Microwave-assisted drying of blueberry (Vaccinium corymbosum L.) fruits: Drying kinetics, polyphenols, anthocyanins, antioxidant capacity, colour and texture
Introduction
Blueberries (Vaccinium corymbosum L.) are one of the most popular soft fruits cultivated mainly in United States, Canada and Poland. These berries are characterized by a strong free radical scavenging properties due to the presence of bioactive compounds i.e. proanthocyanidins (PACs) (Gu et al., 2002) and anthocyanins (Wu et al., 2006). The consumption of blueberries may potentially contribute to the decreased risk of neurodegenerative, cardiovascular and urinary diseases, inhibition of the growth of cancer cells and prevention of vision problems and aging (Seeram, 2008). The seasonal availability of blueberries may limit their consumption and further processing should be employed to extend their availability out of season.
Dehydration of fruits is one of the oldest and the most diverse of unit operations in chemical engineering. It is constantly evolving to offer high quality dried materials and to develop new products for increasing demands of health-conscious consumers. Drying offers preservation of berry fruits, reduction in size and cost of storage and transportation. However, prolonged exposure to elevated temperature, especially during the falling rate period, may result in substantial degradation of product quality (Zielinska, Zapotoczny, Alves-Filho, Eikevik, & Błaszczak, 2013).
Hot air convective drying (HACD) is broadly used in post-harvest technology of agricultural products due to the relatively low cost of the process. Due to a waxy skin layer, HACD of blueberries is limited by the low drying rate that falls rapidly with a gradual reduction of moisture and a long time of drying (Zielinska, Sadowski, & Błaszczak, 2016). Among different initial treatments, freezing/thawing reduces the thickness of blueberry skin and its mechanical resistance. It promotes high moisture transfer during drying, and significantly reduces drying time and specific energy consumption of drying in comparison with drying of blueberries without initial freezing/thawing pretreatment (Zielinska, Sadowski, & Błaszczak, 2015). Additionally, freezing of blueberries before drying yields higher antioxidant activity and improved structural retention compared to drying of fresh blueberries (Pallas, 2011). HACD of blueberries may cause some changes in mechanical properties, colour, structure, volume, porosity and density of fruits (Zielinska et al., 2016). It may have negative effect on the biological activity of some chemical constituents (Pokorny & Schmidt, 2003, chap. 15). Vaghri, Scaman, Kitts, Durance, and McArthur (2000) report that reduced anthocyanin and phenolic content is attributed to the long drying time. Also Somsong, Srzednicki, Konszak, and Lohachoompol (2010) studied HACD in a cabinet dryer operated at temperatures between 70 and 90 °C with single and multistage drying strategies and found that the total anthocyanin content in blueberries was highest in drying at 90 °C followed by 70 °C and multistage drying, respectively. HACD also accelerates the formation of furfural constituents, i.e. 5-hydroxymethylfurfural. This may affect the rate of degradation of the pigment molecule and enhance the negative influence of oxygen on the anthocyanin degradation (Stojanovic & Silva, 2007).
Compared to HACD, microwave drying (MWD) can benefit with its volumetric heating and reduced processing time (Figiel, 2009). On the other side, it may cause overheating of particles and undesired degradation of bioactive compounds (Yongsawatdigul & Gunasekaran, 1996). The use of microwaves under vacuum may significantly prevent from quality degradation of heat-sensitive materials (Mejia-Meza et al., 2008). The vacuum lowers the boiling point of water. The temperature inside the dried particles is higher than that on the surface of the product leading to the increase in partial pressure that drives the evaporating water to the outer layer. Gradient between a vapour pressure in the centre of the material and its surface results in high drying rates (Zhang, Tang, Mujumdar, & Wang, 2006). The absence of air during dehydration eliminates the occurrence of oxidation reactions (Mejia-Meza et al., 2008). MWVD turned out to be relatively better than HACD, in terms of retention of phenolic compounds, anthocyanins and antioxidant capacity (Wojdyło, Figiel, Lech, Nowicka, & Oszmiański, 2013). This method was successfully used for the dehydration of cranberries (Yongsawatdigul & Gunasekaran, 1996) and blueberries (Zielinska et al., 2015, Zielinska et al., 2016). On the other side, the usage of microwaves under reduced pressure increases the drying costs.
Thus, alternative option is a usage of HACD combined with MWVD. MWVD may be applied at the beginning or in the final stage of HACD (Zhang et al., 2006). From the energy efficiency standpoint it would appear that MWVD should be used first followed by HACD. However, the results of Zielinska et al. (2016) showed that MWVD of frozen/thawed blueberries results in a significant leakage of pigmented exudates. Then, initial HACD combined with MWVD was suggested to be used instead of MWVD + HACD. In the final stage, when the drying rate began to fall, the moisture is still present at the centre of the dried particle and with the help of microwave heating at reduced pressure vapour may be quickly forced outside. HACD + MWVD may promote efficient drying of blueberries by enhancing product quality and reducing energy consumption in comparison with HACD (Zielinska et al., 2016). HACD + MWVD results in similar or greater retention of antioxidants than freeze drying (FD) and higher retention of total anthocyanins and polyphenols in Saskatoon berries than HACD (Kwok, Hu, Durance, & Kitts, 2004).
Although Kwok et al., 2004, Zielinska and Markowski, 2016 and Zielinska et al., 2015, Zielinska et al., 2016 provide some results on the microwave-assisted drying of blueberries, further research is required for the evaluation of retention of bioactive compounds of blueberry fruits under different drying conditions. Drying kinetics along with the changes in physicochemical properties of blueberries with drying time need to be clarified for an appropriate assessment of the drying phenomena. Description of drying processes and characterisation of dehydrated blueberries will encourage to: (1) widely use of microwave-assisted drying processes; and (2) increase the production and utility of dried blueberries for developing new products with beneficial health properties.
Taking above into consideration, the aim of study was to examine the effect of hot air convective drying (HACD), microwave vacuum drying (MWVD) and their combination (HACD + MWVD) on the drying kinetics, bioactive compounds, colour and mechanical properties of frozen/thawed blueberries. The changes in moisture ratio, drying rate, material temperature, moisture diffusivity, total polyphenolic compounds, total monomeric anthocyanins, antioxidant capacity, colour density, polymeric colour, percentage polymeric colour, CIE L*a*b* colour, hardness, springiness, cohesiveness, chewiness and gumminess of blueberries were evaluated.
This research study is complementary to our previous works on blueberries drying (Zielinska and Markowski, 2016, Zielinska et al., 2015, Zielinska et al., 2016). Based on the results of Somsong et al. (2010), hot air temperatures used in the present study are 60 and 90 °C, instead the previously used, i.e. 60 and 80 °C.
Section snippets
Chemicals
2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt, cyanidin-3-glucoside, methanol, hydrochloric acid, Folin-Ciocalteu phenol reagent, Trolox and sodium carbonate were from Sigma-Aldrich (Switzerland). All solvent used were of analytical grate.
Material
Fresh blueberries (Vaccinium corymbosum L., cv. Bluecrop) were obtained from a local farm (Czerwinskie, Baranowo, Poland). Fruits were kept frozen (−18 ± 1 °C). The frozen fruits were thawed and warmed up at room temperature for 2 h before
Drying kinetics
Drying methods differed in processing parameters (Table 1) and affected the physical attributes of blueberries (Fig. 1). The initial moisture content of blueberries was 6.22 ± 0.02 kg H2O kg−1 DM, whereas that of dried fruits was between 0.16 ± 0.04 and 0.23 ± 0.03 kg H2O kg−1 DM. HACD at 60 °C resulted in the longest drying time (Fig. 2). The increase in air temperature from 60 °C to 90 °C shortened the drying time by 77%. HACD at 60 °C and 90 °C of blueberries were found to be time-consuming due to the
Conclusions
Drying processes varied in time (from 1 to 23.4 h). No constant drying rate was observed during HACD and HACD + MWVD of frozen/thawed blueberries. The drying rate curves were divided into two falling rate periods, while MWVD was divided into constant and falling rate periods. The values of De ranged from 1.73 × 10−10 (HACD at 60 °C) to 3.11 × 10−9 m2 s−1 (HACD at 90 °C + MWVD). Dried berries contained from 45 to 69% of initial content of TPC. Lower air temperature and prolonged exposure to oxygen resulted
Acknowledgements
The authors would like to thank Piotr Sadowski from the Department of Systems Engineering, University of Warmia and Mazury in Olsztyn for performing compression test.
References (34)
Drying kinetics and quality of vacuum-microwave dehydrated garlic cloves and slices
Journal of Food Engineering
(2009)Drying kinetics and quality of beetroots dehydrated by combination of convective and vacuum-microwave methods
Journal of Food Engineering
(2010)- et al.
Effect of drying parameters on the formation of early and intermediate stage products of the Maillard reaction in different plum (Prunus domestica L.) cultivars
LWT – Food Science and Technology
(2016) - et al.
Effect of thermal processing on anthocyanin stability in foods; mechanisms and kinetics of degradation
Trends in Food Science and Technology
(2010) - et al.
Effective moisture diffusivity of garlic cloves undergoing microwave-convective drying
Journal of Food Engineering
(2004) - et al.
Influence of osmotic concentration, continuous high frequency ultrasound and dehydration on antioxidants, colour and chemical properties of rabbiteye blueberries
Food Chemistry
(2007) - et al.
Influence of air flow velocity on kinetics of convection apple drying
Journal of Food Engineering
(2004) - et al.
Trends in microwave-related drying of fruits and vegetables
Trends in Food Science and Technology
(2006) - et al.
Influence of microwave-assisted drying techniques on rehydration behavior of blueberries (Vaccinium corymbosum L.)
Food Chemistry
(2016) - et al.
Freezing/thawing and microwave assisted drying of blueberries (Vaccinium corymbosum L.)
LWT – Food Science and Technology
(2015)