Block freeze-concentration of coffee extract: Effect of freezing and thawing stages on solute recovery and bioactive compounds
Introduction
Coffee is the most traded food in the world, and its production has great economic and social importance worldwide (Esquivel and Jiménez, 2012, Vignoli et al., 2011). For the consumer, the value of coffee is provided by its sensory and functional properties; for this reason, technologies that promote quality preservation are highly valued in coffee processing. In the production of freeze-dried coffee, freeze concentration (FC) technology is used to remove water from coffee extracts to increase the solid content and reduce the time and cost of the freeze-drying process. At the same time, the sensory properties of the product are preserved using low temperatures (Boss et al., 2004, Joët et al., 2010, Sánchez et al., 2009).
Water removal in FC is achieved by cooling the solution until the ice crystals form and separate (Miyawaki et al., 2005). Three techniques are used according to the ice crystal growth: suspension FC, film FC (progressive or falling film FC) and block FC (total or partial) (Aider and de Halleux, 2009, Sánchez et al., 2009). Suspension FC is a unique technique implemented at the industrial level. Different techniques, such as falling film FC (Chen et al., 1998, Sánchez et al., 2011), progressive FC (Miyawaki et al., 2005) and block FC (Aider and Ounis, 2012, Nakagawa et al., 2010a), are being developed to reduce operational costs.
In the block FC method, also known as freeze–thaw concentration, the solution to be concentrated is completely frozen and then partially thawed to recover a fraction of liquid with a higher concentration (Aider and de Halleux, 2009, Nakagawa et al., 2010b). Block FC consists of three stages: freezing, thawing and separation of the concentrated liquid fraction (Moreno et al., 2013). These stages define the separation efficiency (Nakagawa et al., 2009). Additionally, the process can be repeated in successive cycles to increase the concentration index (Aider and Ounis, 2012).
The technical viability of the block FC method has been proposed recently by several researchers (Gao et al., 2009, Nakagawa et al., 2010a, Aider and Ounis, 2012, Boaventura et al., 2012, Miyawaki et al., 2012, Petzold et al., 2013). During the freezing stage, heat and mass transfer phenomena can modify the solute occlusion, which should be as low as possible. Chen et al. (2001) reported that the solute elution in the freezing front in FC depends on the molecular size of the compounds. Certain authors have reported that the solute separation is controlled by the thawing stage (Nakagawa et al., 2010b). For coffee solutions, Moreno et al. (2013) studied the use of aids in the separation stage. These authors reported the influence of the FC protocol and solution type on solute recovery and the concentration index; for this reason, there is no agreement on the significance of the process variables. The effects of the process variables of block FC on the separation efficiency of coffee extracts have not been reported.
Coffee can be considered to be a functional beverage due to its radical scavenging capabilities (Cheong et al., 2013, Esquivel and Jiménez, 2012). Several studies have reported the health benefits of coffee consumption related to the components with antioxidant activity, such as the group of chlorogenic acids and caffeine. Chlorogenic acid (3-caffeoylquinic acid), cryptochlorogenic acid (4-caffeoylquinic acid), neoclorogenic acid (5-caffeoylquinic acid) and caffeine are the major bioactive compounds present in coffee (Ferruzzi, 2010, Fujioka and Shibamoto, 2008, Sopelana et al., 2013, Vignoli et al., 2011). The block FC method has been shown to retain nutritional and functional properties of the product using low processing temperatures (Belén et al., 2013, Boaventura et al., 2012); however, this effect has not been tested for coffee extracts.
The aim of the present study was to evaluate the effect of the initial coffee mass fraction, the cooling temperature, the heating temperature and the freezing direction on the solute yield and concentration index of block freeze-concentrated coffee extracts. Additionally, the impact of the technique on bioactive compound concentration and the antioxidant activity of the coffee extract was tested.
Section snippets
Materials
Coffee solutions were prepared from freeze-dried soluble coffee supplied by the company Buencafé Liofilizado de Colombia (Colombian Coffee Growers Federation, Colombia) for the FC tests. The coffee was added to distilled water at 35 °C and mixed for 20 min. The samples were stored at 4 °C for 12 h. The solid concentration is expressed as the coffee mass fraction (XS), which is defined as the mass of coffee solids per unit of coffee solution mass. The relationship between Brix degrees and XS is
Temperature profiles
The temperature profiles during FC tests for tests 1 and 8 described in Table 1 are shown in Fig. 3. These tests corresponded to the lowest and highest overall process time; therefore, the other tests were within this time interval. Temperature sensor 1 was located beside the internal wall of the container and sensor 4 was located in the external wall. In test 1, the freezing was achieved from the centre and the thawing from the external wall. For this reason, the temperature dropped first in
Conclusions
Coffee extract was freeze-concentrated by the total block technique. A significant effect of the initial coffee mass fraction, freezing direction and cooling temperature on solute recovery was found. The highest solute recovery was achieved at the lowest coffee mass fraction, when the freezing direction was in counter-flow to the thawing direction and at the highest cooling temperatures. The thawing fractions at which completion of the thawing stage was convenient were found between the values
Acknowledgements
The research was supported by Universidad de La Sabana and COLCIENCIAS with the Project 1230521-28461 (2011). The authors thank Eng. Carlos Osorio of Buencafe Liofilizado de Colombia (Colombian Coffee Growers Federation) for providing the coffee and assisting with the research. The authors thank to Prof. Erlide Prieto, Yomaira Uscategui, Dr. Sergio Cuervo and Dr. Edgar Benitez for assisting with the research. Author Moreno F L thanks COLCIENCIAS for its condonable grant for doctoral studies
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