Fruit Quality as influenced by Wax Application
Half-cooling times for hot (60 °C) and cold (0 °C) dried waxed pears in boxes were identical. Waxed hot dried pears required an additional 21 hours to equilibrate to holding room temperature. Hot or cold dried pears ripened differently. Waxed hot and cold dried pears exhibited lower external, but higher internal concentrations of CO2 than non-waxed fruit. After prolonged storage, waxed cold dried pears required more time to develop the characteristic ripe yellow color and retained firmness longer than either waxed hot dried or non-waxed pears. Waxed hot dried pears were slower to develop yellow color and retained firmness longer than non-waxed pears. Pears waxed immediately after harvest or after 90 days of cold storage demonstrated increased ripening time compared to non-waxed fruit.
The use of waxes to enhance postharvest consumer appeal of fruits and vegetables is a common practice (Schomer and Pierson, 1967; Smock, 1969). Most of the literature on waxing is based upon investigations with older wax formulations and equipment (Hardenburg, 1967). Meheriuk and McPhee (1984) furnish current information on waxing techniques. More recent investigations have been more concerned with apple quality in relation to waxes. Winter pears (d'Anjou) traditionally have been waxed to facilitate packing and to aid in the elimination of abrasions caused by fruit contact with packing equipment. Use of wax on fruit has been shown to reduce water loss (Espelie et al. 1982) and respiration (Trout et al., 1952; Bramlage, 1986; Drake and Nelson, 1990). Meheriuk and Porritt (1972) and Drake and Nelson (1990) found changes in apple respiration rate, and a retention of firmness and weight due to wax application. However, differences were strongly dependent on the cultivar (cv). Trout et al. (1952) noted changes in the ripening pattern of apples due to wax application. Some protection against chilling injury also has been noted with the use of wax (Paull and Rohrbach, 1985).
Two commercial methods are available to dry waxes on fruit. Traditionally hot air (48 to 66 °C) has been used to set and dry waxes on fruit. Recently (the last 5 years) the use of cold air (0 °C) has emerged as a way to set and dry fruit wax with subsequent energy cost savings. Drake and Nelson (1990) investigated the differences in hot and cold drying techniques on apples and noted no advantage in apple fruit quality in relation to drying technique. The purpose of this study was to determine quality differences that may be expected with hot or cold drying of waxed d'Anjou winter pears.
Materials and Methods
D'Anjou pears harvested at commercial maturity from a block of 6 trees were immediately divided into 2 lots. One lot was placed in refrigerated storage (1 °C) for 90 days and then transferred to room temperature 24 hours prior to wax application. The second lot of fruit was waxed 24 hours after harvest, using both hot and cold wax drying techniques. Prior to waxing, the pears were drenched in hot water (40 °C), and passed over rotary dewatering brushes for 30 sec before going into the wax applicator. Using a commercial pear wax formulation (Shieldbrite PR160C, Shield-brite Corp, Kirkland. WA) pears were waxed at the approximate rate of 10 ml/min (6,804 Kg fruit/3.8 liters). Waxed pears were dried in a commercial hot-drying tunnel held at 60 °C (Van Doren Sales, Inc., Wenatchee, WA) or a cold drier operated at 0 °C (Sirron, marketed by MARQ International Packaging Systems, Inc., Yakima, WA) for 2 min. After the fruit surface had dried, 100 pears were packed in pulp fiber trays, enclosed in a box with a polyliner, and placed in cold storage, 1 °C.
Pears waxed immediately after harvest were examined for quality after 60, 90 and 180 days of storage. Pears waxed after 90 days of storage were examined after 120 and 180 total days of storage at 1 °C. Twenty pears (20/wax trt/drying temp/rep) were removed at each examination period. Ten pears were evaluated immediately after removal from storage and the remaining pears after 8 days of ripening at 20 °C.
To study core temperature of pears as a function of drying temperature, iron con stantan thermocouples were inserted into the cores of pears selected from the 2 drying temperatures. Pears with thermocouples were placed in the middle of a box (middle pear, middle tray) and temperatures were monitored using a data logger (Doric Digitural 200, Emerson Electric Corp., San Diego, CA) until a constant holding temperature was established. Individual boxes of pears were randomly distributed about the cold room with a nominal air temperature of 1.9 °C. The core temperature of the centermost fruit was recorded when the boxes were placed in storage and every 3 hours thereafter for 72 hours.
Air temperature of the cold room was also recorded coincident with fruit temperature. Surface temperature of the pears immediately out of the dryers was determined by inserting a fruit thermometer 1 cm into the surface of the fruit. Temperature data were analyzed using the PR C GLM (general linear model) (SAS, 1985). Pears were evaluated for external gases and internal gases (CO2 and C2H4), external and internal color, firmness, titratable acidity and visual disorders. External gases were determined by the procedure described by Drake et al. (1987). Internal gases were determined by the procedure described by Sfakiotakis and Dilly (1973). A Lake City pressure tester, Model EP1 (Kelowna, B.C., Canada) was used to determine firmness and values were reported in Newtons.
External and internal color was determined with a Pacific Scientific, The Color Machine, using the Hunter L, a, b values calibrated with a white CM536 standard. Hunter a and b values were used to calculate hue. Three values for each pear were determined around the circumference of the fruit. Internal color was measured by cutting 10 fruit in half equatorically and placing the exposed flesh surface, blossom end up, immediately in a sample cup. For both, external and internal color, the average values for 10 fruit were reported. Acids were titrated to pH 8.2 with 0.1 N NaOH and expressed as the percentage of malic acid. Visual disorders were determined by laboratory personnel and expressed as percentage of fruit examined. Analysis of variance (ANOVA) of fruit quality was determined by MSTAT (1988) as a factorial design. Based on significant F test, means were separated by Duncan's multiple range test.
Results and Discussion
Fruit from the hot drier entered the cold room at a core temperature of 19.5 °C (±1.86 °C); whereas the fruit from the cold drier treatment measured 12.4 °C [±0.66 °C]) (Figure 1). The time-temperature data were analyzed to determine the half cooling time constant () for the hot and cold waxed fruit. The half cooling time is that time which is required for the fruit to accomplish half of the total possible temperature change. As expected, the half cooling time, , for both treatments was identical and equal to 17.0 hours. The correlations had r2 values of 0.828 and 0.899 for cold and hot treatments, respectively.
While the cooling rates expressed in half cooling times are identical, the important consideration for this study is the time to reach a given temperature. Table 1 lists the time to reach 3 °C for fruit initially at 19.5 °C and 12.4 °C after placement in a cold room at 1.9 °C. Since actual half cooling times are widely variable depending on the cold room and packaging specifics, cooling times associated with values of from 17 to 30 hours are included in the table.
The time to 3 °C was 48 and 69 hours for cold and hot wax drying treatments, respectively. The agreement with the predicted value in Table 1 (=17 hours) is excellent for the waxed hot dried treatment. The waxed cold dried prediction disagrees with the measured time due to the poor temperature stability in experimental cold room which dipped to nearly 0 °C for the first 7 hours and again at 21 hours. The effect of this deviation from the mean air temperature has a much more pronounced effect on the colder fruit since it increases the driving force for cooling by nearly 18%. In analyzing the data, the actual, rather than mean, air temperature was used at each time interval.
Table 1 shows that in all cooling regimes considered, there was a 23% increase in time to cool to 3 °C for the hot dried waxed pears. The effect of this extended cooling time on fruit quality was studied by analyzing various parameters.
Pears waxed immediately after harvest
As shown with apples (Trout, 1952) wax coatings slowed the ripening pattern of d'Anjou pears. This change was dependent upon the method used to dry the wax. Pears that were waxed and cold dried required a longer period of time to develop the characteristic yellow color of ripe pears. In addition, the firmness of waxed, cold dried pears persisted for a longer period of time than either hot waxed or non-waxed pears. Pears waxed and hot dried were between cold dried and non-waxed pears in their ripening pattern. Waxed pears required a longer time to ripen than non-waxed pears resulting in a longer shelf life.
As reported in previous work (Drake and Nelson, 1990) with other types of fruit, the use of wax inhibited CO2 evolution from both the wax coated hot and cold dried fruit (data not shown). Internal CO2 concentration after 180 days of cold storage was greatly elevated in waxed pears regardless of the wax drying temperature when compared to non-waxed pears (21.6, 23.7 and 4.2 mL/L respectively). Elevated internal ethylene (21.8 ppm) was observed, but only in the hot dried waxed fruit.
External color of hot dried waxed pears changed more rapidly than waxed cold dried pears (Table 2). At all evaluation times, the waxed hot dried pears had higher "L" readings than cold dried waxed. There was also a distinct "Hue" difference between the two wax temperatures except for the 90-day evaluation. This difference for "L" and "Hue" between the two wax drying temperatures resulted in a waxed hot dried pear with a lighter, more yellow, less green color than waxed cold dried pears throughout storage. After 180 days of storage, the non-waxed pears were distinctly lighter in color and much more yellow than waxed pears from either drying temperature (Table 3).
When internal color was evaluated there was also a difference in "Hue" due to wax drying treatment (Table 2). Hot dried pears developed a more mature yellow color at a much more rapid rate than cold dried wax pears. Cold dried pears after 180 days of storage displayed an internal "Hue" color equal to waxed hot dried after only 60 days of storage. Internal "Hue" of waxed cold dried pears at 60 and 90 days was distinctly less than all other treatment combinations. When the internal color of waxed pears was compared to non-waxed the non-waxed and waxed hot dried fruit displayed similar "L" color values (Table 3). No difference in internal "Hue" values was evident between waxed and nonwaxed pears.
Firmness is a major quality attribute when considering the acceptability of many fresh fruits. d'Anjou pears are generally harvested when firmness ranges from 14.0 to 16.0 lbs. After they are removed from storage they are allowed to ripen to approximately 2.0 to 3.0 lbs before consumption. Retention of firmness during storage is a major quality consideration. On the other hand, after removal from storage the pear should ripen to the acceptable firmness level in a reasonable and predictable time. Pears that were waxed and hot dried were less firm than waxed pears cold dried regardless of storage time (Table 2). Hot dried pears lost firmness at a gradual rate (11.0 to 7.6 lbs) during storage, whereas cold dried pears were similar at 90 and 180 days of storage. After an 8-day ripening period hot-dried pears lost 61% of their firmness compared to only 41% loss in firmness for cold dried pears. When the firmness of waxed pears was compared to non-waxed pears after 180 days of storage (Table 4), there was significant firmness difference only for the waxed pears cold dried. Both hot dried and non-waxed pears were similar immediately out of storage and after ripening. Cold dried pears were significantly firmer after an 8 day ripening period.
Pears waxed after 90 days storage
At 120 days of storage, waxed pears, both hot and cold dried, were similar in both "L" and "Hue" color values (Table 4). After 180 days of storage however, the hot dried pears had developed a lighter, more yellow, less green appearance than cold dried pears. After 180 days of storage non-waxed pears were lighter in color with more yellow than waxed pears, particularly after an 8 day ambient temperature ripening period.
Internal "L" color values were not influenced by late waxing regardless of storage time or wax drying temperature (Table 4). These data are consistent with the lack of internal "L" color difference for fruit waxed immediately after harvest and stored for 180 days (Tables 2 and 3). There was also no difference in internal "Hue" values for the waxed pears at either 120 or 180 days of storage. No difference was apparent when waxed fruit was compared to nonwaxed after ripening for 8 days after 180 days of storage (Table 4).
Waxed pears cold dried were superior in firmness compared with waxed pears hot dried (Table 4) at either 120 or 180 days of storage. The firmness of cold dried pears after 180 days of storage was greater than that of hot dried pears after 120 days of storage. Waxed pears were firmer regard less of waxing drying temperature, but only immediately after removal from cold storage. After an 8 day ripening period both hot dried and cold dried were firmer than nonwaxed pears. This difference in firmness (late waxing) between hot and cold dried waxed fruit and waxed and non-waxed fruit was also evident in pears waxed immediately after harvest (Table 2 and 3).
There were no differences in visual disorders due to drying temperatures or between waxed and non-waxed pears. Visual disorders, when present, were similar across all treatments and storage.
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The authors thank the Washington Tree Fruit Research Commission for funds partially supporting this project.
Dr. Steve Drake
USDA, ARS Tree Fruit Research Laboratory
1104 N. Western Ave., Wenatchee, WA 98801
FAX: (509) 664-2287
13th Annual Postharvest Conference