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Freeze-drying of acerola (Malpighia glabra L.)
Chemical Engineering and Processing 46 (2007) 451–457
Freeze-drying of acerola (Malpighia glabra L.) Luanda G. Marques, Maria C. Ferreira, Jos´e T. Freire ∗ Department of Chemical Engineering, Drying Center of Pastes, Suspensions and Seeds, Federal University of S˜ao Carlos, P.O. Box 676, 13565-905676, Brazil Received 10 January 2006; received in revised form 10 March 2006; accepted 20 April 2006 Available online 17 September 2006
Abstract Several quality parameters, such as water activity (aw ), glass transition temperature (Tg ), Vitamin C content, shrinkage and rehydration capacity were investigated for the freeze-drying of acerola fruits. The variation of temperature with time at different positions of the samples was measured during the freezing of samples, performed prior to the freeze-drying. Drying kinetic curves were obtained for different types of samples. The extent of shrinkage after freeze-drying was investigated and related to the glass transition temperature, Tg . The variation of water activity (aw ) along the drying and sorption isotherms was obtained for different freezing techniques. It was observed that freeze-dried acerola fruits can be easily reconstituted, and important nutritional parameters are well preserved after the process. They may be considered as a good source of Vitamin C, whose content was best preserved for freeze-drying of fruits at an intermediary stage of ripening. © 2006 Elsevier B.V. All rights reserved. Keywords: Freezing; Vitamin C; Glass transition temperature; Water activity; Rehydration capacity
1. Introduction Acerola, also known as West Indian cherry, Barbados cherry or Cherry of Antilhas (Malpighia glabra L., Malpighia punicifolia L. or Malpighia emarginata DC.) is a fruit original from Antilhas which grows also in the northeast of South America and in Central America. It is round shaped, with diameter varying from 3 to 6 cm. Its fleshy and succulent pulp is encased by a very thin protection peel that quickly ripens. At the initial stages of ripening, the fruit is a full green color, changing to yellow-reddish and finally to red or purple when completely ripened [1]. Its main appealing feature is the high Vitamin C content, which might vary from 1247.10 to 1845.79 mg/100 g [2], but it is also rich in other nutrients such as carotenes, thiamin, riboflavin, niacin, proteins, and mineral salts, mainly iron, calcium and phosphorus [1]. Brazil is the major worldwide producer, consumer and exporter of acerola, with a production of 32,990 tonnes in 1996 [3]. Like other tropical fruits, acerola may be commercialized ‘in nature’ or may be used in the production of frozen pulps and processed juices. The typical weather characteristics that pre-
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dominate at northeast region from Brazil, however, with high relative humidity and temperatures, are not favorable to fruit preservation under natural conditions. Particularly for fruits with high moisture contents (in the case of acerola, the average value is about 91%w.b.), rapid deterioration is commonly observed after cropping. Aiming to offer alternatives for preservation of the fruits and of their original constituents, special attention has been directed to the development of adequate drying techniques. Besides aggregating commercial value to the fruits, drying reduces wastes and post-harvest losses, and might allow their commercialization for extended periods of time, with minor dependence on seasonal conditions. Freeze-drying is a technique that results in high-quality dehydrated products due to the absence of liquid water and the low temperatures required in the process. The solid state of water during freeze-drying protects the primary structure and minimizes changes in the shape of the product, with minimal reduction of volume [4]. In addition, it contributes to preserve constituents as minerals and vitamins, as well as to retain original flavor and aroma [5]. Freeze-drying appears, therefore, as a promising technique for dehydration of thermal-sensitive materials, such as acerola fruits. The final quality of dried fruits can be affected by the physical and structural changes that occur during drying, which may include deformation by shrinking. The true and apparent
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densities, as well as the porosity of the material define the rehydration capacity of a dried product. The drying process may alter these properties, resulting in products with modified texture, optical, thermal and nutritional properties [6]. In various foods, the solids are in an amorphous metastable state that is very sensitive to changes in temperature and moisture content. The change from the glassy to the rubbery state of the matrix occurs at the glass transition temperature (Tg ), which is specific for each material and affect chemical and physical changes during food processing and storage [7]. The activity coefficient expresses the state of water in a solution or a solid [8]. The water activity and the glass transition temperature stay among the parameters which are classically evaluated in the analysis of quality of dehydrated foods. The extent of shrinking and the glass transition temperature are interrelated in that significant changes in volume of the material can be noticed only if the process temperature exceeds Tg , at a particular moisture content. At temperatures higher than Tg the viscosity is considerably reduced, facilitating the product deformation [9]. Another parameter that can be used as a quality index of nutrients during food processing and storage is the ascorbic acid content, because it is an unstable constituent, sensitive to variations on pH, temperature, moisture content, oxygen and light. If the ascorbic acid is retained after processing, other nutrients are likely to be retained [10]. Lin et al. [10] performed a comparative study among vacuum microwave, hot air and freeze-dried carrot slices. They observed that freeze-dried carrots did not present significant losses in the content of Vitamin C, while the samples dried by hot air and vacuum microwave presented losses of 62 and 21%, respectively. Shadle et al. [11] also investigated the Vitamin C content of carrots after convective and freeze-drying, reporting losses of 81.3 and 60.8%, respectively in the dry samples. The rehydration ratio can be considered as a measure of the injuries caused by the processing and drying to the material [12]. It is generally accepted that the rehydration capacity is dependent on the degree of cellular and structural disruption. During drying, Jayaraman et al. [13] observed irreversible cellular rupture and dislocation, resulting in loss of integrity and hence, in a dense structure of collapsed, greatly shrunken capillaries with reduced hydrophilic properties, which are reflected by the inability to imbibe sufficient water to fully rehydrate. In spite of the importance of knowing quality parameters of freeze-dried products, detailed information on these aspects is still lacking in literature, particularly for tropical fruits. The purpose of this work is to determine some quality parameters, such as the Vitamin C content, the glass transition temperature, the water activity and the rehydration capacity for freeze-dried fruits and to investigate the effect of dehydration conditions on these properties.
The fruits, with diameters varying from 1.5 to 2.5 cm were placed in a circular tray of diameter equal to 125 mm and height of 15 mm and submitted to freezing. Three freezing techniques were evaluated: cryogenic freezing with liquid nitrogen, N2(l) ; with vapor of nitrogen, N2(v) , and by putting the samples in a freezer. The crushed pulps and sliced samples previously frozen were subsequently dehydrated in a laboratorial scale freeze-dryer, manufactured by Edwards, L4KR model. Temperatures at different axial positions of the samples were measured on line along the freezing using four K-type thermocouples, inserted at the positions 0, 5, 10 and 15 mm from the bottom of the tray. The freezing rate (θ) was calculated by Eq. (1), listed in Table 1. Freeze-drying tests were performed with vacuum chamber total pressure and temperature equal to 1.3 × 10−1 mbar and −30 ◦ C, respectively. The thermocouple probe located at the bottom of the tray was used to control and monitor the product temperature. The sublimation heat was supplied by a heating plate located under the tray. During the secondary stage of drying, the product reached a final temperature of about 35 ◦ C. Average freeze-drying time was approximately 12 h. The water activity at different moisture contents was obtained in a Thermoconstanter TH200 equipment, manufactured by Novasina, at a constant temperature of 30 ◦ C. As was pointed out by Kitic et al. [14] the time required to reach equilibrium conditions within the sensor chamber depend on several parameters. They include the physical nature of the sample, their water activity level, specific surface and the transport conditions in the free space above the sample. The glass transition temperature of freeze-dried acerola was determined by differential scanning calorimetry using a thermal analysis system DSC 204 (Netzsch). A sample containing 7.6 g of powdery sample was put on an aluminum pan and scanning was performed by heating at 10 ◦ C/min from −100 to 100 ◦ C, in inert atmosphere, with injection of 40 ml/min of N2 . The ascorbic acid contents in the “in nature” and freeze-dried samples were determined using the 2.6 dichlorofenol–indofenol titration [15]. The titration end point was detected visually and all analyses were conducted in triplicate. Ascorbic acid content was then determined by Eq. (2) in Table 1. The freeze-dried acerola fruits were rehydrated by immersing them in a water bath at ambient temperature. The samples were taken from the bath at different immersion times and weighted after being blotted with a tissue paper to remove the excess of water [12].
2. Materials and methods
θ=
The acerola fruits used in this investigation were purchased from a local market in the city of S˜ao Carlos-SP, Brazil.
Aa =
Table 1 Mathematical equations Properties 0 − Tf t (VT )(0.5/Vt )(V )(100) (Va )(m)
(1) (2)
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Fig. 1. Temperature as a function of time at different L. Fig. 2. Temperature as a function of time during freeze-drying.
The initial and final moisture content of the samples were determined by the oven method at (105 ± 3) ◦ C for 24 h. 3. Results and discussion 3.1. Freezing As an example of the temperature variation of the sample during the freezing, Fig. 1 depicts some cryogenic freezing curves for acerola pulps, obtained in freezing with N2(l) . It can be noted that the first position to freeze is the one located at L = 0 mm, because the heat is removed by the liquid nitrogen at the bottom of the tray. Later, the freezing front moves to the positions L = 5, 10 and 15 mm (the later one corresponds to the sample surface), until the whole sample is completely frozen. The freezing of the whole sample was completed in about 6.0 min. When freezing was performed using N2(v) or in freezer, the times for freezing the whole samples were considerably longer, respectively, of 20.0 min and 12.0 h. From Table 2 it can be observed that the freezing rate increases as the freezing front moves from L = 0 to 15 mm. The formation of a superficial layer composed by solutes which were not frozen is a possible reason for this behavior, as reported by Kochs et al. [16]. The eutectic state of this superficial layer might be responsible by the high freezing rate obtained at L = 15 mm. The freezing rates shown in Table 2 were calculated admitting a final freezing temperature equal to −100 ◦ C. The results in Table 2 indicate that the freezing rates for acerola pulps are quite high. As reported by Delgado and Rubiolo [17], the rapid freezing is advantageous since it contributes to maintain the cellular structure of the frozen pulps due to the formation of small-size ice crystals.
3.2. Freeze-drying Fig. 2 shows the variation of product temperature (measured at z ≈ 0 m) as a function of time. An inspection of Fig. 2 shows the presence of two drying stages, the primary one was finished at T = −20 ◦ C, while the secondary stage was finished as the temperature reached approximately 35 ◦ C. Heat was supplied to the freezing material by conduction through the heating plate, at a high enough rate to replace the heat consumed in the water sublimation occurring at the initial stages of freeze-drying. It can be seen in Fig. 2 that in the primary drying stage, the product remained at a constant temperature. When sublimation has virtually ceased, at the end of primary drying stage, the product temperature raises steeply, until reaching a stable final temperature. 3.3. Drying kinetics Fig. 3 represents the drying kinetic data for sliced and crushed acerola pulps. Different behaviors were observed during the sublimation, showing the influence of the sample type on the mass transfer.
Table 2 Freezing rates L (mm)
θ (◦ C/min)
0 5 10 15
23.0 27.8 41.0 73.6
± ± ± ±
0.5 0.5 0.5 0.5
Fig. 3. Moisture content as a function of time.
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The crushed acerola pulp contains a large amount of free water, but in spite of this, it presented lower drying rates. This result can be attributed to the cohesive forces acting in the crushed material. The pulp becomes more compact as the drying proceeds, thus increasing the resistance to mass transfer while the available area for mass exchange is decreased. In the sliced samples, the original structure of the material, with internal and external pores, is maintained during the drying. As a result, the resistance to mass transfer is reduced because the mass exchange occurs through the formerly ice-filled channels. In the secondary stage no effect of the type of sample on the drying rates was detected, because this drying stage is characterized by the removal of bound water. In a previous work, Marques and Freire [18] reported a predominance of the falling rate period in the drying curves of freeze-dried acerola. In their study, the influence of the freezing technique on freeze-drying was also investigated. They observed that slices of acerola freeze-dried after freezing in a freezer or using N2(v) presented higher drying rates than those frozen using N2(l) . According to Delgado and Rubiolo [17] and Delgado and Da-Wen [19], this behavior can be explained by the size and distribution of crystals of ice formed in the process, which depend on the freezing technique. In slow freezing processes, such as those obtained using freezer or N2(v) , the ice crystals formed are larger, facilitating the mass transfer. 3.4. Nutritional characteristics Chemical and physical characteristics of fresh fruit depend on the ripening stage, genotypes differences, soil, climatic conditions, post-harvest practices and handling procedure after harvesting. The ascorbic acid contents of ‘in nature’ and freezedried acerola fruits were determined at three ripening stages: initial (green fruits), intermediate (yellow-reddish fruits) and completely ripened (red fruits). In Table 3 are listed the results obtained for different conditions. A decrease in the Vitamin C content is observed as the ripening evolves. The values obtained agree with those reported in the literature [20]. At an intermediate stage of ripening, the loss of Vitamin C of acerola after freeze-drying was about 13%. However, for the green fruits, this loss was of 69.3%. This difference can be explained by the instability and sensibility of the ascorbic acid (Vitamin C) to moisture content, pH and metallic ions [21], which vary with the fruit maturation stage. The ascorbic acid is also expected to depend on the fruit size and even on the location of the fruit in a tree. Gongatti et al. [20] investigated the composition of acerola fruits according to their maturation stages. They verified that
dark green acerola fruits contains the larger contents of Vitamin C (1822 mg/100 g) and chlorophyll (4.41 mg/100 g), while the lower contents of Vitamin C (1021 mg/100 g) and chlorophyll (0.10 mg/100 g) were observed in the dark red fruits. Larger contents of carotenoids (1.44 mg/100 ml), soluble solids (7.10 ◦ Brix) and soluble sugar (5.05%) were also observed in the dark red fruits. For foods with moderate moisture contents, Lee and Labuza [22] observed that the rates of degradation of Vitamin C increase at high water activity, probably because the reactions occur more easily when the viscosity of aqueous phase is reduced. The final structure of freeze-dried acerola pulps, which were neither collapsed nor sticky, probably contributed for the preservation of Vitamin C in the product. Stickiness is a characteristic that may cause the over-heating of thermal-sensitive substances, resulting in degradation and undesirable sensorial characteristics [23]. The ascorbic acid content retained in the freeze-dried acerola pulps characterizes this product as a valuable source of Vitamin C. In Brazil, the Recommended Daily Intake (RDI) for adults is 60 mg [24]. The small losses of Vitamin C in freeze-dried products are attributed to the low temperatures and to the use of vacuum in the process. It must be pointed out that, due to the highly porous structure of freeze-dried products [25], an inadequate storage may cause oxidative reactions, leading to additional losses of Vitamin C in the final product. 3.5. Water activity and glass transition temperature Fig. 4 depicts the influence of the freezing technique on the variation of water activity during the freeze-drying of acerola fruits. According to Singh and Heldman [26], values of water activity among 0.20 and 0.40 ensures the stability of the product storage against browning and hydrolitical reactions, liquid oxidation, auto-oxidation and enzymatic activity. For samples frozen using N2(l) and N2(v) , the water activity, aw , was significantly reduced after 6 h of freeze-drying, to values equal to 0.22 and 0.19, respectively, which are in the recommended range for storage. For reaching a water activity of about 0.20 for the samples frozen in a freezer, additional time was required, about 8 h. This difference in the rate of decreasing of aw occurs because the different freezing processes result in products with distinct porous structures. Detailed analysis of the states of water and their effects on different physical food properties and quality parameters are presented by Lewicki [27], among other authors [17,19,28,29].
Table 3 Vitamin C content of raw and freeze-dried acerola Maturation stage
Green Yellow-reddish Red
Vitamin C content Raw (mg/100 g)
Raw (mg/g dry solid)
Freeze-dried (mg/g dry solid)
1617.0 ± 175.8 1587.3 ± 95.2 1341.13 ± 27.6
179.7 ± 19.5 176.4 ± 10.6 149 ± 3.1
55.1 ± 3.2 153.4 ± 39.1 72.1 ± 5.0
Loss (%)
Xw.b. (kg/kg)
69.3 13.0 51.6
0.36 0.54 0.07
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Fig. 4. Water activity as a function of time.
Fig. 5 shows typical moisture adsorption isotherms at 30 ◦ C of freeze-dried samples subjected to different freezing techniques. The shapes of sorption isotherms are characteristic of the highsugar fruit and biological materials [26,30,31], which show a sharp increase of sorption capacity at higher water activities and sorbs relatively small amounts of water at low water activities. It is also possible to note that the sorption isotherms are practically not affected by the freezing technique, a justified behavior since at equilibrium conditions the water activities were similar for the three types of freezing investigated. At low water activities, water can be adsorbed only to surface –OH sites of crystalline sugar. Therefore, moisture content is low in the region of low water activity. At high water activities, dissolution of sugar occurs and crystalline sugar is converted into amorphous sugar [8]. The amount of adsorbed water increases greatly after this transition due to the increase in the number of adsorption sites upon breakage of the crystalline structure of sugar [8]. Similar isotherms were determined for powders of freezedried banana [32], papaya [33] and pineapple [34], as well as to freeze-dried fruits such as strawberry [35], guava, mango and pineapple [31], and blueberries [36].
Fig. 5. Moisture content as a function of water activity.
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Fig. 6. Heat flow as a function of temperature.
Fig. 6 shows the range of glass transition temperature for freeze-dried acerola fruits, indicated by the region where the heat flow drops abruptly. The transition is located between an initial (Tg0 ) and a final temperature (Tge ). The adopted value of glass transition temperature is the mean value (Tgm ) and cp indicates the variation of specific heat in this region. The analysis was performed in duplicate, in order to check the reproducibility. Specific heat values of 0.667 and 0.666 J/g K were obtained for samples 1 and 2, respectively, confirming the good reproducibility of data. The powder obtained from acerola samples lyophilizated after freezing in N2(l) presented a (Tgm ) equals to −32.1 ◦ C at a moisture content of 0.25 kg/kg dry solid. It is worth to note that the low value of residual moisture in the powder is an effect of water plasticization, a behavior typically observed in solid foods with low water content. Bonelli et al. [37] investigated the effect of residual moisture content on the structural collapse of freeze-dried sugar matrix (lactose, maltose, sucrose, and trealose). The authors concluded that the residual moisture content promotes a notable change in Tg due to the high sensibility of this parameter to the water plasticization effect in the range of low moisture contents. Several authors determined the glass transition temperature of freeze-dried fruits, such as strawberry [38], apple [39], pineapple [34], persimmon [40] and strawberry, apple and pear [9]. The values obtained by these authors differ significantly, 188 ◦ C, for apple, −135 ◦ C, for strawberry and from −58.4 to −82.6 ◦ C for tomato. According to Khalloufi and Ratti [9] the marked difference between the values of Tg for the different fruits can be explained by the dependence of glass transition on their composition. The low value of Tg obtained for freeze-dried acerola fruits can be attributed to the presence of sorbitol in their chemical composition, a component with very low Tg , equals to −4 ◦ C [41]. Khalloufi and Ratti [9] determined Tg for freeze-dried strawberry, apple and pear, and verified that pear presented the lower value of Tg , probably because it contains the higher content of sorbitol, nearly 14% of the total carbohydrates. Some whole acerola fruits were freeze-dried aiming to investigate the shrinkage during the process. By estimating
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the volume variation, the shrinkage of acerola fruits during freeze-drying was evaluated as being approximately 17%. Ratti [4] verified that the shrinkage of strawberry, raspberry and blackberry was minimal after freeze-drying, 6.59, 6.13 and 16.1%, respectively. The shrinkage observed in the freeze-dried acerola fruits may be justified by relating Tg and the sublimation temperature. As shown in Fig. 2 the sublimation temperature is −20 ◦ C, whereas Tg is equal to −32.1 ◦ C. Since the sublimation temperature is higher than Tg at a moisture content of 0.25 kg/kg dry solid, the viscosity of water present in acerola is reduced, causing high mobility of water in the amorphous matrix and leading, consequently, to small shrinkage. This result agrees with predictions stating that materials with lower values of Tg experiences more intense shrinkage [9]. Shrinkage is also a function of the starch and sugars composition in the material [42]. An additional explanation for the low shrinkage observed in freeze-dried acerola fruits is given by the non-collapse of pores during the process [43], as evidenced by its high porosity, 0.93 [25]. Khalloufi and Ratti [9] verified that Tg increases with the freeze-drying temperatures. The dependence of Tg on freezedrying temperature was statistically significant (P < 0.05) for all the products investigated by them. Thus, it is expected that a small increase in the freeze-drying temperature of acerola results in a higher value of Tg . As a consequence, the transition from glassy to rubbery state will occur near to the final stages of drying, reducing the magnitude of acerola shrinkage. The non-collapse of freeze-dried acerola was easily detected by visual inspection. This is a good characteristic, since stickiness; compaction and crystallization are phenomena associated to the collapse of the material [44]. Ratti [4] observed that the percentage of collapsed samples during freeze-drying increased as the heating plate temperature was raised. At temperatures higher than 50 ◦ C this percentage exceeded 20%, a considerable reduction in the final product quality.
The investigations showed that in the freeze-drying of acerola, a fast previous freezing of the material contributes to preserve the original porous structure of the product and result in a powder material few susceptible to degradation reactions. From the techniques tested in this work, cryogenic freezing using N2(l) is the most recommended technique for freezing the samples. Freeze-dried acerola fruits present minimum shrinkage and the dried powder presented a low glass transition temperature, −32.1 ◦ C. The rehydration capacity of freeze-dried acerola was high, 10.1 kg/kg, because the samples did not suffer cellular rupture. Freeze-dried acerola can be considered a good source of Vitamin C, with a maximum content of 153.4 mg/g dry solid. Vitamin C content was best preserved in freeze-drying of fruits at the intermediary stage of ripening.
3.6. Rehydration capacity
Acknowledgements
Fig. 7 shows the rehydration capacity of freeze-dried acerola. Its elevated rehydration capacity is attributed to the high porosity, 0.93 [25], and the minimal shrinkage attained owing to the use of N2(l) to freeze the samples, as well as to the freeze-drying method itself. Both contribute to preserve the structural and cellular integrity of the acerola fruit. The equilibrium moisture content at saturation, 8.1 kg/kg dry solid, practically reached the moisture content of raw acerola, 10.1 kg/kg dry solid, indicating that the dehydration process is reversible, resulting in a product with large rehydration capacity and excellent color and flavor. Mastrocola et al. [45] verified that the reconstitution of freeze-dried fruit pieces involves, mainly, diffusive phenomena in the first phase of the process, because the product becomes impregnated with liquid and, when diluted solutions are used, high quantities of soluble solids are lost. When the cell wall becomes partially rehydrated, it acts as a semi-
The authors thank the Brazilian funding agencies FAPESP, Proc. 05/54624-9 and CNPq/PRONEX for their financial support.
Fig. 7. Redydration kinetics of acerola.
permeable membrane, being characterized as a ‘pseudo’ osmotic process. 4. Conclusions
Appendix A. Nomenclature
Aa L m t T V X
content Vitamin C (mg/100 g) position (mm) weight (kg) time interval (min) temperature (◦ C) volume (m3 ) moisture content (kg/kg)
Greek symbol θ freezing rate (◦ C)
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Freeze-drying of acerola (Malpighia glabra L.) Luanda G. Marques, Maria C. Ferreira, Jos´e T. Freire ∗ Department of Chemical Engineering, Drying Center of Pastes, Suspensions and Seeds, Federal University of S˜ao Carlos, P.O. Box 676, 13565-905676, Brazil Received 10 January 2006; received in revised form 10 March 2006; accepted 20 April 2006 Available online 17 September 2006
Abstract Several quality parameters, such as water activity (aw ), glass transition temperature (Tg ), Vitamin C content, shrinkage and rehydration capacity were investigated for the freeze-drying of acerola fruits. The variation of temperature with time at different positions of the samples was measured during the freezing of samples, performed prior to the freeze-drying. Drying kinetic curves were obtained for different types of samples. The extent of shrinkage after freeze-drying was investigated and related to the glass transition temperature, Tg . The variation of water activity (aw ) along the drying and sorption isotherms was obtained for different freezing techniques. It was observed that freeze-dried acerola fruits can be easily reconstituted, and important nutritional parameters are well preserved after the process. They may be considered as a good source of Vitamin C, whose content was best preserved for freeze-drying of fruits at an intermediary stage of ripening. © 2006 Elsevier B.V. All rights reserved. Keywords: Freezing; Vitamin C; Glass transition temperature; Water activity; Rehydration capacity
1. Introduction Acerola, also known as West Indian cherry, Barbados cherry or Cherry of Antilhas (Malpighia glabra L., Malpighia punicifolia L. or Malpighia emarginata DC.) is a fruit original from Antilhas which grows also in the northeast of South America and in Central America. It is round shaped, with diameter varying from 3 to 6 cm. Its fleshy and succulent pulp is encased by a very thin protection peel that quickly ripens. At the initial stages of ripening, the fruit is a full green color, changing to yellow-reddish and finally to red or purple when completely ripened [1]. Its main appealing feature is the high Vitamin C content, which might vary from 1247.10 to 1845.79 mg/100 g [2], but it is also rich in other nutrients such as carotenes, thiamin, riboflavin, niacin, proteins, and mineral salts, mainly iron, calcium and phosphorus [1]. Brazil is the major worldwide producer, consumer and exporter of acerola, with a production of 32,990 tonnes in 1996 [3]. Like other tropical fruits, acerola may be commercialized ‘in nature’ or may be used in the production of frozen pulps and processed juices. The typical weather characteristics that pre-
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dominate at northeast region from Brazil, however, with high relative humidity and temperatures, are not favorable to fruit preservation under natural conditions. Particularly for fruits with high moisture contents (in the case of acerola, the average value is about 91%w.b.), rapid deterioration is commonly observed after cropping. Aiming to offer alternatives for preservation of the fruits and of their original constituents, special attention has been directed to the development of adequate drying techniques. Besides aggregating commercial value to the fruits, drying reduces wastes and post-harvest losses, and might allow their commercialization for extended periods of time, with minor dependence on seasonal conditions. Freeze-drying is a technique that results in high-quality dehydrated products due to the absence of liquid water and the low temperatures required in the process. The solid state of water during freeze-drying protects the primary structure and minimizes changes in the shape of the product, with minimal reduction of volume [4]. In addition, it contributes to preserve constituents as minerals and vitamins, as well as to retain original flavor and aroma [5]. Freeze-drying appears, therefore, as a promising technique for dehydration of thermal-sensitive materials, such as acerola fruits. The final quality of dried fruits can be affected by the physical and structural changes that occur during drying, which may include deformation by shrinking. The true and apparent
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densities, as well as the porosity of the material define the rehydration capacity of a dried product. The drying process may alter these properties, resulting in products with modified texture, optical, thermal and nutritional properties [6]. In various foods, the solids are in an amorphous metastable state that is very sensitive to changes in temperature and moisture content. The change from the glassy to the rubbery state of the matrix occurs at the glass transition temperature (Tg ), which is specific for each material and affect chemical and physical changes during food processing and storage [7]. The activity coefficient expresses the state of water in a solution or a solid [8]. The water activity and the glass transition temperature stay among the parameters which are classically evaluated in the analysis of quality of dehydrated foods. The extent of shrinking and the glass transition temperature are interrelated in that significant changes in volume of the material can be noticed only if the process temperature exceeds Tg , at a particular moisture content. At temperatures higher than Tg the viscosity is considerably reduced, facilitating the product deformation [9]. Another parameter that can be used as a quality index of nutrients during food processing and storage is the ascorbic acid content, because it is an unstable constituent, sensitive to variations on pH, temperature, moisture content, oxygen and light. If the ascorbic acid is retained after processing, other nutrients are likely to be retained [10]. Lin et al. [10] performed a comparative study among vacuum microwave, hot air and freeze-dried carrot slices. They observed that freeze-dried carrots did not present significant losses in the content of Vitamin C, while the samples dried by hot air and vacuum microwave presented losses of 62 and 21%, respectively. Shadle et al. [11] also investigated the Vitamin C content of carrots after convective and freeze-drying, reporting losses of 81.3 and 60.8%, respectively in the dry samples. The rehydration ratio can be considered as a measure of the injuries caused by the processing and drying to the material [12]. It is generally accepted that the rehydration capacity is dependent on the degree of cellular and structural disruption. During drying, Jayaraman et al. [13] observed irreversible cellular rupture and dislocation, resulting in loss of integrity and hence, in a dense structure of collapsed, greatly shrunken capillaries with reduced hydrophilic properties, which are reflected by the inability to imbibe sufficient water to fully rehydrate. In spite of the importance of knowing quality parameters of freeze-dried products, detailed information on these aspects is still lacking in literature, particularly for tropical fruits. The purpose of this work is to determine some quality parameters, such as the Vitamin C content, the glass transition temperature, the water activity and the rehydration capacity for freeze-dried fruits and to investigate the effect of dehydration conditions on these properties.
The fruits, with diameters varying from 1.5 to 2.5 cm were placed in a circular tray of diameter equal to 125 mm and height of 15 mm and submitted to freezing. Three freezing techniques were evaluated: cryogenic freezing with liquid nitrogen, N2(l) ; with vapor of nitrogen, N2(v) , and by putting the samples in a freezer. The crushed pulps and sliced samples previously frozen were subsequently dehydrated in a laboratorial scale freeze-dryer, manufactured by Edwards, L4KR model. Temperatures at different axial positions of the samples were measured on line along the freezing using four K-type thermocouples, inserted at the positions 0, 5, 10 and 15 mm from the bottom of the tray. The freezing rate (θ) was calculated by Eq. (1), listed in Table 1. Freeze-drying tests were performed with vacuum chamber total pressure and temperature equal to 1.3 × 10−1 mbar and −30 ◦ C, respectively. The thermocouple probe located at the bottom of the tray was used to control and monitor the product temperature. The sublimation heat was supplied by a heating plate located under the tray. During the secondary stage of drying, the product reached a final temperature of about 35 ◦ C. Average freeze-drying time was approximately 12 h. The water activity at different moisture contents was obtained in a Thermoconstanter TH200 equipment, manufactured by Novasina, at a constant temperature of 30 ◦ C. As was pointed out by Kitic et al. [14] the time required to reach equilibrium conditions within the sensor chamber depend on several parameters. They include the physical nature of the sample, their water activity level, specific surface and the transport conditions in the free space above the sample. The glass transition temperature of freeze-dried acerola was determined by differential scanning calorimetry using a thermal analysis system DSC 204 (Netzsch). A sample containing 7.6 g of powdery sample was put on an aluminum pan and scanning was performed by heating at 10 ◦ C/min from −100 to 100 ◦ C, in inert atmosphere, with injection of 40 ml/min of N2 . The ascorbic acid contents in the “in nature” and freeze-dried samples were determined using the 2.6 dichlorofenol–indofenol titration [15]. The titration end point was detected visually and all analyses were conducted in triplicate. Ascorbic acid content was then determined by Eq. (2) in Table 1. The freeze-dried acerola fruits were rehydrated by immersing them in a water bath at ambient temperature. The samples were taken from the bath at different immersion times and weighted after being blotted with a tissue paper to remove the excess of water [12].
2. Materials and methods
θ=
The acerola fruits used in this investigation were purchased from a local market in the city of S˜ao Carlos-SP, Brazil.
Aa =
Table 1 Mathematical equations Properties 0 − Tf t (VT )(0.5/Vt )(V )(100) (Va )(m)
(1) (2)
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Fig. 1. Temperature as a function of time at different L. Fig. 2. Temperature as a function of time during freeze-drying.
The initial and final moisture content of the samples were determined by the oven method at (105 ± 3) ◦ C for 24 h. 3. Results and discussion 3.1. Freezing As an example of the temperature variation of the sample during the freezing, Fig. 1 depicts some cryogenic freezing curves for acerola pulps, obtained in freezing with N2(l) . It can be noted that the first position to freeze is the one located at L = 0 mm, because the heat is removed by the liquid nitrogen at the bottom of the tray. Later, the freezing front moves to the positions L = 5, 10 and 15 mm (the later one corresponds to the sample surface), until the whole sample is completely frozen. The freezing of the whole sample was completed in about 6.0 min. When freezing was performed using N2(v) or in freezer, the times for freezing the whole samples were considerably longer, respectively, of 20.0 min and 12.0 h. From Table 2 it can be observed that the freezing rate increases as the freezing front moves from L = 0 to 15 mm. The formation of a superficial layer composed by solutes which were not frozen is a possible reason for this behavior, as reported by Kochs et al. [16]. The eutectic state of this superficial layer might be responsible by the high freezing rate obtained at L = 15 mm. The freezing rates shown in Table 2 were calculated admitting a final freezing temperature equal to −100 ◦ C. The results in Table 2 indicate that the freezing rates for acerola pulps are quite high. As reported by Delgado and Rubiolo [17], the rapid freezing is advantageous since it contributes to maintain the cellular structure of the frozen pulps due to the formation of small-size ice crystals.
3.2. Freeze-drying Fig. 2 shows the variation of product temperature (measured at z ≈ 0 m) as a function of time. An inspection of Fig. 2 shows the presence of two drying stages, the primary one was finished at T = −20 ◦ C, while the secondary stage was finished as the temperature reached approximately 35 ◦ C. Heat was supplied to the freezing material by conduction through the heating plate, at a high enough rate to replace the heat consumed in the water sublimation occurring at the initial stages of freeze-drying. It can be seen in Fig. 2 that in the primary drying stage, the product remained at a constant temperature. When sublimation has virtually ceased, at the end of primary drying stage, the product temperature raises steeply, until reaching a stable final temperature. 3.3. Drying kinetics Fig. 3 represents the drying kinetic data for sliced and crushed acerola pulps. Different behaviors were observed during the sublimation, showing the influence of the sample type on the mass transfer.
Table 2 Freezing rates L (mm)
θ (◦ C/min)
0 5 10 15
23.0 27.8 41.0 73.6
± ± ± ±
0.5 0.5 0.5 0.5
Fig. 3. Moisture content as a function of time.
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The crushed acerola pulp contains a large amount of free water, but in spite of this, it presented lower drying rates. This result can be attributed to the cohesive forces acting in the crushed material. The pulp becomes more compact as the drying proceeds, thus increasing the resistance to mass transfer while the available area for mass exchange is decreased. In the sliced samples, the original structure of the material, with internal and external pores, is maintained during the drying. As a result, the resistance to mass transfer is reduced because the mass exchange occurs through the formerly ice-filled channels. In the secondary stage no effect of the type of sample on the drying rates was detected, because this drying stage is characterized by the removal of bound water. In a previous work, Marques and Freire [18] reported a predominance of the falling rate period in the drying curves of freeze-dried acerola. In their study, the influence of the freezing technique on freeze-drying was also investigated. They observed that slices of acerola freeze-dried after freezing in a freezer or using N2(v) presented higher drying rates than those frozen using N2(l) . According to Delgado and Rubiolo [17] and Delgado and Da-Wen [19], this behavior can be explained by the size and distribution of crystals of ice formed in the process, which depend on the freezing technique. In slow freezing processes, such as those obtained using freezer or N2(v) , the ice crystals formed are larger, facilitating the mass transfer. 3.4. Nutritional characteristics Chemical and physical characteristics of fresh fruit depend on the ripening stage, genotypes differences, soil, climatic conditions, post-harvest practices and handling procedure after harvesting. The ascorbic acid contents of ‘in nature’ and freezedried acerola fruits were determined at three ripening stages: initial (green fruits), intermediate (yellow-reddish fruits) and completely ripened (red fruits). In Table 3 are listed the results obtained for different conditions. A decrease in the Vitamin C content is observed as the ripening evolves. The values obtained agree with those reported in the literature [20]. At an intermediate stage of ripening, the loss of Vitamin C of acerola after freeze-drying was about 13%. However, for the green fruits, this loss was of 69.3%. This difference can be explained by the instability and sensibility of the ascorbic acid (Vitamin C) to moisture content, pH and metallic ions [21], which vary with the fruit maturation stage. The ascorbic acid is also expected to depend on the fruit size and even on the location of the fruit in a tree. Gongatti et al. [20] investigated the composition of acerola fruits according to their maturation stages. They verified that
dark green acerola fruits contains the larger contents of Vitamin C (1822 mg/100 g) and chlorophyll (4.41 mg/100 g), while the lower contents of Vitamin C (1021 mg/100 g) and chlorophyll (0.10 mg/100 g) were observed in the dark red fruits. Larger contents of carotenoids (1.44 mg/100 ml), soluble solids (7.10 ◦ Brix) and soluble sugar (5.05%) were also observed in the dark red fruits. For foods with moderate moisture contents, Lee and Labuza [22] observed that the rates of degradation of Vitamin C increase at high water activity, probably because the reactions occur more easily when the viscosity of aqueous phase is reduced. The final structure of freeze-dried acerola pulps, which were neither collapsed nor sticky, probably contributed for the preservation of Vitamin C in the product. Stickiness is a characteristic that may cause the over-heating of thermal-sensitive substances, resulting in degradation and undesirable sensorial characteristics [23]. The ascorbic acid content retained in the freeze-dried acerola pulps characterizes this product as a valuable source of Vitamin C. In Brazil, the Recommended Daily Intake (RDI) for adults is 60 mg [24]. The small losses of Vitamin C in freeze-dried products are attributed to the low temperatures and to the use of vacuum in the process. It must be pointed out that, due to the highly porous structure of freeze-dried products [25], an inadequate storage may cause oxidative reactions, leading to additional losses of Vitamin C in the final product. 3.5. Water activity and glass transition temperature Fig. 4 depicts the influence of the freezing technique on the variation of water activity during the freeze-drying of acerola fruits. According to Singh and Heldman [26], values of water activity among 0.20 and 0.40 ensures the stability of the product storage against browning and hydrolitical reactions, liquid oxidation, auto-oxidation and enzymatic activity. For samples frozen using N2(l) and N2(v) , the water activity, aw , was significantly reduced after 6 h of freeze-drying, to values equal to 0.22 and 0.19, respectively, which are in the recommended range for storage. For reaching a water activity of about 0.20 for the samples frozen in a freezer, additional time was required, about 8 h. This difference in the rate of decreasing of aw occurs because the different freezing processes result in products with distinct porous structures. Detailed analysis of the states of water and their effects on different physical food properties and quality parameters are presented by Lewicki [27], among other authors [17,19,28,29].
Table 3 Vitamin C content of raw and freeze-dried acerola Maturation stage
Green Yellow-reddish Red
Vitamin C content Raw (mg/100 g)
Raw (mg/g dry solid)
Freeze-dried (mg/g dry solid)
1617.0 ± 175.8 1587.3 ± 95.2 1341.13 ± 27.6
179.7 ± 19.5 176.4 ± 10.6 149 ± 3.1
55.1 ± 3.2 153.4 ± 39.1 72.1 ± 5.0
Loss (%)
Xw.b. (kg/kg)
69.3 13.0 51.6
0.36 0.54 0.07
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Fig. 4. Water activity as a function of time.
Fig. 5 shows typical moisture adsorption isotherms at 30 ◦ C of freeze-dried samples subjected to different freezing techniques. The shapes of sorption isotherms are characteristic of the highsugar fruit and biological materials [26,30,31], which show a sharp increase of sorption capacity at higher water activities and sorbs relatively small amounts of water at low water activities. It is also possible to note that the sorption isotherms are practically not affected by the freezing technique, a justified behavior since at equilibrium conditions the water activities were similar for the three types of freezing investigated. At low water activities, water can be adsorbed only to surface –OH sites of crystalline sugar. Therefore, moisture content is low in the region of low water activity. At high water activities, dissolution of sugar occurs and crystalline sugar is converted into amorphous sugar [8]. The amount of adsorbed water increases greatly after this transition due to the increase in the number of adsorption sites upon breakage of the crystalline structure of sugar [8]. Similar isotherms were determined for powders of freezedried banana [32], papaya [33] and pineapple [34], as well as to freeze-dried fruits such as strawberry [35], guava, mango and pineapple [31], and blueberries [36].
Fig. 5. Moisture content as a function of water activity.
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Fig. 6. Heat flow as a function of temperature.
Fig. 6 shows the range of glass transition temperature for freeze-dried acerola fruits, indicated by the region where the heat flow drops abruptly. The transition is located between an initial (Tg0 ) and a final temperature (Tge ). The adopted value of glass transition temperature is the mean value (Tgm ) and cp indicates the variation of specific heat in this region. The analysis was performed in duplicate, in order to check the reproducibility. Specific heat values of 0.667 and 0.666 J/g K were obtained for samples 1 and 2, respectively, confirming the good reproducibility of data. The powder obtained from acerola samples lyophilizated after freezing in N2(l) presented a (Tgm ) equals to −32.1 ◦ C at a moisture content of 0.25 kg/kg dry solid. It is worth to note that the low value of residual moisture in the powder is an effect of water plasticization, a behavior typically observed in solid foods with low water content. Bonelli et al. [37] investigated the effect of residual moisture content on the structural collapse of freeze-dried sugar matrix (lactose, maltose, sucrose, and trealose). The authors concluded that the residual moisture content promotes a notable change in Tg due to the high sensibility of this parameter to the water plasticization effect in the range of low moisture contents. Several authors determined the glass transition temperature of freeze-dried fruits, such as strawberry [38], apple [39], pineapple [34], persimmon [40] and strawberry, apple and pear [9]. The values obtained by these authors differ significantly, 188 ◦ C, for apple, −135 ◦ C, for strawberry and from −58.4 to −82.6 ◦ C for tomato. According to Khalloufi and Ratti [9] the marked difference between the values of Tg for the different fruits can be explained by the dependence of glass transition on their composition. The low value of Tg obtained for freeze-dried acerola fruits can be attributed to the presence of sorbitol in their chemical composition, a component with very low Tg , equals to −4 ◦ C [41]. Khalloufi and Ratti [9] determined Tg for freeze-dried strawberry, apple and pear, and verified that pear presented the lower value of Tg , probably because it contains the higher content of sorbitol, nearly 14% of the total carbohydrates. Some whole acerola fruits were freeze-dried aiming to investigate the shrinkage during the process. By estimating
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the volume variation, the shrinkage of acerola fruits during freeze-drying was evaluated as being approximately 17%. Ratti [4] verified that the shrinkage of strawberry, raspberry and blackberry was minimal after freeze-drying, 6.59, 6.13 and 16.1%, respectively. The shrinkage observed in the freeze-dried acerola fruits may be justified by relating Tg and the sublimation temperature. As shown in Fig. 2 the sublimation temperature is −20 ◦ C, whereas Tg is equal to −32.1 ◦ C. Since the sublimation temperature is higher than Tg at a moisture content of 0.25 kg/kg dry solid, the viscosity of water present in acerola is reduced, causing high mobility of water in the amorphous matrix and leading, consequently, to small shrinkage. This result agrees with predictions stating that materials with lower values of Tg experiences more intense shrinkage [9]. Shrinkage is also a function of the starch and sugars composition in the material [42]. An additional explanation for the low shrinkage observed in freeze-dried acerola fruits is given by the non-collapse of pores during the process [43], as evidenced by its high porosity, 0.93 [25]. Khalloufi and Ratti [9] verified that Tg increases with the freeze-drying temperatures. The dependence of Tg on freezedrying temperature was statistically significant (P < 0.05) for all the products investigated by them. Thus, it is expected that a small increase in the freeze-drying temperature of acerola results in a higher value of Tg . As a consequence, the transition from glassy to rubbery state will occur near to the final stages of drying, reducing the magnitude of acerola shrinkage. The non-collapse of freeze-dried acerola was easily detected by visual inspection. This is a good characteristic, since stickiness; compaction and crystallization are phenomena associated to the collapse of the material [44]. Ratti [4] observed that the percentage of collapsed samples during freeze-drying increased as the heating plate temperature was raised. At temperatures higher than 50 ◦ C this percentage exceeded 20%, a considerable reduction in the final product quality.
The investigations showed that in the freeze-drying of acerola, a fast previous freezing of the material contributes to preserve the original porous structure of the product and result in a powder material few susceptible to degradation reactions. From the techniques tested in this work, cryogenic freezing using N2(l) is the most recommended technique for freezing the samples. Freeze-dried acerola fruits present minimum shrinkage and the dried powder presented a low glass transition temperature, −32.1 ◦ C. The rehydration capacity of freeze-dried acerola was high, 10.1 kg/kg, because the samples did not suffer cellular rupture. Freeze-dried acerola can be considered a good source of Vitamin C, with a maximum content of 153.4 mg/g dry solid. Vitamin C content was best preserved in freeze-drying of fruits at the intermediary stage of ripening.
3.6. Rehydration capacity
Acknowledgements
Fig. 7 shows the rehydration capacity of freeze-dried acerola. Its elevated rehydration capacity is attributed to the high porosity, 0.93 [25], and the minimal shrinkage attained owing to the use of N2(l) to freeze the samples, as well as to the freeze-drying method itself. Both contribute to preserve the structural and cellular integrity of the acerola fruit. The equilibrium moisture content at saturation, 8.1 kg/kg dry solid, practically reached the moisture content of raw acerola, 10.1 kg/kg dry solid, indicating that the dehydration process is reversible, resulting in a product with large rehydration capacity and excellent color and flavor. Mastrocola et al. [45] verified that the reconstitution of freeze-dried fruit pieces involves, mainly, diffusive phenomena in the first phase of the process, because the product becomes impregnated with liquid and, when diluted solutions are used, high quantities of soluble solids are lost. When the cell wall becomes partially rehydrated, it acts as a semi-
The authors thank the Brazilian funding agencies FAPESP, Proc. 05/54624-9 and CNPq/PRONEX for their financial support.
Fig. 7. Redydration kinetics of acerola.
permeable membrane, being characterized as a ‘pseudo’ osmotic process. 4. Conclusions
Appendix A. Nomenclature
Aa L m t T V X
content Vitamin C (mg/100 g) position (mm) weight (kg) time interval (min) temperature (◦ C) volume (m3 ) moisture content (kg/kg)
Greek symbol θ freezing rate (◦ C)
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