Analytical Methods for Food and Dairy Powders

Chapter 1

Dehydration Processes and their Influence on Powder Properties

Most microbial and biochemical changes that alter the quality of food occur in the aqueous phase. Water plays a dual role:

As a solvent, it ensures the transfer of substrates, growth promoters, biological agents and reaction products, which allows reactions to take place in optimal conditions.

As a reaction substrate, it is involved in hydrolysis reactions (proteolysis, lipolysis).

This dual action requires that water is available, which can be characterised by its water activity (aw; cf. 1.2.1.2), i.e. the ratio between the partial pressure of the water vapour of the product and the partial pressure of pure water vapour at the same temperature. Any process that reduces this availability also slows down reaction times.

Water activity can be lowered by the crystallisation of solvent water (freezing) or by the addition of highly hydrophilic solutes that bind water molecules through hydrogen or dipolar interactions (salting, sugaring). It can also be lowered by eliminating the available water (concentration, evaporation and drying); in this case the inhibition generated is removed by dilution or rehydration.

This book deals with the properties of food powders obtained through drying. This preservation method only slightly alters the nutritional and organoleptic qualities during dehydration and any pre-treatments are well controlled with regard to heat and mass transfer.

Given the high latent heat of the vaporisation of water (2258 kJ kg−1 at 100°C), the drying process is often preceded by a concentration of the dry matter within the product to reduce the energy cost of processing. This pre-concentration can be achieved by cross-flow filtration (reverse osmosis for example) or by vacuum evaporation. In reverse osmosis, water is removed without phase change by passing the product through a membrane under the action of a pressure gradient, which reduces the energy cost of water elimination (10–40 kJ kg−1 water). However, the efficiency of the process decreases with increasing viscosity and osmotic pressure resulting from a concentration of dry matter (proteins and molecules of low molecular weight, respectively). Thus, it is generally not possible to concentrate the product beyond 25% (w/w) of dry matter.

Therefore, in this chapter we only deal with concentration by vacuum evaporation and drying, which are the two main unit operations used in the manufacture of dried products.

1.1. Overview of Operations

1.1.1 Concentration by Evaporation

Concentration by evaporation involves exposing a liquid to temperature and pressure conditions that allow vaporisation of the solvent. This process therefore facilitates a concentration of non-volatile elements in the treated product. In the food industry, it is mainly used to remove water from true solutions, emulsions and/or colloidal solutions.

A key aspect of this technique is the energy cost involved, since water is removed by a phase change (liquid–vapour), contrary to separation techniques. Therefore, the concentration at an atmospheric pressure of 1 kg of a 10% sucrose solution, initially at 20°C, to 20% sucrose (elimination of 0.5 kg water) requires a total of 1439 kJ. This energy breaks down as follows: the addition of sensible heat allowing an increase from 20 to 100°C (311 kJ) and the addition of latent heat to vaporise 0.5 kg of water at 100°C (1128 kJ). However, the energy difference between the initial and final systems at 20°C is only 0.5 kJ, which corresponds to an isothermal compression of sucrose molecules. Concentration by evaporation therefore has a very low efficiency, without any energy recovery. Most of the technical developments made were aimed at improving efficiency.

Furthermore, processed food liquids are often heat sensitive. To minimise the biochemical alteration of components, concentration by evaporation is generally carried out under a partial vacuum to reduce the processing temperature by 45–80°C. While the qualitative advantage of this practice may be obvious, the energy gain is in fact low. The alteration of components, according to a time–temperature relationship, can also be reduced by decreasing the residence time in the facility. The physicochemical characteristics of the concentrate (non-denatured protein nitrogen [WPNi; cf. 1.2.1.3], viscosity, insoluble mineral) depend on the length and temperature of the process and the ionic force; these characteristics largely determine the properties and qualities of the final powder.

First the principle of vacuum evaporation is discussed. Then the different techniques to reduce energy consumption are explored.

1.1.1.1 Principle of Vacuum Evaporation

Single-stage vacuum evaporation consists of placing the liquid to be concentrated, which has been brought to its boiling temperature beforehand, into a vacuum chamber (evaporation body; Figure 1.1). The vacuum, obtained by the condensation of spray in contact with a cold source, corresponds to the saturation vapour pressure at the boiling point of the product.

Figure 1.1 Single-stage falling film evaporation.

In this context, any heat applied to the product will result in vaporisation of some liquid. The evaporation body is thus a heat exchanger for providing the product with the latent heat of vaporisation. In practice, the energy supplied to the heat exchanger (tube bundle in general) comes from vapour at a temperature of 5–10°C higher than that of the product.

The liquid–vapour mixture is separated in a separation container attached to the evaporation body. In this way the secondary vapour (still called vapour spray) as well as the concentrated liquid is collected. The energy contained in the vapour mist is usually recovered either to reheat the incoming product or to heat a second evaporation body. This principle of multiple-stage evaporation will be explained in greater detail later.

Each evaporation unit must meet three industry requirements: high evaporation capacity, low specific energy consumption and ability to maintain quality of the concentrate. The types of evaporator differ depending on the liquid flow or the geometry of the heating surfaces, and are more or less adapted to the different food liquids:

1. Climbing film evaporators, used in the sugar industry in particular. The concept of ‘climbing film’ means that the liquid, introduced at the base of the unit, rises while concentrating inside the tubes, thereby ensuring complete wetting of the exchange surfaces.

2. Falling film evaporators (Figure 1.1), used mainly in the dairy industry. The heat transfer is improved compared with climbing film evaporators due to better liquid flow conditions: the liquid, introduced at the top of the unit, runs off by gravity as a thin film (mm range) inside the tubes. However, it is more difficult to achieve a uniform wetting of the heating surface, which can lead to local over-concentrations and fouling.

3. Plate evaporators, with a much smaller footprint and greater ease of disassembly compared with vertical evaporators (climbing or falling film); the limits of this configuration are the high load losses linked to the flow of the concentrate and, in extreme cases, the risk of obstruction of the liquid flow, in the case of a major blockage.

1.1.1.2 Energy

The elimination of water is often an expensive operation in the processing of liquid food. For example, the enthalpy balance of single-stage evaporation, including consideration of energy losses, highlights the following two points:

1. The evaporation of 1 kg of water from a treated product requires the condensation of 1.1 kg of primary vapour. The energy cost is therefore about 2700 kJ kg−1 of evaporated water.

2. The specific enthalpy of the secondary vapour thus obtained is slightly less than that of the primary vapour, corresponding to a temperature drop of 3–5°C.

There are several solutions to reducing the cost of concentration by vacuum evaporation.

The multiple-stage evaporator consists of a set of single-stage evaporation units connected in series (Figure 1.2), whereby the liquid food being concentrated passes from one stage or ‘effect’ to the other. The first stage is heated with direct steam injection while the following ones are heated with vapour mist generated in the preceding stage. The last stage is connected to a condenser, which creates a vacuum in the entire system. The energy cost of removing water in an evaporator with n stages is therefore of water removed (not taking into account sensible heat).

Figure 1.2 Multiple-stage evaporation.

While a temperature difference is necessary between the heating vapour and the product to be concentrated, the evaporation temperature, and therefore the pressure, decreases from one stage to the next. The pressure gradient between the first and last stage is controlled by the vacuum pump, which is connected to the condenser that collects the vapour extracted from the last stage. The limits to multiple-stage evaporators are:

The maximum temperature that the product can withstand in the first stage due to its heat sensitivity. In practice, this temperature is generally between 70 and 90°C for foodstuffs.

The temperature in the last stage, which is limited by the temperature of the condensate and/or the increase in viscosity due to the drop in temperature and the increase in dry matter of the concentrate. In practice, this temperature is generally greater or equal to 40°C.

The drop in the evaporation temperature from one stage to the next. This drop is generally greater or equal to 5°C.

The compromise between energy cost reductions and investment in additional stages (depreciation and maintenance) is concentration facilities comprising three to six stages.

However, modern facilities allow a greater reuse of vapour. The principle consists of compressing the vapour mist, thereby increasing its enthalpy and injecting it back into the stage where it was created. Two methods are used to do this: thermocompression (Figure 1.3) and mechanical vapour recompression (Figure 1.4).

Figure 1.3 Evaporation with steam jet compressor.

Figure 1.4 Evaporation with mechanical compressor.

Thermocompression can easily be integrated into a multiple-stage system and provides an energy gain equivalent to an additional stage at a lower investment cost. Mechanical vapour recompression, even if linked to just one stage, can significantly reduce energy costs.

To summarise, the various measures that can improve the energy costs of the evaporation operation are:

preheating solutions

recovery of heat from condensates or concentrates

multiple-stage evaporation, coupled or not with thermocompression and mechanical vapour recompression.

These factors substantially influence the energy consumption of concentration by vacuum evaporation. Even though the data in the literature is not always consistent (whether or not the following is taken into account: centrifuge pumps, vacuum pumps, condensate, vapour, boiler efficiency, etc.), energy consumption decreases by 2600–3100 kJ kg−1 for a single stage and 260–330 kJ kg−1 for six stages with thermocompression (Kessler, 1986; Westergaard, 2004).

1.1.2 Drying

Dehydration of food products can ensure good stability by lowering the aw and reducing transport and storage costs (Bimbenet and Loncin, 1995). It can be done as follows:

by evaporation at boiling temperature at atmospheric pressure or under partial vacuum (drying by direct contact, on heated rollers for example)

by the combined action of a heat transfer of hot air to the product and a water transfer from the product to the hot dry air (spray drying for example)

sublimation of ice at partial pressures of water below the triple point (610.8 Pa), corresponding to the direct transition from solid to gaseous state. This method, corresponding to freeze-drying of food products (coffee, mushrooms, etc.), is not covered in this book.

Industrial drying of liquid foods has been practised since the early twentieth century. The first heating devices were roller or drum dryers by Just and Hatmaker, patented in 1902. It was not until 1930 that the industry developed spray drying, although the first patent for this was filed in 1865 by Larmont for drying eggs. Such a delay in the development of spray drying is probably due to the low energy and investment cost of roller drying. Spray drying was further developed after the Second World War due to the limited capacity of roller drying facilities in light of the increased level of food drying (e.g. milk surpluses) and the poor quality (nutritional and physical) of powders obtained.

1.1.2.1 Drying by Boiling (Heated Rollers)

1.1.2.1.1 Principles

Drying by boiling involves transmitting a heat flux to the product, which has been brought to its boiling point, via a latent heat exchange surface. As in vacuum evaporation, the evaporation of water under these conditions is directly proportional to the energy input (latent heat of vaporisation). In practice, this input is achieved by conduction via an exchange surface in contact with the product by vapour at a temperature of between 130 and 150°C. According to Fourier's law, the transfer of heat is proportional to the temperature difference between the heat transfer fluid (which usually has a constant temperature) and the boiling liquid at a given pressure.

The device consists of rollers (cylinders) set horizontally next to each other and heated internally by steam (Figure 1.5). The paste-like liquid is poured between the two rollers that rotate slowly in opposite directions. A film forms on the surface of the rollers, which quickly dries and is removed by a scraper blade. The steam is extracted into a hood above the rollers.

Figure 1.5 Roller dryer. 1, Drum. 2, Feed pipe. 3, Knife. 4, Vapour hood. 5, Conveyor.

This method is mostly used in the starch industry, in the preparation of fruit and vegetable flakes (potato, cassava, etc.), and to a lesser extent in the dairy industry.

1.1.2.1.2 Energy

Since the heating surface is a set characteristic of the device, the user can vary two factors to modulate the evaporation capacity of the facility: the temperature of the heating vapour and the overall heat transfer coefficient (by reducing the thickness of the layer).

The specific energy consumption of this type of drying is about 3200 kJ kg−1 of evaporated water. Pilot scale roller dryers in a partial vacuum chamber with mechanical stream recompression reduce energy consumption by up to 900 kJ kg-1 of evaporated water. Roller drying is therefore an appealing technique from an energy-saving point of view.

One of the advantages of products treated on rollers is that they are not in contact with oxygen in the air since a protective layer of water vapour covers the roller for most of the drying and the interface surface generated between the air and the product is limited compared with that created in spray drying; oxidation is thus restricted. Moreover, it is possible to treat products at higher initial concentrations. For example, a sodium caseinate with a dry matter content of 400 g kg−1 dries easily on rollers even though it has a high viscosity.

1.1.2.2 Spray Drying

1.1.2.2.1 Principles

Spray drying (or atomisation) is a particle drying technique. It involves spraying the product, which is in liquid form or in suspension, into a hot gas stream. This is without a doubt the most used drying method for all food sectors combined (charcuterie, fish products, fodder, cereals and vegetable products, fruit, milk, eggs, blood, etc.). Several techniques are involved in this method.

Spray drying involves entrainment. When a wet product is placed in a sufficiently hot and dry stream of air (or another gas), a temperature and partial water pressure gradient spontaneously occurs between the product and the air causing:

a heat transfer from the air to the product due to the temperature difference

a reverse water transfer due to the difference in partial water pressure between the air and the surface of the product (Figure 1.6).

Figure 1.6 Principles of spray drying.