Freeze concentration of must in a pilot plant falling film cryoconcentrator

Abstract

The standard methods of producing concentrated grape must are vacuum evaporation and reverse osmosis. Concentration by freezing is the system that comes closest to the ideal objective of separating water from the food product without affecting the other components. This article examines the process for freeze concentration of white must from Macabeo variety grapes.

The pilot plant equipment designed at the Technical University of Catalonia is based on freezing the water content of fluids in direct contact with a cold surface. Different parameters were studied to allow tracking of the process: evolution of soluble solids and the ice production. Freeze concentration tests with forty five litters of must were made at an initial concentration of 16.4 °Brix. A final concentration of 29.5 °Brix was obtained. An average rate of 1.38 °Brix/h was obtained working at flow rates of 0.8 L/s. The ice production ranged from 1.32 to 1.05 g∙m^− 2∙s^− 1 as the concentration of the must increased.

Industrial relevance

Sometimes the must has low contents in sugars. The freeze concentration can achieve the appropriate level of concentration in some favorable conditions without losses of aroma and sensorial properties. With this purpose INCAVI (Catalonian Institute of vineyard and wine) provided 100 L of grape juice for the test presented in this article. In this work has been applied this technology to concentrate must (grape juice) obtaining promising results.

Introduction

Increasing the sugar content of grape juice is sometimes necessary due to low fruit sugar or for production of a desired wine. Wines produced using added sugars generally have reduced body and sensory characteristics as compared to grapes produced from the same variety that develop higher sugar content. If low sugar juices were concentrated without the loss of flavour and aroma compounds, they might produce wines with improved sensory characteristics.

The standard methods of producing concentrated grape must are vacuum evaporation and reverse osmosis (Mietton-Peuchot, Milisic, & Noilet, 2002). The evaporation process of making concentrated grape must removes more than 90% of the volatiles (Lin, Rouseff, Barros, & Naim, 2002). Furthermore, both processes have high energy consumption and, for must rectification, they require an additional operation of ion-exchange, which leads to severe ecological problems due to resin regeneration and disposal (Santos, Catarino, Geraldes, & de Pinho, 2008).

Reverse osmosis membrane techniques allow winemakers to partially concentrate the sugar, as well as the flavour components of grapes to normal levels without exposing the grape juice to high heat. The level of concentration is limited by osmotic pressure generated by the process itself and by the applied pressure. In a recent study of pilot plant RO filtration of grape juice (Rektor, Kozak, Vatai, & Bekassy-Molnar, 2007) at 40 °C and 50 bar transmembrane pressure, the concentrated must cannot achieve more than 23 °Brix sugar concentration. With recent advances in nanofiltration membrane processing (NF) different new applications in oenology can be foreseen (Massot, Mietton-Peuchot, Peuchot, & Milisic, 2008). For must it is possible to apply the combination of RO and NF and obtain a higher sugar concentration in must, up to 45 °Brix (Kiss, Vatai, & Bekassy-Molnar, 2004).

Cryoconcentration is a technique for concentrating liquid products by means of freezing and the subsequent separation of part of the frozen water from the liquid product. The process involves lowering the temperature of the product to be concentrated to below its freezing point in a controlled manner in order to avoid reaching the eutectic temperature at which all the components of the product solidify at once. The aim is for the ice so formed to be very pure, i.e., only water, without retaining any of the solids in the product. The purpose of removing this ice is to obtain a concentrated liquid product (Vitagliano, 1992).

The two results that are most difficult to obtain in the cryoconcentration of a product are pure ice crystals and their separation from the concentrate. Viscosity and the rate of crystallisation are both parameters that affect the purity of the crystals obtained through the process.

Concentration by freezing is the system that comes closest to the ideal objective of separating water from the food product without affecting the other components. The greatest advantages offered by the use of cryoconcentration rather than other technologies have to do with the low temperatures reached in the process and non-existence of a liquid-vapour interface. There is no loss of volatiles, making this technique very suitable for the concentration of thermo sensitive fluids (Fellows, 1993).

When comparing heat of evaporation (about 2260 kJ/kg under at pressure of 0.1 MPa) with enthalpy of freezing (335 kJ/kg), the process of freeze concentration seems to be cheaper than evaporation from the energy point of view. On the other hand, energy below 0 °C is more expensive than that above this temperature and capital costs are high. Thijssen and Van der Malen, 1981, Thijssen, 1986 considers that the cost of dewatering by means of evaporation and freeze concentration are comparable because the higher capital costs of second are compensated by lower energy costs.

The technique of concentration by freezing is most studied and used in the fruit juice industry (Addison, 1986). There are many studies available on the application of this technique to the concentration of kiwi juice (Valente et al., 1986, Maltini and Mastrocola, 1999) and berry juices (Ghizzoni et al., 1995, Di Cesare et al., 2000), but where it is most commonly applied is to citrus fruits juice (Braddock and Marcy, 1987, Van Nistelrooij, 2005). This concentration technique has also been applied in other food industries, such as dairy products (Van Mil and Bouman, 1990, Hartel and Espinel, 1993, Van Nistelrooij, 2005), brewing (Putman, Vanderhasselt, & Vanhamel, 1997), winery and distilling (Cesare and Maltini, 1993, Patino et al., 1991), and for the concentration of dilute solutions of tea and coffee (Anon, 1993).

On an industrial or commercial scale, the system used for concentrating liquid foods and therefore also juices is based on suspension crystallisation. The system comprises the following equipment (Lemmer et al., 2001, Verschuur et al., 2002): scraped-surface exchanger to form ice nuclei; re-crystalliser for crystal growth based on the Gibbs Thomson effect; crystal separation system (normally a pressurised wash column). The complexity of the system makes it an expensive technology that can only be used in high-volume production operations and with high added value products.

The freeze concentration equipment designed at the food industry pilot plant of the Technical University of Catalonia is based on freezing the water content of fluids in direct contact with a cold surface. That freezing forms a layer of ice on the exchange surface, made up of stainless steel plates through which a refrigerant fluid circulates. The fluid to be concentrated is distributed by a hydraulic system using a drive pump. It has the advantages of simplicity and economy in comparison with the existing freeze concentration methods, since the concentrate is separated from the ice by gravity and no wash columns, centrifuges or presses are needed, and in addition the equipment works at normal atmospheric pressure, unlike the existing systems, which must be pressurised. The ice washing process can be greatly simplified, since the surface area of the ice with plate freezing is much smaller than the surface area of the ice in a suspension crystallisation exchanger. That equipment (Fig. 1) has been described and tested with sugar solutions of sucrose, glucose and fructose (Raventós, Hernández, Auleda, & Ibarz, 2007) and apple and pear juices (Hernández, Raventós, Auleda, & Ibarz, 2009), obtaining concentrations up to 30 °Brix that could be of interest as a pre-concentration system in the food industry.

The principal aim of this paper is to study the process of freeze concentration of grape juice must using a multi-plate device. Different parameters are examined: the variation over time of the content of soluble solids in the juice to be concentrated measured in °Brix and the ice produced during the process; relative impurity of ice and efficiency of concentration. The limits on concentration by this method are also explored. A further aim is to compare the results obtained in previous studies using sugar solutions.