Drying kinetics and quality of beetroots dehydrated by combination of convective and vacuum-microwave methods

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Abstract

Beetroot cubes were dehydrated by convective drying in hot air at 60 °C and by the combination of convective pre-drying (CPD) until moisture content 1.6, 0.6 or 0.27 kg/kg db and vacuum-microwave finish drying (VMFD) at 240, 360 or 480 W. The control samples were obtained by freeze-drying (FD). The drying kinetics of beetroot cubes was described with an exponential function. VMFD significantly reduced the total time of drying and decreased drying shrinkage in comparison with convective method. A critical moisture content divided the temperature profile of samples during VMFD into increasing and falling periods. At the falling temperature period a significant increase in the colour parameters L, a and b was found. VM treated samples as well as FD ones exhibited lower compressive strength, better rehydration potential and higher antioxidant activity than those dehydrated in convection. Increasing the microwave wattage and decreasing the time of CPD improved the quality of beetroot cubes dried by the combined method.

Introduction

Beetroots (Beta vulgaris) are rich in valuable, active compounds such as carotenoids (Dias et al., 2009), glycine betaine, (de Zwart et al., 2003), saponins (Atamanova et al., 2005), betacyanines (Patkai et al., 1997), folates (Jastrebova et al., 2003), betanin, polyphenols and flavonoids (Váli et al., 2007). Therefore, beetroot ingestion can be considered a factor in cancer prevention (Kapadia et al., 1996). However, fresh beetroots are exposed to spoilage due to their high moisture content. One of the preservation methods ensuring microbial safety of biological products is drying (Mathlouthi, 2001). Dried beetroots can be consumed directly in the form of chips as a substitute of traditional snacks, that are rich in trans fatty acids (Aro et al., 1998), or after easy preparation as a component of instant food (Krejcova et al., 2007).

Convective drying in hot air is still the most popular method applied to reduce the moisture content of fruits and vegetables (Lewicki, 2006), including beetroots (Kamiński et al., 2004, Shynkaryk et al., 2008). However, this method has several disadvantages and limitations; for instance, it requires relatively long times and high temperatures, which causes degradation of important nutritional substances (Marfil et al., 2008) as well as colour alteration (Chua et al., 2001). Another disadvantage of that method is shrinkage, which is a result of tissue collapse caused by volume reduction due to the loss of moisture as well as the presence of internal forces (Sjöholm and Gekas, 1995, Mayor and Sereno, 2004).

Some novel drying methods are free of those weaknesses typical for convective drying. Nevertheless, their application in exclusive form involves other problems such as low productivity, high costs or technical inconveniences. Hence, hybrid techniques composed of complementary drying methods which donate their advantages are of the highest interest. Convective drying in hot air is still worth consideration due to the satisfactory efficiency at the initial period of dehydration characterized by relatively high drying rate and large capacity. Therefore convective drying should be followed by a method which can ensure adequate drying rate at the final period of dehydration and high quality of the dried product. Shrinkage and texture are considered to be quality attributes of dried product (Rahman, 1999). Colour is another quality factor of a dried product (Yongsawatdigul and Gunasekaran, 1996), being not only an indicator of the changes occurring in the material during drying (Maskan, 2001), but also an important attribute boosting the attractiveness of a food product (Soysal et al., 2009). The most suitable method satisfying these requirements is drying with application of microwaves under vacuum.

Drying with the microwave method under vacuum is a modern, efficient method of food preservation (Men’shutina et al., 2005). During vacuum-microwave (VM) drying the energy of microwaves is absorbed by water located in the whole volume of the material being dried. This creates a large vapour pressure in the centre of the material, allowing rapid transfer of moisture to the surrounding vacuum and preventing structural collapse (Lin et al., 1998). As a consequence, the rate of drying is considerably higher than in traditional methods of dehydration (Sharma and Prasad, 2004). A decisive factor enhancing drying rate is the wattage of microwaves (Andres et al., 2004, Figiel, 2006). The puffing phenomenon, that accompanies the rapid process of dehydration, creates a porous texture of the food and facilitates obtaining a crispy and delicate texture (Sham et al., 2001), and in this way it reduces the product’s density as well as shrinkage.

The VM technique has already been satisfactory applied to reduce the moisture content of many plant materials, such as carrots (Cui et al., 2004), cranberries (Sunjka et al., 2004), strawberries (Krulis et al., 2005), peanuts (Delwiche et al., 1986), bananas (Mousa and Farid, 2002), apples (Sham et al., 2001), pumpkin (Nawirska et al., 2009) and garlic (Cui et al., 2003). However, at the beginning of VM dehydration the intensive water evaporation from the material being dried may exceed the vacuum pump capacity. This would require a reduction in the raw material subjected to drying or application of a large vacuum installation. This problem can be overcome by pre-drying of the material using convective drying in hot air which is very efficient in the initial period of dehydration. As a result of pre-drying the mass loads of a VM equipment can be radically decreased (Hu et al., 2006). Pre-drying of the material by convective method before VM finish drying (VMFD) reduced the total cost of dehydration and improved the quality of dried tomatoes (Durance and Wang, 2002) and nutritional value of strawberries (Böhm et al., 2006).

No scientific work has yet been reported on the combined drying of beetroots. The combined method consisting of CPD and VMFD (CPD–VMFD) could make a significant contribution to the vegetable processing industry. However, it is not obvious when convective drying should be replaced with VM method and what microwave wattage is supposed to be applied to ensure the optimal conditions of beetroots dehydration. Therefore the aim of this work was to determine the effect of microwave power and the level of CPD on the drying kinetics as well as on some quality factors of VMFD beetroot cubes in terms of shrinkage, texture, colour, rehydration potential and antioxidant activity. The assumption that these quality factors are in some ways interrelated, due to the decisive impact of water content on a large majority of biological material properties, induces the necessity to explain the phenomena which occur within the material subjected to combined drying.

Section snippets

Sample preparation

Beetroots of “Alto F1” variety were cultivated in a field situated close to Wroclaw (Poland). Roots of similar size were washed and cut into 10 mm cubes by using of a cutter equipped with a knife moving perpendicularly to a horizontal base. The base was covered with thick rubber. To ensure proper size of the samples a vertical base was fixed at 10 mm distance from the action plain of the knife. Before drying the cubes were mixed in a plastic container and then divided into 180 g portions.

Drying

The

Drying kinetics

Drying kinetics of beetroot cubes dehydrated by the convective as well as by combined method is shown in Fig. 2. For both the methods the decrease in moisture ratio MR in time was described by an exponential function:MR=a·e-k·t

For convective drying the a value amounted to 1, and Eq. (5) might be simplified to the Lewis’ model:MR=e-k·t

For VMFD the a value was lower than 1 (Table 1). This value was the lower the longer was the time of convective pre-drying.

In the initial phase of convective

Conclusions

The total time of combined drying of beetroot cubes can be considerably shortened by an early introduction of VMFD at high microwave wattage. However, the drying rate is enhanced not only by increasing the microwave power, but also by lengthening the CPD. The course of beetroot temperature during VMFD undergoes a change at critical moisture content, exhibiting both increasing and falling periods. The existence of a critical moisture content might be explained by the thermal balance between the

Acknowledgements

This work was supported by the Polish Ministry of Science and Higher Education under Grant No. N312 031 32/2036 in years 2007–2009.

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