EFFECT OF AIR TEMPERATURE AND AIR VELOCITY ON THE TIME

TECHNICAL BULLETIN 221

ISSN 0070-2315

EFFECT OF AIR TEMPERATURE AND AIR VELOCITY ON THE TIME REQUIRED FOR MECHANICAL DRYING OF TOMATOES

M.C. Kyriacou, P. Polycarpou and C. Gregoriou

AGRICULTURAL RESEARCH INSTITUTE MINISTRY OF AGRICULTURE, NATURAL RESOURCES AND THE ENVIRONMENT LEFKOSIA

CYPRUS

NOVEMBER 2004

Editor - in Chief

Dr C. Papachristoforou, Agricultural Research Institute, Lefkosia, Cyprus.

All responsibility for the information in this publication remains with the author(s). The use of trade names does not imply endorsement of or discrimination against any product by the Agricultural Research Institute. 2

EFFECT OF AIR TEMPERATURE AND AIR VELOCITY ON THE TIME REQUIRED FOR MECHANICAL DRYING OF TOMATOES M.C. Kyriacou, P. Polycarpou and C. Gregoriou

SUMMARY

The time periods required for mechanical drying of tomatoes (cv. FA179 Brilante, Hazera) under airflow temperatures of 50, 55, 60, 65 and 70 oC and airflow velocities of 1.0, 1.5 and 2.0 m/s were determined with the use of a mechanical hot airflow cabinet. Model curves for the dehydration of tomato halves were established and a sorption isotherm relating water activity (Aw) at 20 oC to the product’s moisture content was produced. Moisture content at Aw 0.650 was 29.6% and was referred to as the termination point for the dehydration process. Time periods required for completing the process of dehydration for fresh tomatoes were accordingly deduced under all the above-mentioned airflow temperature and velocity regimes. Drying times ranged from 83.3 hrs at 1.0 m/s and 50 oC to 24.8 hrs at 2.0 m/s and 70 oC. Drying time decreased with increasing airflow velocity and temperature, yet the effect of velocity decreased with increasing temperature. At 70 oC differences in drying time between velocities 1.0, 1.5 and 2.0 m/s were marginal. At 70 oC non-enzymatic browning impaired the quality of the final product. ΠEPIΛHΨH Οι χρονικές περίοδοι που απαιτούνται για τη ξήρανση νωπής τοµάτας (cv. FA179 Brilante, Hazera) µε ροή αέρα σε θερµοκρασίες 50, 55, 60, 65 και 70 oC και ταχύτητες 1.0, 1.5 και 2.0 m/s, καθορίστηκαν µε χρήση µηχανικού ξηραντηρίου µε κλειστό κύκλωµα ροής. Παρήχθησαν πρότυπες καµπύλες ξήρανσης για την τοµάτα και καθορίστηκε η ισόθερµη καµπύλη συσχέτισης της περιεκτικότητας υγρασίας και της ενεργότητας υγρασίας (Aw) του προϊόντος στους 20 oC. Σε ενεργότητα υγρασίας Aw=0.650 το ποσοστό υγρασίας του προϊόντος ήταν 29.6% και εκλήφθηκε ως το σηµείο τερµατισµού της ξήρανσης. Ακολούθως καθορίστηκε η απαιτούµενη χρονική διάρκεια της ξήρανσης για νωπή τοµάτα κάτω από όλους τους πιο πάνω συνδυασµούς θερµοκρασίας και ταχύτητας ροής αέρα. Οι χρόνοι ξήρανσης κυµάνθηκαν από 83.3 ώρες στο 1.0 m/s και 50 oC µέχρι 24.8 ώρες στα 2.0 m/s και 70 oC. Ο χρόνος ξήρανσης µειώθηκε µε την αύξηση της ταχύτητας και θερµοκρασίας ροής του ξηραντηρίου. Εντούτοις η επίδραση της ταχύτητας στο χρόνο ξήρανσης µειώθηκε µε την αύξηση της θερµοκρασίας. Στους 70 oC η διαφορά στο χρόνο ξήρανσης µεταξύ των ταχυτήτων 1.0, 1.5 και 2.0 m/s ήταν περιορισµένη αλλά η ποιότητα του προϊόντος επηρεάστηκε αρνητικά από µη ενζυµατικό αποχρωµατισµό.

INTRODUCTION

Tomato is an important crop in Cyprus with a total annual production of about 40 000 tons (Agricultural Statistics, 2001) often marked by significant surplus. Dehydration is potentially an important value-adding method for processing various fresh fruits and vegetables, including tomatoes, destined for long-term storage. Such processing may take place either by direct exposure to solar radiation or by means of mechanical driers. The former method compares favourably to the latter in terms of energy input require-

ment but is prone to quality loss due to photo-oxidation and uncontrolled non-enzymatic reactions (Zanoni et al., 1999). The main advantages of the latter method are time efficiency and finer control of the end product’s quality through accurate monitoring and regulation of the main parameters of the drying process, namely temperature and velocity of airflow (Akritides, 1993). Storage temperature, moisture content and water activity of processed fruits and vegetables are the main parameters affecting the physical, biochemical and microbiological changes taking place during their storage 3

(Lahsasni et al., 2002). Water activity is a quotient denoted by the ratio of vapour pressure at the solid-gas interface of a product to the vapour pressure of liquid pure water, at the same temperature. It is a determinant of the growth of microorganisms and of the rate of degradation reactions of chemical, enzymatic and physical nature (Maltini et al., 2003). The purpose of this work was to generate model data useful in the future design of an industrial-scale mechanical drier. Accordingly, the main objectives were to determine the time periods required for drying fresh tomatoes under a range of airflow temperatures (50-70 oC) and velocities (1.02.0 m/s) and under the above regimes, to establish model curves for the dehydration process. A sorption isotherm relating the water activity (Aw) of the final product at 20 oC to its moisture content was produced in order to define the termination point of the dehydration process in terms of the product’s moisture content. In turn, the time periods required for completing the process of dehydration for fresh tomatoes were deduced under all the above-mentioned airflow temperature and velocity regimes. MATERIALS AND METHODS

For the drying tests, fully ripe (U.S.D.A. Visual Aid TM-L-1, Red) fresh tomatoes of FA179 Brilante (Hazera) cultivar were selected for size (6.0-7.0 cm in diameter) and absence of defects. They were cut vertically into halves, parenchyma and seeds left intact. Tomato halves were dried in a pilot-plant mechanical drier designed by EKEFE Democretos (Athens, Greece) and built by Airtechnic Hatzoudis (Athens, Greece). Fourteen tomato halves were placed in single layers on each of three perforated stainless steel trays positioned vertically to the drying cabinet’s airflow. The other complementary 14 tomato halves were desiccated in a Sanyo OMT Gallenkamp Oven for 24 hours at 110 oC in order to determine dry matter and initial moisture content of the fruits. Airflow was recycled inside the cabinet along a top-tobottom direction in relation to the trays’ positioning. Relative humidity inside the cabinet remained constant at about 15%. Total tomato weight of each tray was monitored and 4

recorded on an A&D GX-4000 digital balance during the drying process. Drying was carried out at airflow temperatures of 50, 55, 60, 65 and 70 oC and airflow rates of 1.0, 1.5 and 2.0 m/s. Tomatoes were dried to about 17% final moisture content. A sorption isotherm at 20 oC for the dried tomatoes was produced by measuring the water activity (Aw) of fruits at various stages of dehydration. To that effect fruits were allowed to equilibrate in the sensor chamber of a Rotronic Hygrolab2 water activity meter with a Hygroclip HK25 relative humidity sensor and a MOK-02-B5 cable. Equilibration time ranged from 45 to 70 minutes. The sensor chamber was connected via a flowing circuit to a water bath at 20 oC. At the end of each Aw measurement, the sample was weighed and desiccated in an oven as described above in order to determine its moisture content. Moisture content corresponding to Aw 0.650 was used to define the termination of the drying process. It is widely established that reducing Aw down to 0.600-0.650 constitutes an effective control against the growth of osmophilic microorganisms by limiting water available to support their growth (Maltini et al., 2003). The time period required for the dehydration of tomatoes down to Aw 0.650 was deduced from the dehydration curves established, after converting moisture content at Aw 0.650 to relative weight. RESULTS AND DISCUSSION

The dehydration of tomatoes of the FA179 Brilante (Hazera) cultivar is expressed in Figures 1, 2 and 3 as relative weight over drying time at constant air velocities of 1.0, 1.5 and 2.0 m/s, respectively. In each case, the drying process is depicted at airflow temperatures of 50, 55, 60, 65 and 70 oC. Airflow temperature was limited to a maximum of 70 oC in the course of experimentation, in view of the undesirable non-enzymatic browning (Maillard reaction) and oxidative heat damage manifested at that temperature (Zanoni et al., 1999). Therefore, in order to obtain a dried product of acceptable quality, airflow temperature inside the drying cabinet must be maintained below 70 oC. It is apparent from Figures 1, 2 and 3 that the process of dehydration is accelerated as air-

Figure 1. Effect of temperature on tomato dehydration at airflow velocity of 1.0 m/s.



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flow temperature and velocity are increased. The time period required for satisfactory dehydration was quantified at each velocitytemperature combination in order to compare the effects of the two parameters on the dehydration process. The termination point for the dehydration process is defined as the moisture content (MCf) that will deter the product’s spoilage during long-term storage at ambient temperature (20 oC). Water activity Aw is a measure of the product’s moisture available to microbial growth, hence a measure of its susceptibility to spoilage (Maltini et al., 2003). At Aw levels below 0.650 the

potential for microbial growth is limited. The isothermal relation between water activity and moisture content at 20 oC was defined as shown in Figure 4. The product’s final moisture content (MCf) at Aw 0.650 was 29.6% and the product’s final dry matter content (DMf) was 70.4%. The initial moisture content (MCi) and initial dry matter content (DMi) of the product, defined at the start of each drying period, were 94.2% and 5.2%, respectively. Given that dry matter remains constant throughout the dehydration process, the product’s final relative weight as derived from Equations 1 to 3 is 0.074.

Figure 4. Tomato moisture content isotherm at 20 oC.

Figure 5. Effect of airflow temperature and velocity on drying time.

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Equation 1 Equation 2 Equation 3

DMi=DMf 0.052 x Win = 0.704 x Wf RWf = Wf / Win = 0.074

The effect of airflow temperature and velocity on total drying time, determined at RWf 0.074, is shown in Figure 5. In agreement to the findings of Park et al. (2002) related to mint leaves, both airflow temperature and velocity appear to affect total drying time negatively. In the case of tomato halves, the effect of airflow velocity on the total drying time decreases as airflow temperature increases. At 50 oC drying times between 1.0 and 2.0 m/s differ by 25 hours while at 70 oC they differ by only 1.5 hours. It is therefore apparent that when mechanical drying of tomatoes is performed at temperatures as high as 65 oC the time required to achieve an adequately dehydrated product (Aw 0.650) relies much more on the drying temperature than on the velocity of airflow. This is perhaps due to the lower surface to volume ratio of tomato halves and the higher energy demand in order to transfer moisture from the centre of the product to the surface. ACKNOWLEDGEMENTS

The authors wish to thank Dr I.M. Ioannides for facilitating part of the above work in the Molecular Biology Laboratory of the Agricultural Research Institute and Mr. I. Photiades for his technical assistance.

REFERENCE

Agricultural Statistics. 2001. Agricultural Statistics: Series 2 Report No.33. Department of Statistics and Research, Minisrty of Finance of the Government of Cyprus.

Akritides, K.B. 1993. Drying and Storage of Agricultural Products. Yiachoudi-Yiapouli Publications. pp.4-10.

Lahsasni, S., M. Kouhila, M. Mahrouz, and N. Kechaou. 2002. Experimental study and modeling of adsorption and desorption isotherms of prickly pear peal (Opuntia ficus indica). Journal of Food Engineering 55:201-207.

Maltini, E., D. Torreggiani, E. Venir, and G. Bertolo. 2003. Water activity and the preservation of plant foods. Food Chemistry 82:79-86.

Park, K.J., Z. Vohnikova, and F.P.R. Brod. 2002. Evaluation of drying parameters and desorption isotherms of garden mint leaves (Mentha crispa L.). Journal of Food Engineering 51:193-199.

Zanoni, B., C. Peri, G. Giovanelli, and R. Nani. 1999. Study of oxidative heat damage during tomato drying. Acta Horticulturae ISHS 1999:395-399.

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