Concentration of apple and pear juices in a multi-plate freeze concentrator

Abstract

In this article we will examine the process for concentration of apple and pear juices as well as concentration of sugar solutions modelling pear juice (simulation fluid) using a descending film multi-plate freeze concentrator. It has been determined in advance the freezing point of those fluids in the working concentration and temperature intervals. In addition, different parameters were studied to allow tracking of the process of freeze concentration, such as ice accumulation, variation of the content of soluble solids in the solution and in the ice removed, ice production and energy consumption. The apple and pear juices tested showed similar behaviour, while the mixture of sugars used for simulation showed better behaviour, due perhaps to the absence of foam in the process and to the quicker formation of ice. It has been obtained concentrations of 30.2 and 30.8 °Bx with the apple and pear juices, respectively, and up to 32.7 °Bx with the simulation fluid.

Industrial relevance

The clarified juices usually are concentrated on an evaporation stage. In this stage the juice is subjected to high temperatures that cause undesirable reactions, such as non enzymatic browning and destruction of nutritive compounds. The freeze concentration is a technology that allows eliminating water from the juices at temperatures below the water’s freezing point, what allows obtaining products of better quality. In this work has been applied this technology to concentrate apple and pear juices, obtaining promissory results.

Introduction

Freeze concentration is based on the separation of soluble solids from a liquid phase by means of freezing the water content of the liquid. It is presented as an alternative to the standard techniques used in food industries at present such as evaporation and membrane technologies, given the low temperatures used in the process. Concentration of liquid foods by freezing is a selective process that avoids the problem of loss of volatile components that occurs in evaporation and the need for frequent replacements in membrane technology. Considering this advantages, freeze concentration technology has been used for concentration of liquids foods (Thijssen, 1986, Braddock and Marcy, 1987, Van Nistelrooij, 2005, April/May).

It has been applied to concentration of fruit juices (Thijssen, 1986, Valente et al., 1986, Bayindirli et al., 1993, Van Weelden, 1994, Lee and Lee, 1998, Maltini and Mastrocola, 1999, Jansen et al., 2001, Chiampo and Conti, 2002, Ramos et al., 2005), tomato juice (Miyawaki, Liu, & Hayakawa, 1999 ), sugar cane juice (Patil, 1993, Rane and Jabade, 2005, Rane et al., 2005), beer (Davies et al., 1987, Patino et al., 1991) and milk products (Ahmed and Luksas, 1990, Van Mil and Bouman, 1990, Hartel and Chung, 1993, Hartel and Espinel, 1993, Zhang and Hartel, 1995, Chen and Chen, 2000, Aider et al., 2007, Burdo et al., 2008).

According to various researchers (Müller and Sekoulov, 1992, Flesland, 1995b, Chen et al., 1998, Miyawaki, 2001, Shirai et al., 2001), there are two basic methods for ice crystal formation in solutions. The first consist of indirect cooling on a cold surface, forming an ice layer (Müller and Sekoulov, 1992, Flesland, 1995a). The other is known as suspension crystallisation (Huige and Thijssen, 1972, Hartel and Chung, 1993) (Fig. 1). Table 1 shows the main distinguishing characteristics of each type of technology.

Recently block cryoconcentration has been described (Burdo et al., 2008) in which the solution was cooled from the external surface. In this technique a liquid food solution is introduced in a crystallizer chamber and partially frozen from the centre by introducing a pipe in which a cooling agent is circulated. The physical analogue of the block freezer with a horizontal plate crystallizer has been considered. They showed that transition to the technology of growing crystals in the condition of an ice block has allowed reducing power expenses on the process to 0.55–0.6 MJ/kg of ice. The important parameter, determining the meaning of the separation factor, is the density of a packing of crystals in the block ice. The higher the density of the packing the better the solution separates. The results of the experimental investigations of acoustic oscillations in the process of the block freezing of water out of food solutions have been given. The maximum effect is achieved in the range of frequencies from 15 to 50 kHz. This technology was successfully used to concentrate milk whey, fruit juices, coffee extract, and aromatic compounds from grape seeds (Aider & de Halleux, 2009).

On an industrial or commercial scale, the system used for concentrating liquid foods and therefore also juices is based on suspension crystallisation. There has been a great deal of research on this subject since the works of Huige and Thijssen (1972) and Huige (1972), showing that it is very important to obtain ice crystals that are long enough for the separation stage to be very simple (Omran and King, 1974, Hartel and Chung, 1993, Kobayashi et al., 1996, Verschuur et al., 2002). The system comprises the following equipment (Lemmer et al., 2001, Jansen et al., 2001, Verschuur et al., 2002): scraped-surface exchanger to form ice nuclei; recrystalliser 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.

Another type is a direct cooling where the refrigerant in liquid form under pressure, vaporizes and provides refrigeration effect, and causes formation of ice crystals within the product (Table 2). This type is rarely used in the food industry for different reasons. The most important is that a vapour–liquid interface is created, resulting in a subsequent loss of volatile flavours and aromas (Hartel, 1992). Some references with liquid nitrogen have been found for the freeze concentration of vinegar (Patent WO2/066594 A1).

The freeze concentration equipment designed at the food industry pilot plant of the School of Agriculture Barcelona 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 highly 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. 2) has been described and tested with 3 sugar solutions (sucrose, glucose and fructose), obtaining concentrations up to 30 °Bx that could be of interest as a pre-concentration system in the food industry (Raventós, Hernández, Auleda, & Ibarz, 2007).

The principal aim of this paper is to study the process of freeze concentration of apple and pear juices using a multi-plate device. To do so we will examine the behaviour of certain variables that can define the efficiency of this type of concentration by freezing the water content of solutions. Average concentration rate, efficiency of concentration, average ice production, relative impurity of ice, and energy consumption were analyzed. The concentration limits by this method vas also explored.