that these measurements were made with insensitive methods and, probably, there was an increase in internal ethylene before the external levels could be measured (Biale, 1960). A more recent investigation (Kosiyachinda and Young, 1975) used a method of higher sensitivity to measure both internal and external ethylene in the cherimoya. It was reported that external ethylene production rose after the climacteric CO 2 peak and that internal ethylene concentration was lower than 0.1 ppm until a point where respiration had already increased substantially (Fig. 3). No evidence was found for the induction of the climacteric rise by endogenously produced ethylene since the internal ethylene concentration at the beginning of the respiratory rise was found insufficient to induce the climacteric. Thus, ethylene does not appear to be the major controlling factor and there should be other agents involved in the climacteric induction. Some authors have stated that early respiratory rise in Annona fruits appears to

POSTHARVETS

EVENTS

IN C H E R I M O Y A

[95

8OO

800

150'

100,

SO-

ETHYLENE I

I

I

I

1

2

4

6

8

10

D A Y S AFTER H A R V E S T

Fig. 3. Ethylene rise and respiration in the Chaffey variety of cherimoya. Adapted from Kosiyachinda and Young, 1975.

be ethylene independent (Bruinsma and Paull, 1984). In the case of whole A. muricata, the second respiratory rise coincides with autocatalytic ethylene evolution and isolated discs were found to follow the same pattern. These authors concluded that postharvest respiration consists of a climacteric increase, as normally encountered in fruits with autocatalytic ethylene production, preceded by a probable harvest-induced transient respiratory rise. Although this may not be the case in cherimoya, in which the first rise is accompanied by typical ripening changes until the onset of the second rise, the hypothesis of a harvest-induced rise in CO 2 is appealing since it implies that the induction of an early increase in CO, may be due to the shift from exogenously supplied assimilates toward internally accumulated starch as a source of respiration substrates. This shift may lead to a temporary 'overshooting' of the pathway of starch degradation (Bruinsma and Paull, 1984). Unfortunately, there are no studies of starch evolution in the cherimoya to shed light on this hypothesis. There have been few studies related to the role of ethylene in cherimoya ripening. Although external applications of this hormone has provoked an increase in respiration (Solomos and Laties, 1976), it is not clear which ripening characteristics are controlled or affected by ethylene. Ripening in the cherimoya is characterized by softening, increased sugar and acidity content, and development of aroma and flavor. These changes appear to be more related to climacteric respiratory rise than to ethylene. Ethylene does not have an effect on soluble solids and acidity in many fruits (Watada, 1986). This seems to be the case of A. muricata (Paull, 19821 and A. atemoya (Wills et al., 1984) where the rise in acidity and sugars occur before the onset of ethylene increase.

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In the cherimoya, best eating quality is attained at maximal sugar content and acidity (Reginato, 1980), and this coincides with the ethylene peak (Brown et al., 1988). Thus, even though there are no reports of a correlation between soluble solids and acidity with ethylene production, it might be thought that the same situation described for A. muricata and A. atemoya could be applicable to the cherimoya. Softening can be due to the activation of cellular hydrolases induced by ethylene. Although cherimoya softening begins days before the onset of ethylene increase, the firmness drops dramatically during its production (Brown et al., 1988). Ethylene has been found to soften fruits without affecting other physiological processes in pears (Watada, 1986). It could have a softening role in the cherimoya without controlling soluble solids and acidity. Most fruits become more sensitive to exogenous ethylene with time. This supports the idea that sensitivity is the limiting factor to the response to ethylene (McGlasson, 1985). The observation that external ethylene production of early season cherimoyas follows the rise in respiration after a longer period than late season fruits (Kosiyachinda and Young, 1975) suggests increasing sensitivity to ethylene with age. POSTHARVEST PROBLEMS Ethylene management. Attempts have been made to control fruit ripening for commercial advantage either by adding ethylene to achieve uniform maturity or absorbing it to prolong shelf life. Due to the lack of an adequate maturity index for harvesting cherimoyas, fruits picked on the same day inevitably are at different maturity stages and, therefore, ethylene might be useful for producing uniformly ripened fruit. Ethylene and cyanide have been reported to increase respiration rate in the cherimoya fruit (Solomos and Laties, 1976). Moreover, the commercial use of controlled ripening with 100 ppm of ethylene gas for 24-36 hours after harvest has been reported to promote uniform ripening of Annona fruits and to give optimal eating quality (Brown et al., 1988). On the other hand, there have been cases in which ethylene treatments failed to produce uniform ripening (Abascal, 1989), though this case was probably due to CO 2 accumulation which reduced the effectiveness of ethylene (Reid, 1985b). In A. squamosa, ethylene applications have also had no apparent effect on ripening; this species does not seem to respond either to addition or removal of ethylene (Broughton and Guat, 1979). Cherimoyas have a high ethylene production rate (Table 4). All available literature agrees with Reid (1985b) that the peak of ethylene production in cherimoya is higher than 100 ~L kg-~ h-1, thus explaining why ethylene absorbers have been tried as a way of delaying ripening. Unfortunately, results are confusing. In two studies that were made using potassium permanganate (Toro, 1989) and activated charcoal (Pardo, 1988) absorbers, no difference in the ripening rate was induced. It is possible that in these cases a delay was not induced because the

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TABLE 4 Ethylene production of the cherimoya Variety

Temperature (°C)

C2H 4 production ( ~ L k g - I h ~)

Reference

Delicosa Baldwin Chaffey Booth Booth

20 20 20 20 15

140 115 650 186 150

( 1) (1) (2) (3) (3)

References: (1) Brown et al., 1988; (2) Kosiyachinda and Young, 1975; (3) Biale et al., 1954.

modified atmosphere inside the test chambers could have depressed the typical accelerating effect of ethylene on ripening (Reid, 1985b). Other work with potassium permanganate showed a positive effect at 3.5 g of the absorber per fruit kg at 8.5°C, but at higher doses the respiration rate and the ethylene production were greater than the untreated fruits. This unexpected behaviour was explained by the authors as the trapping of an inhibitor of the ethylene synthesis by the absorber, leading to a rise in ethylene production (De la Plaza et al., 1989).

Enzymatic browning. Enzymatic browning in plants has been defined as an oxidative process catalyzed by polyphenoloxidase (PPO). PPO has two different actions: (a) hydroxylation of monophenolic, and (b) oxidation of ortho-di-phenolic substrates (Mayer and Harel, 1979). This enzyme (EC 1.10.3.1) hydroxylates monohydric phenols to dihydric phenols and oxidizes them to quinones (Reddy and Rameshwar, 1984). Browning is the result of this phenol oxidation followed by a nonenzymatic polymerization of the quinones into tannins or melanins (Dilley, 1971). One of the most important problems for marketing cherimoyas is the rapid peel browning occurring a few days after harvesting (Martlnez-Cayuela et al., 1988c) a n d / o r darkening caused by physical injury from handling, transportation, etc. Browning is observed also when fruit are cut or peeled. These problems result from enzymatic oxidation due to phenoloxidases (Boscan, 1969). Reports of PPO in cherimoya deal with these problems separately. There are studies of epicarp PPO (Martlnez-Cayuela et al., 1986; Martfnez-Cayuela et al, 1988a, b, c) as well as the enzyme in pulp and juice (Boscan, 1969; Abufom, 1985; Abufom and Olaeta, 1986). Epicarp PPO. Epicarp PPO in the cherimoya shows both catalytic activities, monophenolase activity being lower than that of dihydroxyphenolase (MartinezCayuela et al., 1988c). Effects of reductants and phenolic carboxylic acids on epicarp PPO were studied (Martinez-Cayuela et al., 1988a, b, c). None of these compounds, including organic acids, aliphatic carboxylic acids and sugars, showed any effect on enzyme activity at 0.1-1 mM concentrations.

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Pulp PPO. A very obvious difficulty blocking industrialization of the cherimoya is maintenance of the original color of the pulp (Boscan, 1969). This might be solved with chemical inhibition, or treatments that decrease PPO activity (Gardiazabal and Rosenberg, 1986). Darkening occurs in pulp and suspended pulp in juice if it is left in contact with oxygen. Sodium bisulphite, at concentrations higher than 0.1%, prevents this darkening more effectively than ascorbic acid (Boscan, 1969). The optimal pH for PPO in cherimoya is 5.0 to 5.5, while the optimum temperature is 25-30°C. A thermal treatment of 5 min at 80°C is sufficient to inactivate the enzyme. This enzyme easily oxidizes o-diphenols. It acts slightly on gallic and tannic acids but does not show activity on monophenols, m- or p-diphenols, or o-phenylenediamine (Boscan, 1969). Similar specificity has been reported in A. squamosa pulp peroxidase, the enzyme supposedly responsible for darkening in this fruit (Sastry et al., 1961), but it was found that peroxidase activity decreased during ripening so it is quite likely that PPO, which increases during ripening, may play a more active role than peroxidase in pulp browning (Reddy and Rameshwar, 1984). PPO of A. squamosa shows activity on both L-DOPA and catechol, the former exhibiting the highest activity due to greater affinity. The level of L-DOPA increases during ripening and decreases slightly in overripe fruit, resulting in lower levels of total and o-diphenols. However, PPO does not show any significant change in activity during ripening. Indole acetic acid and ascorbic acid oxidase activity have not been detected in either ripe or unripe fruits (Reddy and Rameshwar, 1984). Practical attempts have been made to control enzymatic browning in frozen pulp (Abufom and Olaeta, 1986). Pasteurization (15 min at 75°C) and the addition of antioxidants (0.15% ascorbic acid + 0.2% citric acid + 0.02% EDTA) have been tried. Pasteurization led to reduced quality aspects, but antioxidants inhibited enzymatic browning while maintaining color, texture and appearance for up to 120 days (Abufom and Olaeta, 1986). Immersion for 2 min in ascorbic acid at 0.6% plus citric acid at 1% prevented enzymatic browning of frozen pieces of cherimoya fruit (Abufom, 1985). Also, pulp mixed with sugar in a 2:1 proportion did not darken (Boscan, 1969); the same was found for frozen pieces of fruit in orange juice (Univ. Cat61. Valparaiso, 1988). Chilling injury. Tropical and subtropical plants suffer a physiological dysfunction when exposed to low but nonfreezing temperatures below 10-12°C (Lyons, 1973; Jackman et al., 1988; Parkin et al., 1989). This is chilling injury (CI) and cherimoya fruits are highly sensitive to it. A major concern in postharvest cherimoyas is high perishability and the easiest method to prolong shelf life would be refrigerated storage. Unfortunately, this tool is limited as a result of propensity to CI. Temperature and exposure time are prime variables in CI damage. For all fruits, at any temperature down to freezing, there seems to be some minimum time to induce irreversible injury (Couey, 1982) and cherimoya does not appear to be an exception to this rule. This fruit does not show a change in either its acceptability or its appearance when stored for a short period (18 h) at 0°C (Irarr~zaval, 1984). As with other fruits, there is a difference in sensitivity to CI between varieties of

POSTHARVETS EVENTS IN (THERIMOYA

1C)9

the cherimoya. For example, fruits of the Bronceada variety are more susceptible than the Concha Lisa variety; 8°C is a hazardous storage temperature for the former but safe for the latter (Univ. Cat61. Valparafso, 1988). Although there is not complete agreement, the disruption of cellular membranes is thought to play a role in CI sensitivity (Lyons, 1973; Wang, 1982; Parkin et al., 1989). The differential sensitivities between cherimoya varieties may be due to variation in cellular membrane composition (Reginato and Lizana, 1980a), but no studies on such membranes in cherimoyas have been reported. Even though symptoms of C1 in the cherimoya have been described and some morphological changes have been found to occur (Loyola, 1988), the physiological reason for chilling injury has not been studied, It has been speculated that development of CI and senescence may share some common features (Kuo and Parkin, 1989). Support for this idea lies in the fact that skin spotting, a CI symptom in the cherimoya, has been found in the Concha Lisa (Reginato and Lizana, 1980b) and Bronceada (Peralta, 1984) varieties harvested late in the season and without refrigerated storage.

Symptoms of CI in cherimoya. Symptoms that develop following chilling exposure are not unique to this stress (Morris, 1982). They vary with plant tissue and severity of injury and usually develop increasingly after a return to nonchilling temperatures (Lyons, 1973; Noguera, 1988). This occurs in cherimoyas, where similar symptoms could be due to senescence (Reginato and Lizana, 1980b; Peralta, 1984), and symptoms differ depending on the severity of chilling (Loyola, 1988). Table 5 summarizes reports of chilling injury found in the literature. Fruit with CI symptoms exhibited brachischlereid lignification and cell wall breakage that resulted in high levels of starch and tannins in intercellular spaces (Univ. Cat61. Valparafso, 1988). Brachischlereids are normally present in healthy parenchyma of cherimoyas, as in pears and quinces, but are fewer in number. It is believed that they have a protective function and they may respond to environmental stress, but the exact mechanism of this process has yet to be established (Univ. Cat61. Valparafso, 1988). Except for lengthy storage at chilling temperatures, cherimoyas can show CI symptoms externally but still ripen normally. Normal values of soluble solids, acidity, pH, flavor, acceptance and pulp color result if the skin is peeled (Fuster and Prestamo, 1980; Irarrfizaval, 1984). Treatments. Treatments for reducing CI in other fruits include conditioning to temperatures near chilling before refrigeration, intermittent warming, increased CO 2 during chilling, pretreatments with calcium or ethylene, and holding under hypobaric conditions (Morris, 1982). Intermittent warming and pretreatment with ethylene do not seem useful for cherimoyas due to their high respiration rates and perishability. Other alternatives may be able to prevent CI in this fruit. High CO-. levels during chilling can prevent CI in avocados (Couey, 1982) but this has not been attempted in cherimoyas. One way of modifying the storage performance is

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T. PALMAET AL.

TABLE 5 Symptoms of chilling injury reported in cherimoya Symptom

Description a

Pitting

Brown spots with slight depressions; irregular thickening of cell walls

Skin spotting

Small brown areas only on skin

Days until symptom

Variety b

Reference a

0 5 7 9

7 21 21 20

CL CL CL CL

(3) (4) (4) (l)

5 5 7 7 9

5 21 28 21 20

FJ, C c CL CL CL CL

(2) (4) (4) (6) (1)

11

14

CL

(6)

Storage Temperature (°C)

Epidermal browning

Dark, depressed areas of irregular shape = 2-3 cm wide

5 7 7 7 9

30 25 30 35 20

FJ, C c CL CL CL CL

(2) (5) (4) (6) (1)

Hard fruit

Nonripening fruit; totally dark, remains hard

5 5 7 7

35 40 35 40

CL CL CL CL

(4) (1) (4) (4)

Crystallized pulp

Found in 'hard fruits'; watery appearance of the pulp

5 9

35 40

CL CL

(1) (4)

Subepidermal darkening

Small dark areas between epidermis and pulp; not found in CL variety

7

9

B

(6)

a b c a

A complete description including histological plates is given in Loyola, 1988. CL, Concha Lisa; FJ, Fino de Jete; C, Campa; B, Bronceada. Fruit stored at 40% relative humidity. References: (1) Concha, 1988; (2) Fuster and Prestamo, 1980; (3) Irarrfizaval, 1984; (4) Loyola, 1988; (5) Noguera, 1988; (6) Reginato and Lizana, 1980b.

by w a x i n g t h e fruit. In c h e r i m o y a s this r e d u c e s t h e p e r c e n t a g e o f t h e a r e a a f f e c t e d by C I in t h e C o n c h a L i s a v a r i e t y ( I r a r r f i z a v a l , 1984). I n t o m a t o e s , a c c l i m a t i o n to c h i l l i n g t e m p e r a t u r e s p r e v e n t e d t h e d e v e l o p m e n t o f c h i l l i n g injury s y m p t o m s d u r i n g p o s t s t o r a g e r i p e n i n g ( M a r a n g o n i et al., 1990). P e r h a p s this c o u l d b e a m e t h o d to p r e v e n t C I s y m p t o m s in t h e c h e r i m o y a . C a l c i u m a p p l i c a t i o n s a n d h y p o b a r i c c o n d i t i o n s h a v e h e l p e d in t h e c a s e o f a v o c a d o s ( C o u e y , 1982) a n d m a y w o r k f o r c h e r i m o y a s .

P O S T H A R V E T S EVENTS IN C H E R I M O Y A

21)1

Postharuest diseases.

There are few reports about postharvest diseases in the cherimoya. Most researchers have dealt with diseases that attack the cherimoya plant but not the fruit. Fungi attacking cherimoya fruits that have been reported include Botrytis cinerea Pers. ex. Fr., Rhizopus stolonifer (Fr.) Lind., Penicillium expansum (Link.) Thorn., Phomopsis t~exans (Sacc. y Syd) Harter, Alternaria sp. and Fusarium sp. (Morales and Cooper, 1977; Sotomayor, 1988; Snowdon, 1990). Botrytis, Phomopsis and Rhizopus are described as the most important pathogens while Penicillium and Alternaria are secondary pathogens (Sotomayor, 1988). Penicillium, Botrytis and Rhizopus rots develop as soft and wet rots, while Phomopsis rot causes spots over the epidermis that turns leather-like and finally looks scaly (Morales and Cooper, 1977). Botrytis, Rhizopus, Penicillium and Phomopsis penetrate through wounds into the epidermis. Their damage to the fruit increases at higher temperatures. Botrytis is probably the most aggressive due to its capability to penetrate with no need for wounds and its resistance to low temperatures (Morales and Cooper, 1977). Control of these diseases in the cherimoya has been described using immersion treatments. These treatments have shown to be effective and do not have collateral effects of epidermal injury (Sotomayor, 1988). Since the cherimoya fruit is susceptible to chilling injury, the use of low temperature for fungal control is not feasible (Sommer, 1985). Another fungus present in postharvest cherimoya is Fumago spp. It is a by-product of the presence of an insect (Pseudococcus sp.) whose exudates supports growth of this fungi (P6rez de Castro, 1987). It may be considered only a cosmetic problem for the fruit, since its presence is indicated by black areas in the peel. POSTHARVEST MANAGEMENT Since the cherimoya has been shown to be a highly perishable fruit, the major concern in its handling is prolonging shelf life. There are three major methods to prolong shelf life of fruits and vegetables (Shewfelt, 1986): (a) avoiding mechanical injury, (b) optimizing environmental conditions, and (c) applying additives. The first of these is quite important in the cherimoya because of its green skin color and the presence of active polyphenoloxidases. Any mechanical damage is readily apparent as dark spots. It should be noted that there are differences among varieties, e.g., smooth peeled varieties (Fig. 4a) are less delicate during handling than those which have hillocks on their skin (Fig. 4b) (De la Rocha, 1967). Attempts to reduce mechanical damage during transportation have been tried using different packaging materials (Noguera, 1988; Univ. Cat61. Valparaiso, 1988; Giancaspero, 1989; Bravo, 1989). Except for a package that includes an expanded polystyrene tray wrapped with a polyethylene film, nonsignificant differences in the reduction of mechanical injury were found among the various packaging methods (Noguera, 1988; Giancaspero, 1989). However, a distinction was seen between a smooth skin variety (Concha Lisa) and 'hillocky' skin variety (Bronceada) (Univ. Cat61. Valparaiso, 1988).

202

T, P A L M A E T AL.

f c e ,eL ec

CONCHA LISA

BRONCEADA

Fig. 4. Differencesamonga smoothpeel variety(A) and a 'hillocky'variety(B). The second method to prolong shelf life is the optimization of the environmental conditions. This usually involves lowering respiration of the fruit and reducing the growth of decay organisms without inducing physiological damage (Shewfelt, 1986). This approach includes modifying variables such as temperature, relative humidity, CO2, 0 2 and ethylene levels, which have different effects singly or in combination, although the most effective ratio of 0 2 to CO 2 is not yet known. Temperature is the major factor in prolonging the shelf life of cherimoya. It has been shown that refrigerated storage can double or triple the shelf life (De la Plaza, 1980; Fuster and Prestamo, 1980; Reginato, 1980; Reginato and Lizana, 1980a; Irarrfizaval, 1984; Peralta, 1984; Concha, 1988; Univ. Cat61. Valparaiso, 1988), but minimum storage temperatures are 7-12°C, depending on variety, and these are not sufficiently low to inhibit either mould growth or dehydration. Low temperatures reduce respiration rate of the cherimoya fruit (Peralta, 1984) and softening is delayed (Fuster and Prestamo, 1980). After refrigerated storage at nonchilling temperatures, cherimoyas will ripen normally and will develop acceptable eating quality as long as a time factor during storage is not exceeded (Univ. Cat61. Valparaiso, 1988). Since the length of storage has an effect, this temperature/length storage combination is different for each variety and defines the ripening velocity after transfer to higher temperatures. Modifying the gaseous composition of the atmosphere is another possibility to optimize the environment for prolonging shelf life. Rising CO 2 levels, lowering 0 2 levels, or scavenging ethylene with chemical ethylene absorbents are common practices used with other fruits that either by their own or in combination delay ripening. Controlled atmosphere storage has been shown to delay ripening (De la

POSTHARVETS EVENTS IN CHERIMOYA

203

Plaza, 1980; Moreno and De la Plaza, 1983; Palma, 1991), but there are still questions to be answered in relation to the cherimoya's responses to high CO: and low 0 2 concentrations. Reports dealing with susceptibility to high CO 2 or low O , injuries in cherimoya were not found in the literature. In A. squamosa it has been shown that storage in 10% CO 2 produces normal taste and appearance while 15% CO 2 made the fruit incapable of ripening (Broughton and Guat, 1979). Results of research that examined the effects of CO 2 suggest that this gas can prolong storage life of the cherimoya for at least 3 weeks (Martlnez-Cayuela et al., 1986). Coating of the fruit has also been tried (Irarrfizaval, 1984; Univ. Cat61. Valparaiso, 1988), but the results were contradictory. Irarrfizaval (1984) found that waxing delayed ripening in fruit stored at room temperature and suggested that this was due to a modified atmosphere around the fruit. This treatment also reduced the area affected by chilling injury in refrigerated storage. However, other research showed that, although waxing had an effect in reducing dehydration, il did not delay ripening (Univ. Cat61. Valparaiso, 1988). More work is needed to determine the optimal CO z and 0 2 concentrations for controlled atmosphere a n d / o r design of modified atmosphere storage for cherimoyas. The concept of 'active packaging', recently recommended for the fruit and vegetable industry (Labuza and Breene, 1989), could be useful in prolonging shell: life of cherimoyas but a great deal of research is needed before it can be commercially applicable. The final approach to prolong shelf life is the application of additives. Growth regulators such a Alar TM and gibberellic acid have been tried in cherimoya (Vidal, 1987; Univ. Cat61. Valparaiso, 1988), but neither had a significant effect on maturity. Gibberellic acid showed a positive effect on organoleptic quality, while Alar produced bitterness in the fruit (Univ. Cat61. Valparaiso, 1988). It should be noted that additives such as these often generate controversy with regards to health questions. Calcium has been shown to help prevent senescence in other fruits and control physiological disorders (Poovaiah, 1986). There are no reports of postharvest applications of calcium to cherimoyas but this could be a useful postharvest management tool. Gamma irradiation of cherimoyas has been shown to control respiration activity and prolong shelf life by 30%. Above 3000 rad, gamma irradiation produced a fungistatic effect and doses up to 6000 rad had no effect on organoleptic characteristics of the cherimoya (Diaz et al., 1969). Doses of 3000 rad extended shelf life of the cherimoya by 2 days, a large effect since this is considered to be 30% of the normal storability, but the commercial significance of this treatment is limited. Chemical fungicides have been used to prevent decay (Sotomayor, 1988). A! present, with the limitations of refrigerated storage due to chilling injury, fungicides may be the only way to control fungal attack on the cherimoya. CONCLUSIONS

This review has demonstrated that too little is known concerning postharvest events in the cherimoya. Essentially, it is a highly perishable climacteric fruit with

204

T. P A L M A E T AL.

high respiration and ethylene production rates, although the primary role of ethylene in the cherimoya's ripening is not clear. The presence of polyphenoloxidase activity leads to rapid discoloration following mechanical damage. Fruit ripening is accompanied by increases in sugars, acidity and softening, but the primary causes of ripening remain illusive, as does control of this event. While low temperatures can prolong shelf life, the cherimoya fruit is sensitive to chilling injury. There are varietal differences in CI sensitivity and, except for extreme cases, CI mainly affects appearance and not ripening processes or taste characteristics. Also, symptoms similar to CI have been found in nonchilled, senescent fruits, but the relation between senescence and CI is currently unknown. Obviously, much more needs to be known about the cherimoya before effective strategies to manage commercially important operations such as storage and transportation can be devised. A maturity index is required that allows harvesting of fruit that can be handled safely but still develop the highest quality in terms of appearance and flavor. Acceptable methods of prolonging the shelf life of cherimoyas have yet to be developed. Preventing or delaying CI effects would enable the use of lower temperatures that could reduce spoilage due to moulds. However, this awaits a more complete understanding of physiological responses of this fruit to chilling. Combinations of nonchilling temperatures with other methods to prolong shelf life, such as modified atmospheres, may prove useful. A study of responses to different gas concentrations in order to find safe and useful atmospheres would be welcome. Finally, postharvest research contributions that have greatest impact on the fruit industry are those that can translate understanding of physiology into practical ways of maintaining quality and extending shelf life (Shewfelt, 1986). In this sense, there is a lack of knowledge of basic cherimoya physiology which precludes development of successful management strategies for the cherimoya fruit. ACKNOWLEDGEMENTS This work was supported in part by the Natural Sciences and Engineering Research Council and the Ontario Ministry of Agriculture and Food. REFERENCES Abascal, A. (1989) Uniformizacidn de la madurez de chirimoyas (Annona cherimola Mill) mediante la exposicidn de la fruta a etileno gaseoso. Tesis Ing. Agr. Santiago, Pontif. Univ. Catdl. Chile. Abufom, J.C. (1985) Efectos de aditivos, mondados y tiempo de almacenaje en la calidad de pulpa y rodelas congeladas de chirimoya (Annona cherimola Mill) cvs. Concha Lisa y Bronceada. Tesis Ing. Agr. Quillota, Univ. Cat61. ValparMso. Abufom, J.C. and Olaeta, J.A. (1986) Congelado de chirimoya (Annona cherimola Mill). Alimentos 11(2), 27-33. Biale, J.B. (1960) The postharvcst biochemistry of tropical and subtropical fruits. Adv. Food Res. 10, 293-354. Biale, J.B. and Barcus, D.E. (1970) Respiratory patterns in tropical fruits of the Amazon Basin. Trop. Sci. 12(2), 93-104.

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2(15

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Herrera, D.J. (1988) Determinaci6n de las curvas de crecimiento y verificaci6n de un indice de cosecha en chirimoya (Annona cherimola Mill) para los cultivares Concha Lisa y Bronceada en la zona de Coquimbo. Tesis Ing. Agr. Quillota, Univ. Cat61. Valparaiso. Idstein, H., Herres, W. and Schreier, P. (1984) High-resolution gas chromatography - mass spectrometry and Fourier transform infrared analysis of cherimoya (Annona cherimola Mill) volatiles. J. Agric. Food Chem. 32, 383-389. INE (1988) Anuario Estadlstico de Espafia 63, Instituto Nacional de Estadfstica. Irarrfizaval, M.J. (1984) Respuesta de la chirimoya (Annona cherimola Mill) al tratamiento de frio y encerado en postcosecha. Tesis Ing. Agr. Santiago, Pontif. Univ. Cat61. Chile, 60 pp. Jackman, R.L., Yada, R.Y., Marangoni, A., Parkin, K.L. and Stanley, D.W. (1988) Chilling injury. A review of quality aspects. J. Food Qual. 11,253-278. Kawamata, S. (1977) Studies on determining the sugar composition of fruits by gas-liquid chromatography. Bull. 10, Tokyo-to Agricultural Experiment Station, Tachikawa, Japan, pp. 53-67. Cf. Hortic. Abstr. (1978) 48 (4), 270. Kosiyachinda, S. and Young, R.E. (1975) Ethylene production in relation to the initiation of respiratory climacteric in fruits. Plant Cell Physiol. 16, 595-602. Kuo, S. and Parkin, K.L. (1989) Chilling injury in cucumbers (Cucumis satiL,a L.) associated with lipid peroxidation as measured by ethane evolution. J. Food Sci. 54, 1488-1491. Labuza, T.P. and Breene, W.M. (1989) Applications of 'active packaging' for improvement of shelf-life and nutritional quality of fresh and extended shelf-life foods. J. Food Proc. Preserv. 13, 1-69. Loyola, E. (1988) Identificaci6n y caracterizaci6n morfol6gica de los des6rdenes fisi61ogicos: pitting, moteado y cristalizaci6n de la pulpa en chirimoya cv. Concha Lisa. Tesis Ing. Agr. Quillota, Univ. Cat61 Valparaiso, 134 pp. Lyons, J.M. (1973) Chilling injury in plants. Annu. Rev. Plant Physiol. 24, 445-466. Marangoni, A.G., Butuner, Z., Smith, J.L. and Stanley, D.W. (1990) Physical and biochemical changes in the microsomal membranes of tomato fruit associated with acclimation to chilling. J. Plant Physiol. 135, 653-661. Martinez-Cayuela, M., Plata, M.C., Sanchez de Medina, L. Gil, A. and Faus, M.J. (1986) Evoluci6n de diversas actividades enzim~ticas durante la maduraci6n del chimimoyo en atm6sfera controlada. ARS Pharm. 27, 371-380. Cf. Hortic. Abstr. (1989) 59(6), 605. Martinez-Cayuela, M., Plata, M.C., Faus, M.J. and Gil. A. (1988a) Effect of some phenolic carboxylic acids on cherimoya (Annona cherimolia) polyphenoloxidase activity. J. Sci. Food Agric. 45, 215-222. Mart~nez-Cayuela, M., Faus, M.J. and Gil, A. (1988b) Effect of some reductants on the activity of cherimoya polyphenoloxidase. Phytochemistry 27, 1589-1592. Martlnez-Cayuela, M., Sanchez de Medina, L., Faus, M.J. and Gil, A. (1988c) Cherimoya (Annona cherimola Mill) polyphenoloxidase: monophenolase and dehydroxyphenolase activities. J. Food Sci. 53, 1191-1194. Mayer, A.M. and Harel, E. (1979) Polyphenoloxidases in plants. Phytochemistry 18, 193-215. McGlasson, W.B. (1985) Ethylene and fruit ripening. HortScicnce 20, 51-54. Morales, A. and Cooper, T. (1977) Identificaci6n de hongos causantes de pudriciones en chirimoyas (Annona cherimola Mill) almacenadas a baja temperatura. Inv. Agricola (Chile) 3(2), 75-76. Moreno, J. and De la Plaza, J.L. (1983) The respiratory intensity of cherimoya during refrigerated storage: a special case of climacteric fruit? Acta Hortic. 138, 179-186. Morris, L.L. (1982) Chilling injury of horticultural crops: an overview. HortScience 17, 161-165.

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Noguera, B. (1988) Efecto del acondicionamiento interno del embalaje sobre la apariencia externa y la madurez de frutos de chirimoyo (Annona cherimola Mill) cultivar Concha Lisa en dos estados de madurez. Tesis Ing. Agr. Quillota, Univ. Cat61. Valparaiso. Palma, T. (1991) Bases para el disefio de atm6sferas modificadas para el almacenaje de chirimoyas (Annona cherimola Mill.). Tesis Ing. Agr. Santiago, Pontif. Univ. Cat61 Chile. Pardo, H.A. (1988) Almacenaje refrigerado de chirimoyas (Annona cherimola Mill) cv. Concha Lisa. Efecto de indices de cosecba y un absorbedor de etileno. Tesis lng. Agr. Quillota, Univ. Cat61. Valparaiso. Parkin, K.L., Marangoni A., Jackman R.L., Yada R.Y. and Stanley D.W. (1989) Chilling injury. A review of possible mechanisms. J. Food Biochem. 13, 127-153. Paull, R.E. (1982) Postharvest variation in composition of soursop (Annona muricata L.) fruit in relation to respiration and ethylene production. J. Am. Soc. Hortic. Sci. 1(17, 582-585. Paull, R.E., Deputy, J. and Chen, N.J. (1983) Changes in organic acids, sugars and headspace volatiles during fruit ripening of soursop (Annona muricata L.) J. Am. Soc. Hortic. Sci. 108, 931-934. Pavez, M.L. (1985) Respuesta a la polinizaci6n artificial y determinaci6n de cambios fisicos y quimicos del fruto de chirimoya (A. cherimola Mill) en diferentes cultivares de la zona de La Cruz. Tesis Ing. Agr. Quillota, Univ. Cat61. Valparaiso. Peralta, M.S. (1984) Maduraci6n y almacenamiento de chirimoya (Annona cherimola Mill) cvs. Concha Lisa y Bronceada. Tesis Ing. Agr. Santiago, Univ. Cat61. Chile. P6rez de Castro, M.J. (1987) Descripci6n de la calidad de chirimoyas (Annona cherimola Mill) y plfitanos (Musa sp.) comercializadas en supermercados del gran Santiago. Tesis lng. Agr. Santiago, Pontif. Univ. Cat61. Chile. Poovaiah, B.W. (1986) Role of calcium in prolonging the storage life of fruits and vegetables. Food Technol. 40(5), 86-89. Reddy, R.S. and Rameshwar, A. (1984) Oxidases of custard apple fruits. Plant Physiol. Biochem. 11, 118-125. Reginato, G. (1980) Comportamiento de la chirimoya cn frio (Annona cherirnola Mill.). Tesis Ing. Agr. Santiago, Univ. Chile. Reginato, G. and Lizana, L.A. (1980a) Comportamiento de chirimoya (Annona cherimola Mill) cv. Concha Lisa en almacenaje refrigerado. Simiente 50(3-4), 138-145. Reginato, G. and Lizana, L.A. (1980b) Alteraciones detectadas en chirimoyas (Annona cherimola Mill) durante el almacenamiento. Inv. Agricola (Chile) 6(3): 97-101. Reid, M.S. (1985a) Product maturation and maturity indices. In: Postharvest Technology of Horticultural Crops. Spec. Publ. 3311, Division of Agriculture and Natural Resources, University of California. Reid, M.S. (1985b) Ethylene in postharvest technology. In: Postharvest Technology of Horticultural Crops. Spec. Publ. 3311, Division of Agriculture and Natural Resources, University of California. Rokba, A.M., El-Wakeel, A.T. and Ali, N.H. (1977) Studies and evaluation of some Annona varieties. Agric. Res. Rev. 55(3), 13-29. Sastry, L.V., Bhatia, B.S. and Oirdhari, L. (1961) Custard apple peroxidase. J. Food Sci. 26, 244-247. Schroeder, C.A. (1956) Cherimoyas, sapotes, and guavas in California. Calif. Avoc. Soc. Yearb. 40, 49-51. Shewfelt, R.L. (1986) Postharvest treatments for extending the shelflife of fruits and vegetables. Food Technol. 40(5), 70-76. Snowdon, A.L. (1990) A colour atlas of post-harvest diseases and disorders of fruits and

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