Duration of hydrothermal treatment and peeling of 'Murcott' tangor

CROP SCIENCE

Duration of hydrothermal treatment and peeling of 'Murcott' tangor

Ana Luiza Pinheiro^I; Angelo Pedro Jacomino^I; Maria Cecília de Arruda^II; Ricardo Alfredo Kluge^III

^IUSP/ESALQ – Depto. de Produção Vegetal, C.P. 09 – 13418-900 – Piracicaba, SP – Brasil

^IIAPTA Polo Regional Centro-Oeste, Av. Rodrigues Alves, 40 – 17030-000 - Bauru, SP – Brasil

^IIIUSP/ESALQ – Depto. de Ciências Biológicas

^{Corresponding author}

ABSTRACT

Hydrothermal treatment facilitates the peeling of 'Pera' sweet orange fruit and does not alter its quality. The aim of this work was to adapt the technology of peeling for the use of hydrothermal treatment in 'Murcott' tangor and to evaluate its influence in the CO₂ production and the physicochemical, microbiologic and sensorial characteristics of fruits. The peeling time, the yield of marketable fruits and the internal temperature of fruits during the treatment were also evaluated. The hydrothermal treatment consisted of placing the fruits in a water-bath at 50 ºC for 5, 10, 15, 20, 25 and 30 min. Fruits were peeled by first opening a gap in the peduncle region with a knife and then manually removing the flavedo and albedo. Fruits were stored at 5 ºC for six days. Hydrothermal treatment caused changes in the fruits' CO₂ production for only the first few hours after processing. Internal fruit temperature after 30 min of treatment reached 35 ºC. There were no changes in the physicochemical and microbiologic characteristics of the fruits. The treatment did not change the flavor, improved the fruits' appearance, decreased the peeling time of the treated fruits by 57 % and increased the yield of marketable fruits. In conclusion, the hydrothermal treatment accomplished from 5 to 30 min at 50 ºC can be used as part of the peeling process for 'Murcott' tangor.

Keywords: fresh-cut, heating, quality, appearance, easy peeling.

Introduction

Tangerines rank second in economic importance among other citrus fruits. Among the tangerines the 'Ponca' and the 'Murcott' varieties stand out (Pio et al., 2005). 'Murcott' tangor which is also known as Honey Tangerine is a hybrid between sweet orange (Citrus sinensis) and tangerine (Citrus reticulada). The fruit has high commercial value because it is juicy, sweet and a little acidic flavor (Figueiredo, 1991). However, the residual odor transferred to the consumer hands, when they are peeled, limits their consumption. 'Murcott' tangor has a thin and adhered peel to the juice portion, being difficult to peel. Therefore, it has a great potential of being marketed as a fresh-cut product convenient to assist to the consumer's demands (Damiani et al., 2008).

Hydrothermal treatment at 50 ºC for 8-30 min was found to be effective in maintaining the quality of the 'Pera' sweet Orange fruit (Arruda et al., 2008; Pinheiro et al., 2009). This technique can be extended to other citrus fruits difficult to peel such as 'Murcott' tangor. The aim of this work was to adapt the technology of peeling for the use of the hydrothermal treatment in 'Murcott' tangor and to evaluate its influence in this fruit quality.

Materials and Methods

Fruits with uniform size were used, weighting around 300 ± 50 g and with a shine skin. 'Murcott' tangors were used with a soluble solids/titratable acidity ratio around 16. 'Murcott' tangor were washed, sanitized with chlorinated water (200 mg of active chlorine L^–1) for 10 min and cooled at 5 ºC for 12 h. Then, fruits were submitted to hydrothermal treatment and peeled. This treatment consisted of putting the fruits in heated water at around 50 ºC, for 0 (control) 5, 10, 15, 20, 25 and 30 min. Fruits were then peeled by first opening cut the peduncle region with a knife and after that, manually removing the flavedo and albedo. Fruits were put in expanded polystyrene trays, covered with polyvinyl chloride film of 17 µm of thickness and stored at 5 ± 1 ºC and 80 ± 5 % of relative humidity for six days.

CO₂ production of both fresh-cut fruits submitted and those not submitted to hydrothermal treatment as well as for whole fruit with peel. Fruits were put in hermetic containers of glass that were kept closed for 30 min. Air samples measuring 1 mL were collected inside the containers with a Hamilton^Ò Gastight syringe. Samples were injected into a ThermoScientific gas chromatographer, (Trace 2000GC), equipped with a flame ionization detector (250 ºC), an injector (100 ºC), a Porapack N column of 4 m (100 ºC) and a methanator (350 ºC). CO₂ production was determined considering the container's volume, the fruit mass, the period of time the container remained closed and the initial concentration of CO₂ inside of containers. The experimental design was entirely casualized with six replicates of one fruit for each treatment. Measurements were carried out every 2 h for 10 h after peeling and every 24 h for six days.

Internal fruit temperature was evaluated at 2 cm depth in relation to the epicarp during the hydrothermal treatment to determine the curve of heating of the fruit. For the measurement, type-T thermocouples were inserted (copper-constantan) in the equatorial region of the cooled fruits. Fruits were immersed in a water-bath at 50 ºC for 30 min. Temperature data were recorded with a data acquisition system (Micrologger CR7, Campbell Scientific, Inc., Logan USA). Eight replicates were conducted of one fruit. Water temperature was also monitored. Fruit were characterized for peel thickness, whose replicates' average was 3.2 mm, and degree of humidity, whose average was 85.3 %.

Physicochemical analyses were carried out on the day of processing and on the 6^th day of storage at 5 ºC. The external color was determined by a Minolta colorimeter (CR-300). Two measurement media were considered for each fruit. Results were expressed in Lightness (L). Soluble solids, titratable acidity and ascorbic acid content were determined according to the methodology described by Carvalho et al. (1990). The experimental design was entirely casualized with four replicates of two fruits each treatment.

Microbiologic analyses were carried out on the day of processing and on the 6^th day of storage at 5 ºC. Analyses included total count of aciduric microorganism (yeasts, molds and aciduric bacteria), total count of lactic bacteria and the most probable number (MPN) of total coliforms and thermo-tolerant coliforms according to Silva et al. (2007). Acidified Potato Dextrose Agar was used to count the levels of contamination from aciduric microorganisms using spread-plate technique. Plates were incubated to 25 ºC for five days. The number of lactic bacteria was quantified by using pour plate technique. Orange Serum Agar was used. Plates were incubated to 30 ºC for 48-72 h. Most probable number (MPN) of total coliforms and thermo-tolerant coliforms were determined by using multiple-tube method.

Sensorial analyze was carried out for attributes flavor and appearance on the first and sixth days of storage at 5 ºC. The fruits treated for 30 min were compared as a control in three tests. For flavor, the triangle test was made, which is used to determine whether a noticeable difference exists between two products by comparing three samples, with two of them being identical. Then, 30 untrained judges were asked to identify the different sample (Ferreira et al., 2000). For the appearance, a paired-preference test to evaluate the choice of a sample over another was applied (Ferreira et al., 2000), as was a visual quality test. For the last test, the following score was used: 5 – excellent; 4 – good; 3 – fair; 2 – bad; 1 – very bad.

For measuring the effect of treatment on peeling time, five people peeled ten fruits that had undergone treatment, totaling 350 fruits. The average of the time spent peeling each fruit and the yield of marketable fruits were evaluated. Fruits showing injuries (breaking of the segments' membranes with juice leakage) of 1 cm² or more, or showing more than three injuries smaller than 1 cm^2, were considered unsuitable for commercialization. The design used randomized blocks, with each person considered a block. The sequence of the treatments in each block was randomized.

The results obtained in each experiment were submitted to variance analysis and averages compared by Tukey's test (5 %). Regarding the sensorial analysis, the statistical analysis accomplished with the triangle test was based on number of correct judgments compared with the number of total judgments (30 judges). The minimum number of judges with correct answers needed to establish a difference between samples is 15 at a 5 % significance level (O'Mahony, 1986). For paired-preference test, the statistical analysis was based on the number of judgments of the suitable sample as a favorite. In the case of 30 judges, the minimum number of correct judgments in favor of a certain sample that is needed to establish a significant difference at 5 % is 21 (Meilgaard et al., 1991). For the visual quality test, the results were submitted to the variance analysis and the averages were compared using Tukey's test (5 %).

Results and Discussion

CO₂ production of 'Murcott' tangors that had gone through hydrothermal treatment increased with time. However, this effect was limited to the first hours following the treatment. The CO₂ production of treated fruits equaled that of the untreated fruits in the eighth hour following the processing (Figure 1A) (p > 0.05). Similar results were found by Pinheiro et al. (2009), who verified that the CO₂ production of treated 'Pera' sweet oranges (50 ºC for 10 to 30 min), equaled that of untreated fruits on the fourth hour following processing. A day after processing, fresh-cut 'Murcott' tangors had already presented the same CO₂ production as that presented by whole fruits with peel (Figure 1B) (p > 0.05). The stress effect of processing did not remain during the fruits' storage.

Internal fruit temperature increased (p ≤ 0.01) during hydrothermal treatment up to 35 ºC at a depth of 2 cm. There was greater increase of the temperature in the first minutes of treatment, which became less intense over time (Figure 2). The internal temperature of the 'Pera' sweet oranges treated (50 ºC for 10 to 30 min) reached 35 ºC approximately, at a depth of 2 cm (Pinheiro et al, 2009). Internal temperature of peaches treated with hot water had larger increase in first minutes of treatment (Zhou et al., 2002).

The pulp temperature of fruits with 30 min of heating (35 ºC) was similar to the temperature commonly observed in fruits at the moment of the crop and during other parts of the production chain and commercialization process. Therefore, it is believed that this temperature does not result in the alteration of the quality of the fresh-cut fruits. Additionally, immediately after peeling, these fruits are again cooled (Pinheiro et al, 2009).

Neither hydrothermal treatment nor storage time has influenced external color, soluble solids, titratable acidity and ascorbic acid (p > 0.05) (Figure 3A, 3B, 3C e 3D). Fresh-cut 'Ponca' tangerines had a decrease of the L-value on the 6^th day of storage at 0 ºC and 10 ºC (Damiani et al., 2008). Fresh-cut 'Murcott' tangors, stored at 6 ºC for nine days, did not present difference in soluble solids in storage (Kluge et al., 2003). Vitamin C level in fresh-cut 'Ponca' tangerines decreased during storage at 5 ºC (Pinto et al., 2007).

The levels of contamination from aciduric microorganisms (yeasts, molds and aciduric bacteria) were as low for as those from lactic bacteria reaching at the highest 2.9 × 10² CFU g^–1 (table 1). There is no legislation setting a maximum amount of microorganisms or bacteria allowed to be present in food. However, fresh-cut products with high amounts of these microorganisms (> 10⁵ CFU g^–1) are inappropriate for consumption (Morton, 2001).

All of samples presented an absence of total coliforms and thermo-tolerant coliforms. In agreement with the microbiologic patterns established by ANVISA, fresh fruits (peeled, selected or fractional) should present at the most 5.0 × 10² thermo-tolerant coliforms g^–1 (Anvisa, 2010).

Regarding the sensorial analysis, the number of correct answers indicating that hydrothermally treated fruits had a flavor different from that of the control samples was 9 and 12 in 1^st and 6^th day respectively. This demonstrates that the hydrothermal treatment did not interfere in flavor of the fruits. The minimal number of correct answers to characterize a difference between samples at 5 % significance is 15 (O'Mahony, 1986).

The flavor of 'Pera' sweet oranges submitted to hydrothermal treatment (50 ºC for 8 to 30 min) was not altered (Arruda et al, 2008; Pinheiro et al., 2009). To judge appearance, two tests were accomplished: a paired-preference test and a visual quality test. The paired-preference test showed that judges preferred hydrothermally treated fruits to the control fruits. During the 1^st day of storage at 5 ºC, 21 judges preferred the hydrothermally treated sample to the control sample. On the sixth day, 23 judges preferred the treated sample. A minimum of 21 answers in favor of a certain sample is necessary to characterize a significant difference between samples at 5 % significance level (Meilgaard et al., 1991).

According to the visual quality test, hydrothermally treated fruit presented superior reviews (p ≤ 0.05). While these fruits obtained ratings between 4 and 5 (between good and excellent), the untreated fruits obtained ratings between 3 and 4 (between fair and good) (Figure 4). The appearance of 'Pera' sweet oranges peeled through the use of hydrothermal treatment for eight to 30 min had the consumers' preference. The smaller immersion times enabled the total removal of the albedo, while the fruits without this treatment presented albedo traces (Arruda et al., 2008; Pinheiro et al., 2009).

Hydrothermal treatment decreased the peeling time of 'Murcott' tangors (p ≤ 0.05) and increased the yield of marketable fruits (p ≤ 0.05) (Figure 5). Peeling an untreated 'Murcott' tangor required an average time of 1.43 min. For the fruits that had undergone 5 min of hydrothermal treatment, the average peeling time was 0.85 min (what corresponds a decrease of 41 % in the time) and the average peeling time was only 0.62 min for fruits treated for 30 min (decrease of 57 %). No treated fruits differed of the others (p ≤ 0.05), and the fruits treated for 5 min differed of those treated with 30 min (p ≤ 0.05).

The yield of marketable fruits was greater for treated fruits (p ≤ 0.05). The yield for peeled fruits with no treatment reached a level of 88 % and the yield for peeled fruits with hydrothermal treatment reached 98-100 % (Figure 4). Treated fruit did not differ among them in the yield marketable fruits (p > 0.05).

'Pera' sweet orange submitted to hydrothermal treatment (50 ºC) for 10 to 30 min had a reduction in peeling time of up to 78 % from the amount of time used to peel untreated oranges. Peeled untreated oranges produced a yield of 60 %, and the yield of treated peeled oranges reached levels in a range of 90-96 % (Pinheiro et al., 2009).

Conclusions

Hydrothermal treatment performed for 5 to 30 min at 50 ºC can be used as peeling technique for 'Murcott' tangors. This treatment makes peeling of fruits easier and increases the yield of marketable fruits even when using the shortest immersion time. Additionally, it is found to improve the appearance of the fruits. Hydrothermal treatment neither alters the physicochemical and microbiologic characteristics nor the flavor of the fruits. Changes in the fruits' CO₂ production are not persistent.

Acknowledgements

To FAPESP (Sao Paulo Research Foundation) for providing Master's scholarship, to P.A.R. 'Frutas Boni', for supplying fruits for the purposes of the experiments, and to Dr. Rafael Vasconcelos Ribeiro, for help with measurement of internal fruit temperature.

Edited by: Ana Cláudia Carraro Alleoni

Received July 08, 2010

Accepted December 20, 2010

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Publication Dates

• Publication in this collection
  02 Feb 2012
• Date of issue
  Dec 2011

History

• Received
  08 July 2010
• Accepted
  20 Dec 2010