Water removal is the key to concentration of all liquid food products. Various methods are available to remove this water. They can be divided into three main categories:
- 1) Evaporation converts water (and other components) into a vapor.
- 2) Membrane technology provides a barrier that allows water (and all smaller molecules) to pass through.
- 3) Crystallization converts the water into solid ice crystals. Solid-liquid separators are required to remove the ice (GEA Messo PT, 2013).
Evaporation is the most common and the most applied technique for concentration. The limited selectivity and high temperatures generally result in relatively poor retention of the original product quality. Membranes can provide low operational costs but provide a relatively poor concentration factor and limited selectivity. Crystallization provides the highest selectivity toward water removal and, due to the low operating temperatures, the activity of sensitive nutritional and flavor components are maintained (GEA Messo PT, 2013).
Traditionally, fruit purees and juices have been concentrated by water elimination (evaporation) using heat with or without vacuum application. Evaporation is considered to be the most economical and most widely used method of concentration. However, it is not suited for food products with very delicate flavors (Deshpandea et al., 1984). Fruit purees and juices are high-acidity materials rich in glucose and fructose: It is well known that reducing sugars react with amino acids in the Maillard reaction, a series of reaction that can deeply change the color (browning) and the taste (carameli- zation) of those vegetal materials. HTST treatments slightly improved the organoleptic features without allowing a complete solution of those problems.
As compared to the conventional evaporation processes, concentration by freezing is potentially a superior and economic process for aroma-rich liquid foods. In the past, the process, however, was seldom used because of the investment cost and the considerable loss of concentrate in the withdrawn ice, and hence, the quality. Recent technological developments have minimized these two drawbacks associated with the earlier freeze concentration processes. In the coming decade, freeze concentration is seen as a potentially attractive method for the concentration of aroma-rich liquid foods, including fruit juices, coffee, and tea (Deshpandea et al., 1984).
Crystallization of water from liquid products has commonly been referred to as freeze concentration. The process has been applied for centuries. In its earliest form, it was as simple as leaving a barrel filled with product outside in the winter and draining the remaining liquid as concentrated product. The ice is formed as pure water crystals and everything else remains in the liquid (GEA Messo PT, 2013).
Freeze concentration technology provides the highest quality retention with a relatively high concentration factor against reasonable cost. It is commercially applied in the citrus industry for concentrating orange, grapefruit, and mandarin juices and other fruit juices like strawberry juice, grape juice, lemon juice, black/red current juice, raspberry juice, blue/black berry juice, grape juice, peach juice, banana juice, cranberry juice, and others with delicate flavor and/or color. The maximum achievable concentration is in the range of 35° to 50°Brix (GEA Messo PT, 2013).
High product quality is a result of the following:
- • Low processing temperature: The concentration takes place at the freezing point of the product (e.g., -8°C). All microbiological, biochemical, and chemical reactions have virtually stopped. There is no thermal damage to the product.
- • Efficient separation of the water: The separated ice crystals are 100% pure ice without any inclusions. The separation of ice crystals in the unique wash column separator is 100% efficient so that all the original components remain in the concentrated product.
- • No contact with air: The process operates as a pressurized liquid-filled system. Consequently, all contact with air/oxygen is eliminated and the potential for oxidation is minimized.
- • No need for intermediate cleaning: The process operates 24 hours per day and can go for weeks without intermediate cleaning. Throughput is flexible, between 0 and 100% of design capacity (GEA Messo PT, 2013).
Freeze concentration is the removal of pure water in the form of ice crystals at subzero temperatures. Figure 18.104.22.168 shows the complete process.
Figure 22.214.171.124 Diagram of freeze concentration process. (Image Credit: GEA Process Engineering).
This single-stage process consists of one crystallizer (1) and one wash column (2). The crystallizer is a vessel with a cooling jacket. The inner wall of the vessel is scraped. The outer wall is cooled by a circulating refrigerant. Ice production and crystal growth take place inside the crystallizer. By creating residence time, ice crystals grow, creating an optimal crystal size distribution for efficient separation. In the wash column, the concentrated liquid is separated efficiently from the ice crystals. A compressed ice crystal bed is washed with melted ice to remove all traces of concentrated liquid.
Freeze concentration ensures that all original product characteristics remain in the concentrate.