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Influence of ripeness and air temperature on changes in banana texture during drying
Journal of Food Engineering 55 (2002) 115–121 www.elsevier.com/locate/jfoodeng
Influence of ripeness and air temperature on changes in banana texture during drying N. Boudhrioua a, C. Michon b, G. Cuvelier b, C. Bonazzi a
a,*
JRU for Food Process Engineering, Cemagref, ENSIA, INA PG, INRA, 1 Avenue des Olympiades, F-91744 Massy Cedex, France b Biophysics Laboratory, Food Science Department ENSIA, 1 Avenue des Olympiades, F-91744 Massy Cedex, France Received 16 March 2001; accepted 28 December 2001
Abstract This study aims to characterize how the rheological properties of slices of Cavendish Grande naine bananas change during the hot air drying process. Since the degree of ripeness of fresh banana considerably affects the rheological properties of the dried product, changes in fresh banana were first of all monitored during storage at room temperature and humidity. This study made it possible to determine the parameters which discriminated between degrees of fruit ripeness––moisture content, sugar content in the pulp, firmness of the banana with peel (S) and peel color (a ). Two non-linear regression correlations linked S and a to storage time. Glucose content was correlated with the values of S, a and moisture content in order to avoid this measurement very time consuming. Secondly, the changes in rheological properties were monitored by penetrometry throughout the drying experiments. This study showed that a radical change in the rheological behavior of the slices occurred depending on the fruit ripeness after 4, 6 or 8 h of drying at 80 C: they lost their deformability and became brittle. An analysis of the thermo-mechanical properties of the slices by dynamic mechanical thermal analysis showed that this abrupt change in the properties must be related to the product going below the glass transition temperature (Tg ) as it is cooled after drying. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Banana; Drying; Rheological properties; Glass transition; Ripeness
1. Introduction Several authors have studied how banana quality changes during ripening so as to prolong its longevity after harvesting and make it more marketable (Collin & Dalnic, 1991; Kojima, Naoki, Susumu, & Akira, 1994; Kojima, 1996; Meng, Slaughter, & Thompson, 1997; Prabha & Bhagyalak, 1998; Golding, Bhearer, McGlasson, & Wyllie, 1999). Drying makes it possible not only to stabilize the product by reducing its moisture content or water activity, but also to create new ranges of products. A large number of studies have been carried out to study how the process affects the quality of dried banana (Cosio, 1997 (freeze-drying); Waliszewski, Corzo, Parpio, & Garcia, 1999; Waliszewski, Cortes, Parpio, & Garcia, 1999 (osmotic dehydration); McDonald & Schaschke, 2000 (vacuum drying)). However,
few authors have taken an interest in monitoring how quality kinetics change during the process (Krokida, Tsami, & Maroulis, 1998; Krokida, Karathanos, & Maroulis, 2000) which would help to improve the understanding of the phenomenon and make it possible to develop a dynamic model of how the product changes. The aim of this study was to monitor how the rheological properties of banana slices changes during hot air drying. The first part of the study was devoted to characterizing the ripeness of fresh banana, which has a significant influence on textural properties of banana during drying (Cosio, 1997). After having selected the measurements discriminating between degrees of fruit ripeness, the rheological properties of the fruit were examined during drying. 2. Materials and methods 2.1. Raw material
*
Corresponding author. Tel.: +33-1-6993-5026; fax: +33-1-69935185. E-mail address: [email protected] (C. Bonazzi).
The bananas used in this study belong to the Musa acuminata species, Cavendish group, Grande naine
0260-8774/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 2 ) 0 0 0 2 5 - 0
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N. Boudhrioua et al. / Journal of Food Engineering 55 (2002) 115–121
Nomenclature B banana d day DM dry matter DMTA dynamic mechanical thermal analysis FB fresh banana h hour P position a redness or blueness b yellowness or greenness C glc concentration of glucose (g/100 g FB) C average concentration of sugar (g/100 g FB) dXpl =dt drying rate (kg water/kg DM s) e thickness (mm) E0 storage modulus (Pa) E00 loss modulus (Pa) F force measured versus displacement (N) Fp force at perforation (N)
variety, a high quality category from Martinique. Yellow–green bananas were purchased from a local market. Measurements were taken from the purchase date which was taken to be day d0. The variations in parameters (moisture content (Xpl ) and sugar content in the pulp, firmness (S) and color (a ) of the banana with peel) were monitored over a period of two weeks during ripening at room temperature and humidity. Three bananas were analyzed each day for color, firmness, sugars and moisture contents analysis. The moisture content of the pulp (Xpl ) was measured by gravimetry in samples of approximately 5 g before and after dehydration for 24 h in a ventilated oven at 105 C. It was expressed in kg water/kg DM. 2.2. Drying of banana slices The drying experiments were carried out in a pilot hot air drier which allows to carry out long-term kinetics under strictly defined and perfectly-controlled temperature (20 < Ta < 160 C), air velocity (0 < Va < 3m s1 ) and air humidity conditions (room humidity up to 0.3 kg H2 O/kg air s by injection of vapor). The drier works as an open-loop system and is controlled by a computer, with temperature, relative air velocity and relative air humidity as adjustable parameters. The weight and the surface temperature of the product were recorded over time. Moisture content of the pulp (Xpl ) versus time was deduced from product weight and initial moisture content. Drying rate (dXpl =dt) was calculated by deriving a sliding polynomial of second order fitted over five points (Abud Archila, Courtois, Bonazzi, & Bimbenet, 2000).
L S
lightness work applied during punching of banana (N mm) t time (s) T oven temperature (C) Ta air temperature (C) tanðdÞ loss angle (tanðdÞ ¼ E00 =E0 ) Tp product temperature (C) Tg glass transition temperature (C) Va air velocity (m s1 Þ Xpl moisture content of the pulp (kg water/kg DM) Subscripts exp experimental cal calculated
The monitoring of the internal (using a thermocouple in the center of a slice) and surface (using an infrared sensor) temperatures of the product showed that the difference between both temperatures was not significant. Product temperature was therefore estimated directly from the surface temperature. The drying experiments were carried out under preselected conditions and with thin layers, with banana slices (5 mm thick) distributed homogeneously over a mesh. The drying kinetics duration was set at 24 h. The experiments were carried out at a constant air velocity (2 m/s) without steam injection and at air temperatures of 40, 60 or 80 C. Each drying experiment was repeated three times. The influence of initial ripeness was studied for drying at 80 C, air temperature at which an abrupt of the rheological behavior of the product was observed. Banana slices were sampled at regular time intervals in order to analyze the rheological properties. 2.3. Variations in sugar contents of banana during storage Glucose, fructose and sucrose contents were measured using an enzymic kit (fructosidase, hexokinase, isomerase and glucose dehydrogenase), which transforms glucose, fructose and sucrose into D -gluconate6-phosphate with production of one NADPH. The NADPH formed in this reaction is stoichiometric to the amount of glucose and is measured by means of its light absorbance at a wavelength of k ¼ 340 nm (Spectrophotometer JASCO V-550 UV–VIS). Soluble sugars were separated in an aqueous solution by clarifying and precipitating macromolecules. The
N. Boudhrioua et al. / Journal of Food Engineering 55 (2002) 115–121 Table 1 Results of the variance analysis of the L , a , b parameters measured on banana peel Parameters
Factors
Level of significance
L
Day Position (on banana) Banana (in batch) Day Position (on banana) Banana (in batch) Day Position (on banana) Banana (in batch)
a
b
F––Fisher number. Three replications. p < 0:05, p < 0:01 and
Fexp =Fcal
p-value at 95%
1.69 2.06
0.11 0.1
1.98
0.04
731.36 0.135
<0.0001 0.94
0.76
0.65
43.28 42.22
<0.0001 <0.0001
6.2
<0.0001
p < 0:001.
extract was centrifuged and 0.1 ml of the filtrate was recovered for analysis. Enzyme solutions were added successively according to the procedure described in the enzymic kit manual. 1 Sugars contents were measured at the rate of one measurement per day during the first week of storage and one measurement every two days during the second week. Each analysis was repeated two or three times on different bananas of the set.
2.4. Variation in banana color during storage Color measurements were taken using a Minolta CR300 colorimeter. The surface analyzed by the apparatus was a disc of 8 mm diameter. A D65 lighting was used and the L , a , b space was chosen for results representation. The color measurements were taken on three bananas with peel at four positions, P1 (by the peduncle) to P4 (by the apex). The average of the 12 measurements of each color parameters (L , a , b ) was reported. An analysis of variance (ANOVA) was performed on the colorimetric measurements (L , a , b ) in order to discriminate the influence of the storage time, of the measurement position on the banana with peel and of the variability between bananas of the same batch (Table 1).
1 Enzymic kit glucose/fructose/sucrose, ref 720216, Boehringer Mannheim, France.
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Three batches of banana were used to this analysis. It shows that the change in a during storage was significant at 95%. Moreover, the variation in a measured on a banana is independent of the position and is greater than variation between bananas of a same batch. Parameter a was the most discriminant colorimetric parameter of fruit ripeness and was therefore the only one kept for characterizing fresh banana. 2.5. Assessment of rheological properties of banana 2.5.1. Monitoring of firmness change of banana with peel during storage In order to estimate the initial firmness of fresh banana and analyze how it changes during storage, penetrometry experiments were carried out using a rheological properties analyzer TAXT2i (XT RAD, Rheo), connected to a data acquisition system, fitted with a 25 kg force sensor and equipped with a needle probe moving at a rate of 2 mm/s. Penetration depth was set at 50% of the diameter of the fruit with peel. The work applied at perforation, S, is defined as the product of the force applied F (in N) by the displacement d (in mm) and was recorded as a function of time. Three S measurements were taken at the center of the banana with peel. Three bananas were analyzed and the mean value of the nine measurements was reported. 2.5.2. Analysis of rheological properties of banana slice during drying In order to estimate initial firmness and the way in which the firmness of banana slices changes during drying, penetrometry experiments were carried out using the TAXT2i equipped with a flat-ended probe (Ø ¼ 1 mm) which moves down vertically and crosses over the slice placed on the tray. The force (F ) measured during displacement (to 5 mm at least), at a rate of 0.1 mm/s was recorded. The maximum in the curve of force (F ) versus displacement is called Fp . The value of Fp was systematically recorded in order to assess how hard the slices were during drying. Three slices were sampled after 2, 4, 6 (or 8) and 24 h of drying and cooling to room temperature for 30 to 45 min in a desiccator. Each slice was then analyzed at three points at the center of each carpel, and the average value of the nine measurements of Fp was reported. 2.5.3. Measurement of the glass transition temperature by dynamic mechanical thermal analysis In order to determine the glass transition temperature of the product (Tg ) versus drying time, the DMTA was investigated using a DMTA MK IIIE (Rheometric Scientific). The principle of the method is based on applying to the sample an uniaxial force which is a sinusoidal
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Fig. 1. Variation in the storage modulus E0 , the loss modulus E00 and the loss angle tan(d) versus increasing temperature measured by DMTA: heating rate: 1 C/min, strain amplitude: 2 mm 15%, frequency: 0.01 to 200 rad/s.
function of time. The resulting strain is measured. In the linearity range, the strain signal is also a sinusoidal function of time. The tests were performed in tensile mode on almost parallelipedic samples prepared from banana slices dried for 2, 6, 8 or 24 h at 80 C. Samples were coated with Teflon grease in order to minimize dehydration. The storage modulus E0 , loss modulus E00 and loss factor tan(d) were measured at 1 rad/s when increasing temperature from 0 to 80 C (rate 1 C/min). The transition from glassy to rubbery state occurred over a rather wide temperature range, and was associated with a drop of at least two order of magnitude of E0 (from 2:9 109 to 1:71 106 Pa) and a peak of tan(d) (see Fig. 1). In order to compare samples properties, the criterion for determining the glass transition temperature (Tg ) used was the maximum of the peak of tan(d) (see Fig. 1).
Fig. 2. Variation in the chromatic parameter a and the firmness (S) of banana with peel measured on banana peel during storage.
ing at a value of 10 N mm from the eighth day of storage. S and a are rapid and simple to measure. The variations of these two parameters were described using empirical correlations versus the number of days elapsed from d0 (Eqs. (1) and (2)): S ¼ 42:5 expð0:09d 2 Þ þ 11:6 expð0:0003d 2 Þ
ð1Þ
2
ðR ¼ 0:94Þ a ¼ 11:4 expð0:46dÞ þ 3 expð0:16ðd 4ÞÞ
ð2Þ
2
ðR ¼ 0:87Þ Fig. 2 shows that the experimental points were well fitted to the empirical correlations, which were then validated for a batch of bananas purchased one month later. The deviation between the experimental measurements taken and the estimated values was calculated to be very small; it decreased over time in the case of S (from 1.4 on d2 to 0.36 on d8) whereas it increased in the case of a (from 0.2 on d2 to 2.3 on d8). Characterization of banana became less and less accurate after d8.
3. Results and discussion 3.1. Changes in fresh banana during storage 3.1.1. Monitoring and modeling changes in color and firmness The variations in a and S versus the time of storage were reported in Fig. 2. The parameter a increased rapidly from 10 3 (green) to þ5 2 (yellow) during the first eight days of storage after purchase. During the following days, it increased steadily at a constant rate up to a value of over 8 0:5 (yellow with black spots). Concerning firmness, Fig. 2 also shows the changes of the work applied during the punching of banana with peel, S, versus time of storage. S dropped rapidly in the first five days and then more slowly thereafter, stabiliz-
3.1.2. Variations in sugar contents Measurements presented in Table 2 show a significant variation in the sugar contents (glucose, fructose and sucrose) between the second and the thirteenth days of storage at 20 C. Values of the standard deviation of sugar contents analyzed in bananas of equal ripeness (yellow (d2) or yellow black (d13)) belonging to a same batch or to different batches of bananas were of the same order of magnitude (Table 2). The total sugar content measured on the date of purchase was of approximately 12 g/100 g FB (fresh banana). By monitoring the way in which sugar contents in the pulp of fresh banana changed during storage it was possible to see that the glucose (respectively fructose) contents increased from 1.6 to 4 g/100 g FB whereas the
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Table 2 Sugar concentrations in fresh banana on the second and thirteenth days of storage Sugars
Glucose Fructose Sucrose
Banana (d2)
Banana (d13)
C (g/100 g FB) (within a batch)
C (g/100 g FB) (different batches)
C (g/100 g FB) (within a batch)
C (g/100 g FB) (different batches)
1.7 0.15 1.64 0.03 10.0 1.2
1.77 0.08 1.65 0.01 10.60 0.6
4.81 0.02 4.7 0.3 5.7 0.9
4.80 0.01 4.6 0.2 5.4 0.4
C: Average concentration (two or three replications). Table 3 Correlations between glucose content and S, a and Xpl measurements Linear regressions
R2
Cglc exp Cglc cal (g/100 g FB)
Cglc ¼ 5:63 þ 0:1S Cglc ¼ 3:05 þ 0:27a Cglc ¼ 4:3 þ 2:5Xpl
0.88 0.8 0.72
From 0.02 to 0.5 From 0.02 to 0.7 From 0.03 to 0.7
Cglc exp : Experimental concentration of glucose. Cglc cal : Calculated concentration of glucose.
Fig. 3. Variation in glucose, fructose and sucrose concentrations in the pulp of banana during storage.
sucrose content decreased gradually from 9.4 to 7.4 g/100 g FB. Glucose and fructose increased almost linearly during the first five days of storage (R2 ¼ 0:98 for glucose; R2 ¼ 0:99 for fructose). These values then stabilized at values above 4.5 g/100 g FB (Fig. 3). The increase of sucrose content in the first three days of storage was explained by starch hydrolysis into small sugars like sucrose and maltose. Then sucrose decreased because of its hydrolysis into glucose and fructose. This is in agreement with results obtained by Collin and Dalnic (1991). The analysis of sugars in banana pulp can be limited to monitoring the glucose or fructose. We have correlated the variation in glucose content in the pulp and the variations of firmness (S), color (a ) and moisture content of the pulp (Xpl ). 3.1.3. Correlations between changes in glucose content and firmness, color and moisture content of the pulp Glucose and fructose contents changed linearly during the first week of storage. However, using the enzymic pathway to measure sugars contents was a costly and time-consuming process (>1 h). For this reason, change in glucose content was empirically correlated with the measurements of firmness (S), color (a ) of the peel and moisture content of the pulp (Xpl ). The relationships are presented in Table 3. The accuracy of glucose content determined on the basis of these relationships was estimated by the deviation from the experimental values. It varied from 0.02 g at d0 to 0.7 g at d12 (Table 3). The maximal deviation was of 0.7 g and was of the same
order of magnitude as the variability observed between three bananas from a same batch analyzed on the same day (Table 2). The accuracy of the glucose content estimation determined using this method was therefore deemed to be satisfactory for estimating the initial sugar content in banana before drying. 3.2. Changes in the rheological properties of banana slices during the drying process Drying rate (dXpl =dt) changed as a function of the moisture content of the product and showed that the higher the air temperature, the more rapidly the product dried and the shorter the processing time (Fig. 4). It was also observed that the more ripe banana would be dried faster, which has consequences on product quality changes during drying and particularly on rheological properties. 3.2.1. Influence of drying air temperature on rheological properties Fig. 5 represents the curves of the force (F ) measured versus displacement (d) recorded at different drying times at 80 C. The curves (F versus d) recorded by the texture analyzer at the beginning of drying at 80 C (variable time depending on the initial ripeness 2, 4 or 6 h) showed bell-shaped curves with a maximum corresponding to an Fp value which increased with drying time. Similar curves were obtained during drying at 40 and 60 C. But different curves with several peaks were observed after a certain time of drying at 80 C (4, 6 or 8 h depending on the initial ripeness). Such curves (presented for 8 h at 80 C on Fig. 5) brought to the fore a rupture in the rheological behavior of the product. The variation in the force applied at perforation (Fp ) for different drying times and different temperatures is
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Fig. 4. Variation in drying rate versus moisture content at 40, 60 and 80 C.
Fig. 6. Variation in the force applied until perforation (Fp ) versus drying time.
Fig. 5. Variation in the force applied (F ) during a compression test performed with a texture analyzer (TAXT2i) after different drying times at 80 C. Uniaxial displacement rate: 0.1 mm/s.
Fig. 7. Variation in the glass transition temperature (Tg ) and of the product temperature (Tp ) versus drying time at 80 C.
shown in Fig. 6. At 40 and 60 C, Fp increased linearly during drying. The resistance of the material to penetration, corresponding to a higher Fp , increased with air temperature (Fig. 6). At 80 C, Fp increased during the first few hours of drying, during which the product lost more than 90% of its moisture content. However, after 4, 6 or 8 h of drying at 80 C, depending on ripeness, the Fp value dropped and the shape of the curve recorded (F versus distance) underwent a radical change (Fig. 5). The slices became stiff and brittle and this behavior was no doubt linked to the fact that the slices exceeded the glass transition temperature (Tg ). In order to test this assumption, banana slices were put, after 24 h of drying at 80 C, in a waterproof sachet sealed under vacuum, which was placed for 20 min in a water bath at 80 C. Under these conditions, the product underwent an increase in temperature from ambient temperature to 80 C without drying. When the sachet was taken out of the water bath the slices were soft, and they soon hardened as they cooled down. This experiment qualitatively showed that the product underwent the glass transition
temperature between 80 C, which was the temperature in the drier, and room temperature ( 20 C). 3.2.2. Interpretation of the abrupt change in mechanical properties at 80 C In order to confirm the above result, we determined the glass transition temperature (Tg ), of the product versus time of drying at 80 C by DMTA. Values of Tg obtained for each drying time at 80 C were reported in Fig. 7. The Tg value increased rapidly from 30 to 35 C when banana was dried from 0 to 6 h at 80 C. These results confirm that, after 6 h of drying at 80 C, the banana slice was still in rubbery state. When it was cooled down at room temperature ( 20 C) it went through the glassy transition and became stiff and brittle. The moment at which this transition occurred depended on the initial ripeness of the banana as shown in Fig. 6. In fact, Fp dropped after 4 h of drying at 80 C in the case of yellow–black banana (d16) and after 6 h in the case of green banana (d2). The variation in Tg as a function of moisture content in the pulp seemed there-
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fore to depend on the ripeness of the fruit. The more ripe banana the richer it was in sugars and moisture. These two components were known to act as plasticizers and to reduce the Tg value.
4. Conclusions The study of the changes of fresh banana during storage at room temperature, facilitated the selection of properties that discriminated between degrees of banana ripeness: sugar contents, color parameter (a ), firmness (S) and moisture content in the pulp (Xpl ). By correlating the variation in glucose content with those in a , S and Xpl , the glucose content of fresh banana could be estimated on the basis of simple and rapid measurements and removed the need to measure it by biochemical assay. The study of the influence of drying on banana slices showed that drying kinetics, as great changes in rheological properties, depended mainly on the degree of fruit ripeness and on the air temperature during drying: • Drying occurred faster when the product was ripe. • After 4, 6 or 8 h of drying at 80 C, a split in the rheological behavior of the slices was observed which was related to an increase in the glass transition temperature (Tg ) of the product.
Acknowledgements We would like to thank Mr. Serges Thomas, director of the Bratigny Company of the RUNGIS market who guaranteed a regular supply of Cavendish bananas from Martinique to us during this study.
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References Abud Archila, M., Courtois, F., Bonazzi, C., & Bimbenet, J. J. (2000). A compartmental model of thin-layer drying kinetics of rough rice. Drying Technology, 18(7), 1389–1414. Collin, M. N., & Dalnic, R. (1991). Variation de quelques criteres physico-chimiques de la banane plantain (cultivar Orishele) au cours de la maturation. Fruits, 46(1), 13–17. Cosio, R. (1997). Lyophilisation sous vide de la banane (Cavendish Poyo). Incidence du procede sur la qualite (pp. 130–140). Ph.D. Thesis, Institut National Agronomique, Paris-Grignon. Golding, J., Bhearer, D., McGlasson, W. B., & Wyllie, S. G. (1999). Relationship between respiration, ethylene and aroma production in ripening banana. Journal of Agricultural Food and Chemistry, 47(4), 1646–1651. Kojima, K. (1996). Softening of banana fruit: relationship between firmness and chemical composition. Japan Agriculture Research Quarty, 30(4), 269–274. Kojima, K., Naoki, S., Susumu, K., & Akira, K. (1994). Changes in firmness and chemical constituents of banana fruits during ripening. Japan Journal of Tropical Agriculture, 38(4), 293–297. Krokida, M. K., Tsami, E., & Maroulis, Z. B. (1998). Kinetics on color changes during drying of some fruits and vegetables. Drying Technology, 16(3–5), 667–685. Krokida, M. K., Karathanos, V. T., & Maroulis, Z. B. (2000). Effect of osmotic dehydration on viscoelastic properties of apple and banana. Drying Technology, 18(4–5), 951–966. McDonald, L., & Schaschke, C. J. (2000). Combined effect of high pressure, temperature and holding time on polyphenoloxidase and peroxidase activity in banana (Musa acuminata). Journal of Science and Food Agriculture, 80, 719–724. Meng, L. I., Slaughter, D. C., & Thompson, J. E. (1997). Optical chlorophyll sensing system for banana ripening. Post HarvestBiological Technology, 12(3), 273–283. Prabha, T. N., & Bhagyalak, S. N. (1998). Carbohydrate metabolism in ripening banana fruit. Phytochemistry, 48(6), 915–919. Waliszewski, K. N., Corzo, C., Parpio, V. T., & Garcia, M. A. (1999). Effect of proteolytic enzymes on color changes in banana chips during osmotic dehydration. Drying Technology, 17(4–5), 947– 954. Waliszewski, K. N., Cortes, H. D., Parpio, V. T., & Garcia, M. A. (1999). Color parameter changes in banana slices during osmotic dehydration. Drying Technology, 17(4–5), 955–960.
Influence of ripeness and air temperature on changes in banana texture during drying N. Boudhrioua a, C. Michon b, G. Cuvelier b, C. Bonazzi a
a,*
JRU for Food Process Engineering, Cemagref, ENSIA, INA PG, INRA, 1 Avenue des Olympiades, F-91744 Massy Cedex, France b Biophysics Laboratory, Food Science Department ENSIA, 1 Avenue des Olympiades, F-91744 Massy Cedex, France Received 16 March 2001; accepted 28 December 2001
Abstract This study aims to characterize how the rheological properties of slices of Cavendish Grande naine bananas change during the hot air drying process. Since the degree of ripeness of fresh banana considerably affects the rheological properties of the dried product, changes in fresh banana were first of all monitored during storage at room temperature and humidity. This study made it possible to determine the parameters which discriminated between degrees of fruit ripeness––moisture content, sugar content in the pulp, firmness of the banana with peel (S) and peel color (a ). Two non-linear regression correlations linked S and a to storage time. Glucose content was correlated with the values of S, a and moisture content in order to avoid this measurement very time consuming. Secondly, the changes in rheological properties were monitored by penetrometry throughout the drying experiments. This study showed that a radical change in the rheological behavior of the slices occurred depending on the fruit ripeness after 4, 6 or 8 h of drying at 80 C: they lost their deformability and became brittle. An analysis of the thermo-mechanical properties of the slices by dynamic mechanical thermal analysis showed that this abrupt change in the properties must be related to the product going below the glass transition temperature (Tg ) as it is cooled after drying. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Banana; Drying; Rheological properties; Glass transition; Ripeness
1. Introduction Several authors have studied how banana quality changes during ripening so as to prolong its longevity after harvesting and make it more marketable (Collin & Dalnic, 1991; Kojima, Naoki, Susumu, & Akira, 1994; Kojima, 1996; Meng, Slaughter, & Thompson, 1997; Prabha & Bhagyalak, 1998; Golding, Bhearer, McGlasson, & Wyllie, 1999). Drying makes it possible not only to stabilize the product by reducing its moisture content or water activity, but also to create new ranges of products. A large number of studies have been carried out to study how the process affects the quality of dried banana (Cosio, 1997 (freeze-drying); Waliszewski, Corzo, Parpio, & Garcia, 1999; Waliszewski, Cortes, Parpio, & Garcia, 1999 (osmotic dehydration); McDonald & Schaschke, 2000 (vacuum drying)). However,
few authors have taken an interest in monitoring how quality kinetics change during the process (Krokida, Tsami, & Maroulis, 1998; Krokida, Karathanos, & Maroulis, 2000) which would help to improve the understanding of the phenomenon and make it possible to develop a dynamic model of how the product changes. The aim of this study was to monitor how the rheological properties of banana slices changes during hot air drying. The first part of the study was devoted to characterizing the ripeness of fresh banana, which has a significant influence on textural properties of banana during drying (Cosio, 1997). After having selected the measurements discriminating between degrees of fruit ripeness, the rheological properties of the fruit were examined during drying. 2. Materials and methods 2.1. Raw material
*
Corresponding author. Tel.: +33-1-6993-5026; fax: +33-1-69935185. E-mail address: [email protected] (C. Bonazzi).
The bananas used in this study belong to the Musa acuminata species, Cavendish group, Grande naine
0260-8774/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 2 ) 0 0 0 2 5 - 0
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N. Boudhrioua et al. / Journal of Food Engineering 55 (2002) 115–121
Nomenclature B banana d day DM dry matter DMTA dynamic mechanical thermal analysis FB fresh banana h hour P position a redness or blueness b yellowness or greenness C glc concentration of glucose (g/100 g FB) C average concentration of sugar (g/100 g FB) dXpl =dt drying rate (kg water/kg DM s) e thickness (mm) E0 storage modulus (Pa) E00 loss modulus (Pa) F force measured versus displacement (N) Fp force at perforation (N)
variety, a high quality category from Martinique. Yellow–green bananas were purchased from a local market. Measurements were taken from the purchase date which was taken to be day d0. The variations in parameters (moisture content (Xpl ) and sugar content in the pulp, firmness (S) and color (a ) of the banana with peel) were monitored over a period of two weeks during ripening at room temperature and humidity. Three bananas were analyzed each day for color, firmness, sugars and moisture contents analysis. The moisture content of the pulp (Xpl ) was measured by gravimetry in samples of approximately 5 g before and after dehydration for 24 h in a ventilated oven at 105 C. It was expressed in kg water/kg DM. 2.2. Drying of banana slices The drying experiments were carried out in a pilot hot air drier which allows to carry out long-term kinetics under strictly defined and perfectly-controlled temperature (20 < Ta < 160 C), air velocity (0 < Va < 3m s1 ) and air humidity conditions (room humidity up to 0.3 kg H2 O/kg air s by injection of vapor). The drier works as an open-loop system and is controlled by a computer, with temperature, relative air velocity and relative air humidity as adjustable parameters. The weight and the surface temperature of the product were recorded over time. Moisture content of the pulp (Xpl ) versus time was deduced from product weight and initial moisture content. Drying rate (dXpl =dt) was calculated by deriving a sliding polynomial of second order fitted over five points (Abud Archila, Courtois, Bonazzi, & Bimbenet, 2000).
L S
lightness work applied during punching of banana (N mm) t time (s) T oven temperature (C) Ta air temperature (C) tanðdÞ loss angle (tanðdÞ ¼ E00 =E0 ) Tp product temperature (C) Tg glass transition temperature (C) Va air velocity (m s1 Þ Xpl moisture content of the pulp (kg water/kg DM) Subscripts exp experimental cal calculated
The monitoring of the internal (using a thermocouple in the center of a slice) and surface (using an infrared sensor) temperatures of the product showed that the difference between both temperatures was not significant. Product temperature was therefore estimated directly from the surface temperature. The drying experiments were carried out under preselected conditions and with thin layers, with banana slices (5 mm thick) distributed homogeneously over a mesh. The drying kinetics duration was set at 24 h. The experiments were carried out at a constant air velocity (2 m/s) without steam injection and at air temperatures of 40, 60 or 80 C. Each drying experiment was repeated three times. The influence of initial ripeness was studied for drying at 80 C, air temperature at which an abrupt of the rheological behavior of the product was observed. Banana slices were sampled at regular time intervals in order to analyze the rheological properties. 2.3. Variations in sugar contents of banana during storage Glucose, fructose and sucrose contents were measured using an enzymic kit (fructosidase, hexokinase, isomerase and glucose dehydrogenase), which transforms glucose, fructose and sucrose into D -gluconate6-phosphate with production of one NADPH. The NADPH formed in this reaction is stoichiometric to the amount of glucose and is measured by means of its light absorbance at a wavelength of k ¼ 340 nm (Spectrophotometer JASCO V-550 UV–VIS). Soluble sugars were separated in an aqueous solution by clarifying and precipitating macromolecules. The
N. Boudhrioua et al. / Journal of Food Engineering 55 (2002) 115–121 Table 1 Results of the variance analysis of the L , a , b parameters measured on banana peel Parameters
Factors
Level of significance
L
Day Position (on banana) Banana (in batch) Day Position (on banana) Banana (in batch) Day Position (on banana) Banana (in batch)
a
b
F––Fisher number. Three replications. p < 0:05, p < 0:01 and
Fexp =Fcal
p-value at 95%
1.69 2.06
0.11 0.1
1.98
0.04
731.36 0.135
<0.0001 0.94
0.76
0.65
43.28 42.22
<0.0001 <0.0001
6.2
<0.0001
p < 0:001.
extract was centrifuged and 0.1 ml of the filtrate was recovered for analysis. Enzyme solutions were added successively according to the procedure described in the enzymic kit manual. 1 Sugars contents were measured at the rate of one measurement per day during the first week of storage and one measurement every two days during the second week. Each analysis was repeated two or three times on different bananas of the set.
2.4. Variation in banana color during storage Color measurements were taken using a Minolta CR300 colorimeter. The surface analyzed by the apparatus was a disc of 8 mm diameter. A D65 lighting was used and the L , a , b space was chosen for results representation. The color measurements were taken on three bananas with peel at four positions, P1 (by the peduncle) to P4 (by the apex). The average of the 12 measurements of each color parameters (L , a , b ) was reported. An analysis of variance (ANOVA) was performed on the colorimetric measurements (L , a , b ) in order to discriminate the influence of the storage time, of the measurement position on the banana with peel and of the variability between bananas of the same batch (Table 1).
1 Enzymic kit glucose/fructose/sucrose, ref 720216, Boehringer Mannheim, France.
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Three batches of banana were used to this analysis. It shows that the change in a during storage was significant at 95%. Moreover, the variation in a measured on a banana is independent of the position and is greater than variation between bananas of a same batch. Parameter a was the most discriminant colorimetric parameter of fruit ripeness and was therefore the only one kept for characterizing fresh banana. 2.5. Assessment of rheological properties of banana 2.5.1. Monitoring of firmness change of banana with peel during storage In order to estimate the initial firmness of fresh banana and analyze how it changes during storage, penetrometry experiments were carried out using a rheological properties analyzer TAXT2i (XT RAD, Rheo), connected to a data acquisition system, fitted with a 25 kg force sensor and equipped with a needle probe moving at a rate of 2 mm/s. Penetration depth was set at 50% of the diameter of the fruit with peel. The work applied at perforation, S, is defined as the product of the force applied F (in N) by the displacement d (in mm) and was recorded as a function of time. Three S measurements were taken at the center of the banana with peel. Three bananas were analyzed and the mean value of the nine measurements was reported. 2.5.2. Analysis of rheological properties of banana slice during drying In order to estimate initial firmness and the way in which the firmness of banana slices changes during drying, penetrometry experiments were carried out using the TAXT2i equipped with a flat-ended probe (Ø ¼ 1 mm) which moves down vertically and crosses over the slice placed on the tray. The force (F ) measured during displacement (to 5 mm at least), at a rate of 0.1 mm/s was recorded. The maximum in the curve of force (F ) versus displacement is called Fp . The value of Fp was systematically recorded in order to assess how hard the slices were during drying. Three slices were sampled after 2, 4, 6 (or 8) and 24 h of drying and cooling to room temperature for 30 to 45 min in a desiccator. Each slice was then analyzed at three points at the center of each carpel, and the average value of the nine measurements of Fp was reported. 2.5.3. Measurement of the glass transition temperature by dynamic mechanical thermal analysis In order to determine the glass transition temperature of the product (Tg ) versus drying time, the DMTA was investigated using a DMTA MK IIIE (Rheometric Scientific). The principle of the method is based on applying to the sample an uniaxial force which is a sinusoidal
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Fig. 1. Variation in the storage modulus E0 , the loss modulus E00 and the loss angle tan(d) versus increasing temperature measured by DMTA: heating rate: 1 C/min, strain amplitude: 2 mm 15%, frequency: 0.01 to 200 rad/s.
function of time. The resulting strain is measured. In the linearity range, the strain signal is also a sinusoidal function of time. The tests were performed in tensile mode on almost parallelipedic samples prepared from banana slices dried for 2, 6, 8 or 24 h at 80 C. Samples were coated with Teflon grease in order to minimize dehydration. The storage modulus E0 , loss modulus E00 and loss factor tan(d) were measured at 1 rad/s when increasing temperature from 0 to 80 C (rate 1 C/min). The transition from glassy to rubbery state occurred over a rather wide temperature range, and was associated with a drop of at least two order of magnitude of E0 (from 2:9 109 to 1:71 106 Pa) and a peak of tan(d) (see Fig. 1). In order to compare samples properties, the criterion for determining the glass transition temperature (Tg ) used was the maximum of the peak of tan(d) (see Fig. 1).
Fig. 2. Variation in the chromatic parameter a and the firmness (S) of banana with peel measured on banana peel during storage.
ing at a value of 10 N mm from the eighth day of storage. S and a are rapid and simple to measure. The variations of these two parameters were described using empirical correlations versus the number of days elapsed from d0 (Eqs. (1) and (2)): S ¼ 42:5 expð0:09d 2 Þ þ 11:6 expð0:0003d 2 Þ
ð1Þ
2
ðR ¼ 0:94Þ a ¼ 11:4 expð0:46dÞ þ 3 expð0:16ðd 4ÞÞ
ð2Þ
2
ðR ¼ 0:87Þ Fig. 2 shows that the experimental points were well fitted to the empirical correlations, which were then validated for a batch of bananas purchased one month later. The deviation between the experimental measurements taken and the estimated values was calculated to be very small; it decreased over time in the case of S (from 1.4 on d2 to 0.36 on d8) whereas it increased in the case of a (from 0.2 on d2 to 2.3 on d8). Characterization of banana became less and less accurate after d8.
3. Results and discussion 3.1. Changes in fresh banana during storage 3.1.1. Monitoring and modeling changes in color and firmness The variations in a and S versus the time of storage were reported in Fig. 2. The parameter a increased rapidly from 10 3 (green) to þ5 2 (yellow) during the first eight days of storage after purchase. During the following days, it increased steadily at a constant rate up to a value of over 8 0:5 (yellow with black spots). Concerning firmness, Fig. 2 also shows the changes of the work applied during the punching of banana with peel, S, versus time of storage. S dropped rapidly in the first five days and then more slowly thereafter, stabiliz-
3.1.2. Variations in sugar contents Measurements presented in Table 2 show a significant variation in the sugar contents (glucose, fructose and sucrose) between the second and the thirteenth days of storage at 20 C. Values of the standard deviation of sugar contents analyzed in bananas of equal ripeness (yellow (d2) or yellow black (d13)) belonging to a same batch or to different batches of bananas were of the same order of magnitude (Table 2). The total sugar content measured on the date of purchase was of approximately 12 g/100 g FB (fresh banana). By monitoring the way in which sugar contents in the pulp of fresh banana changed during storage it was possible to see that the glucose (respectively fructose) contents increased from 1.6 to 4 g/100 g FB whereas the
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Table 2 Sugar concentrations in fresh banana on the second and thirteenth days of storage Sugars
Glucose Fructose Sucrose
Banana (d2)
Banana (d13)
C (g/100 g FB) (within a batch)
C (g/100 g FB) (different batches)
C (g/100 g FB) (within a batch)
C (g/100 g FB) (different batches)
1.7 0.15 1.64 0.03 10.0 1.2
1.77 0.08 1.65 0.01 10.60 0.6
4.81 0.02 4.7 0.3 5.7 0.9
4.80 0.01 4.6 0.2 5.4 0.4
C: Average concentration (two or three replications). Table 3 Correlations between glucose content and S, a and Xpl measurements Linear regressions
R2
Cglc exp Cglc cal (g/100 g FB)
Cglc ¼ 5:63 þ 0:1S Cglc ¼ 3:05 þ 0:27a Cglc ¼ 4:3 þ 2:5Xpl
0.88 0.8 0.72
From 0.02 to 0.5 From 0.02 to 0.7 From 0.03 to 0.7
Cglc exp : Experimental concentration of glucose. Cglc cal : Calculated concentration of glucose.
Fig. 3. Variation in glucose, fructose and sucrose concentrations in the pulp of banana during storage.
sucrose content decreased gradually from 9.4 to 7.4 g/100 g FB. Glucose and fructose increased almost linearly during the first five days of storage (R2 ¼ 0:98 for glucose; R2 ¼ 0:99 for fructose). These values then stabilized at values above 4.5 g/100 g FB (Fig. 3). The increase of sucrose content in the first three days of storage was explained by starch hydrolysis into small sugars like sucrose and maltose. Then sucrose decreased because of its hydrolysis into glucose and fructose. This is in agreement with results obtained by Collin and Dalnic (1991). The analysis of sugars in banana pulp can be limited to monitoring the glucose or fructose. We have correlated the variation in glucose content in the pulp and the variations of firmness (S), color (a ) and moisture content of the pulp (Xpl ). 3.1.3. Correlations between changes in glucose content and firmness, color and moisture content of the pulp Glucose and fructose contents changed linearly during the first week of storage. However, using the enzymic pathway to measure sugars contents was a costly and time-consuming process (>1 h). For this reason, change in glucose content was empirically correlated with the measurements of firmness (S), color (a ) of the peel and moisture content of the pulp (Xpl ). The relationships are presented in Table 3. The accuracy of glucose content determined on the basis of these relationships was estimated by the deviation from the experimental values. It varied from 0.02 g at d0 to 0.7 g at d12 (Table 3). The maximal deviation was of 0.7 g and was of the same
order of magnitude as the variability observed between three bananas from a same batch analyzed on the same day (Table 2). The accuracy of the glucose content estimation determined using this method was therefore deemed to be satisfactory for estimating the initial sugar content in banana before drying. 3.2. Changes in the rheological properties of banana slices during the drying process Drying rate (dXpl =dt) changed as a function of the moisture content of the product and showed that the higher the air temperature, the more rapidly the product dried and the shorter the processing time (Fig. 4). It was also observed that the more ripe banana would be dried faster, which has consequences on product quality changes during drying and particularly on rheological properties. 3.2.1. Influence of drying air temperature on rheological properties Fig. 5 represents the curves of the force (F ) measured versus displacement (d) recorded at different drying times at 80 C. The curves (F versus d) recorded by the texture analyzer at the beginning of drying at 80 C (variable time depending on the initial ripeness 2, 4 or 6 h) showed bell-shaped curves with a maximum corresponding to an Fp value which increased with drying time. Similar curves were obtained during drying at 40 and 60 C. But different curves with several peaks were observed after a certain time of drying at 80 C (4, 6 or 8 h depending on the initial ripeness). Such curves (presented for 8 h at 80 C on Fig. 5) brought to the fore a rupture in the rheological behavior of the product. The variation in the force applied at perforation (Fp ) for different drying times and different temperatures is
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Fig. 4. Variation in drying rate versus moisture content at 40, 60 and 80 C.
Fig. 6. Variation in the force applied until perforation (Fp ) versus drying time.
Fig. 5. Variation in the force applied (F ) during a compression test performed with a texture analyzer (TAXT2i) after different drying times at 80 C. Uniaxial displacement rate: 0.1 mm/s.
Fig. 7. Variation in the glass transition temperature (Tg ) and of the product temperature (Tp ) versus drying time at 80 C.
shown in Fig. 6. At 40 and 60 C, Fp increased linearly during drying. The resistance of the material to penetration, corresponding to a higher Fp , increased with air temperature (Fig. 6). At 80 C, Fp increased during the first few hours of drying, during which the product lost more than 90% of its moisture content. However, after 4, 6 or 8 h of drying at 80 C, depending on ripeness, the Fp value dropped and the shape of the curve recorded (F versus distance) underwent a radical change (Fig. 5). The slices became stiff and brittle and this behavior was no doubt linked to the fact that the slices exceeded the glass transition temperature (Tg ). In order to test this assumption, banana slices were put, after 24 h of drying at 80 C, in a waterproof sachet sealed under vacuum, which was placed for 20 min in a water bath at 80 C. Under these conditions, the product underwent an increase in temperature from ambient temperature to 80 C without drying. When the sachet was taken out of the water bath the slices were soft, and they soon hardened as they cooled down. This experiment qualitatively showed that the product underwent the glass transition
temperature between 80 C, which was the temperature in the drier, and room temperature ( 20 C). 3.2.2. Interpretation of the abrupt change in mechanical properties at 80 C In order to confirm the above result, we determined the glass transition temperature (Tg ), of the product versus time of drying at 80 C by DMTA. Values of Tg obtained for each drying time at 80 C were reported in Fig. 7. The Tg value increased rapidly from 30 to 35 C when banana was dried from 0 to 6 h at 80 C. These results confirm that, after 6 h of drying at 80 C, the banana slice was still in rubbery state. When it was cooled down at room temperature ( 20 C) it went through the glassy transition and became stiff and brittle. The moment at which this transition occurred depended on the initial ripeness of the banana as shown in Fig. 6. In fact, Fp dropped after 4 h of drying at 80 C in the case of yellow–black banana (d16) and after 6 h in the case of green banana (d2). The variation in Tg as a function of moisture content in the pulp seemed there-
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fore to depend on the ripeness of the fruit. The more ripe banana the richer it was in sugars and moisture. These two components were known to act as plasticizers and to reduce the Tg value.
4. Conclusions The study of the changes of fresh banana during storage at room temperature, facilitated the selection of properties that discriminated between degrees of banana ripeness: sugar contents, color parameter (a ), firmness (S) and moisture content in the pulp (Xpl ). By correlating the variation in glucose content with those in a , S and Xpl , the glucose content of fresh banana could be estimated on the basis of simple and rapid measurements and removed the need to measure it by biochemical assay. The study of the influence of drying on banana slices showed that drying kinetics, as great changes in rheological properties, depended mainly on the degree of fruit ripeness and on the air temperature during drying: • Drying occurred faster when the product was ripe. • After 4, 6 or 8 h of drying at 80 C, a split in the rheological behavior of the slices was observed which was related to an increase in the glass transition temperature (Tg ) of the product.
Acknowledgements We would like to thank Mr. Serges Thomas, director of the Bratigny Company of the RUNGIS market who guaranteed a regular supply of Cavendish bananas from Martinique to us during this study.
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