Eggs

Spray Drying and Pretreatment of Dried Egg Products

Niro is as one of the only suppliers in the world which can deliver complete processing lines for eggs - utilizing the most advanced equipment for pretreatment and spray drying of eggs.

The largest consumers of eggs have long recognized that shell eggs are unsuitable as raw material in a food industry geared to modernization. Frozen eggs have been found much more reliable and convenient. In the course of modernization, however, large egg consumers, faced with the need to further improve product quality and expand product varieties, had to reduce manufacturing costs. More efficient processing techniques were sought. This led to a fuller use of spray drying due to spray dried products being found a most suitable dry raw material for the food industry. The advantage to the food industry of spray dried egg products can be summarized as follows:

• Spray dried products are successfully handled in fully automated processing lines.
• Spray dried products possess more standard constant compositions and overall properties than any other previous form of egg products.
• Spray dried products are readily tailormade to meet required product specifications. This means it is possible to ensure the standardized properties of bakery constituents, an important aspect for mechanized product handling and packaging.
• Spray dried products are always ready for use. (They require no overnight swelling or melting etc.).
• The costs of product transport and storage are considerably reduced, and thus long distance and even overseas transport is possible.

The above points are the main reasons why spray dried whole egg, egg yolk and egg white has been widely preferred by manufacturers.

The main users of egg products are bakeries, confectionery, macaroni, and noodle manufacturers, the meat handling industry, large scale catering, hotels, etc.

For these users NIRO A/S can offer a variety of dried products, such as normal whole egg, egg yolk, and egg white powder, bacteriological or enzymatic fermented albumin powder, instant sugared whole egg or egg white powder, and many others.

Even though the spray drying in these cases will be the main process, it cannot be emphasized too much that the pretreatment of the various products have a very great influence on the quality of the finished product and, therefore we shall also deal with these processes.

Spray Drying Process

The spray drying process enables removal of nearly all the water from a heat sensitive biological product like eggs in such a way that the valuable constituents are not only unaffected but even further refined. The basic feature of spray drying that enables this is the atomization of the liquid egg product into a spray of droplets that is dispersed into hot air. The spray has an extensive surface area and moisture evaporation is virtually instantaneous. Because of that the product temperature is maintained well below levels that cause potential heat damage and deterioration in the valuable properties of egg products.

Successful application of spray drying requires obtaining a spray of well defined characteristics (droplet size and size distribution) and optimum spray-air mixing through control of atomization, drying air properties and air distribution.

Atomization of liquid to form a spray can be achieved by several methods. Each one results in sprays of different characteristics. The most important ones utilize centrifugal pressure, or kinetic energy.

Rotary atomization based on centrifugal energy is the type of atomization that most readily produces the required characteristics for egg products. Sprays are formed from a rotating wheel. Atomizer wheels have the advantage of flexibility by successfully handling liquids of widely varying properties. Atomizer performance is controlled by the wheel diameter and speed of rotation. Values of the variables are selected according to the liquid product and required dried properties.

Careful selection of all processing stages and especially close control of the spray drying stage enable the meeting of desired dried egg properties. These include moisture content, particle and bulk density, particle size distribution, colour, baking and foaming properties, etc.

The spray drying process has been used for a long time within the egg processing industry, however, there are other methods of drying eggs. The oldest method is tray drying, which has been used for many decades to produce a so-called crystallized egg white. This has now been replaced by spray dried egg white, which besides the same excellent functional properties has a superior microbiological quality and is more ready-to-use. Some other drying principles than spray drying have been investigated recently, for example foam drying (inert gas is introduced in the feed prior to drying) and freeze drying (under high vacuum the water content of the frozen product is removed by sublimation). With the continuing advances in spray drying technology, it seems unlikely that these other techniques will replace spray drying in the future.

Design of Spray Dryer

To select the correct type of plant, practical research has taken place, and it has been found that a conventional drying chamber with a conical bottom is the most suitable.

For separation of the fines in the drying air, cyclones, or bag filters are used and powder from the drying chamber and the powder separators is conveyed by a pneumatic conveying system whereby the total amount of powder is discharged at a single bagging-off point.

Two basic designs for whole egg and egg white drying are shown in figures 1 and 2. The main features of the plants are:

Filtered atmospheric air is heated in the air heater (1) and is introduced into the drying chamber (3) through the air disperser (2).

The egg pulp is pumped by the feed pump (4) to the centrifugal atomizer (5) where it is atomized and brought into contact with the hot drying air. The dried product is discharged from the drying chamber into a pneumatic conveying system (8), which conveys the powder to a powder separator (9). The powder is separated from the conveying air and passes through a sifter (10) for bagging-off.

The drying air with entrained fines passes through a main cyclone (6) where the fines are separated and discharged into the pneumatic conveying system. The exhaust air is removed by the fan (7).

In the case of a spray dryer for egg white all drying air and powder are passed through a bag filter (6). The bag filter discharges the product into a pneumatic cooling system.

Figure 1 Spray dryer for egg product

Figure 2 Spray Dryer for Egg White

The main difference between the two designs concerns the equipment for separation and recovery of the airborne egg product leaving the drying chamber. The very light egg white powder requires a more efficient separation system than egg yolk/whole egg powder. In both designs all surfaces in contact with egg product are of stainless steel and plant layout meets the hygienic standards required for egg processing.

Pretreatment of Albumin

Fermentation

Whereas the pretreatment of whole egg and yolk pulp is fairly simple, it is necessary to expose the albumin pulp to a comprehensive pretreatment, i.e. a fermentation and ammonium treatment.

The fermentation serves to break down the glucose in the egg pulp, because the glucose is detrimental to the quality of the dried egg products. In the storage, heating, and drying stages, the glucose will react with the protein and give the so-called "Maillard Reaction". The reaction will cause a brownish colour and some insoluble compounds. Furthermore, the amount of glucose affects the whipping properties of powdered egg white and the keeping quality of the powdered product. As a rule of thumb the shelf life of the egg powder is doubled if made from fermented products.

As for the albumin, it is important to preserve its whipping properties and the albumin is therefore always fermented before drying. For whole egg and yolk powder the quality requirements of the relevant market will decide whether the two products must be fermented before drying.

Fermentation processes for albumin, whole egg and yolk are largely identical though with minor differences. The different fermentation methods used for albumin are described in the following. The description conclusively lists a number of points on which the fermentation of whole egg and yolk differs.

As mentioned formerly the fermentation serves to remove the glucose in the albumin. Fresh albumin contains only a few hundredths % of fat, but during storage of shell eggs some part of the yolk fat will extrude and blend with the albumin. Some small parts of yolk may also be found in the albumin after the breaking and add to the fat content in the albumin. The fermentation process also serves to remove this extra fat from the albumin. To that end a strong foaming is created during the fermentation process causing the fat and other undesirable elements to lodge in the foam and will then be removed together with the layer of foam. In order to get the right intensity of foaming up, the fermentation tanks must comply with very specific dimensioning standards.

The fermentation is a biological process, which can be split up in three groups depending on whether yeast, bacterial or enzyme fermentation is used. A flow-sheet is shown in fig. 3.

Figure 3 Process Flow for Eggs

Yeast Fermentation

This process, for which very specific kinds of yeast are used, has not been found as popular as the two others. The reason is that a yeasty flavour or odour is usually evident in the product. In the fermentation process glucose is converted into alcohol and carbon dioxide. These elements are volatile and will escape during the drying.

Bacterial Fermentation

This process has for many years been preferred because the finished egg powder has excellent whipping qualities, solubility, odour, and taste.

As fermentation agent the bacteria inherent in the albumin may be used, but the process will be difficult to control. Hence, special bacteria cultures are used, though these may vary in composition from one egg processing plant to another. The cultures are either single strain cultures or mixed cultures. They must not contain proteolytic organisms.

The albumin is heated in the plate heat exchanger to approx. 30°C, and then led to the fermentation tanks. The pH-value is adjusted here to about 7.0 for example by adding acetic acid. This may be adjusted and controlled automatically by a pH-unit. The acetic acid is added during agitation and at a slow rate so that a local denaturation of the serum proteins is avoided. During the fermentation process which takes about 36-48 hours, some organic acids and carbon dioxide will develop depending on the type of bacteria used. Some of these elements are volatile and will escape during the drying process.

The bacterial fermentation gives a product with a very high bacterial count. It is therefore essential to keep a constant rigid control of the glucose content during the processing. When the glucose has been removed the fermentation must be stopped in order to avoid undesirable transformations of the albumin. Countermeasures are taken in the form of after-treatment and pasteurization.

Enzymatic Fermentation

This process is gaining ever wider acceptance because the process is faster and easier to control and moreover precludes undesirable transformations. One other important advantage is that the processing is possible not only at 30°C, but also in the temperature range from 2°C to 15°C. The fermentation time will be longer in the lower temperature ranges. In the low temperature area the rapid bacterial development in the albumin which would occur at the higher temperatures is avoided.

The enzymatic fermentation involves adding of glucose oxidase, which through oxidation transforms the glucose into gluconic acid. The liquid egg products contain only a small percentage of the oxygen required for the glucose removal. Normally, hydrogen per-oxide is added to provide the extra oxygen.

In order to release the oxygen from that the enzyme catalase must be present. Normally, catalase has been added in the enzymatic agents prepared specially for egg products fermentation purposes. The catalase ensures at the same time that the hydrogen peroxide has fully decomposed on completion of the fermentation. The pH-value is reduced to 6.5-7.3 before the fermentation is started by adding for example nitric acid to albumin at the appropriate temperature. A portion of the hydrogen peroxide is then added followed by the enzymatic agent. Stirring must take place for proper dispersion of the agents. The foaming starts shortly after the enzymes have been added. Limited amounts of hydrogen peroxide are added continuously to the albumin under the fermentation process, which is completed within 10-16 hours. The doses are reduced stepwise according to plan. In this manner hydrogen peroxide is added according to the requirements only. Incessant stirring must be kept up to disperse the hydrogen peroxide and in order also to give the enzymes new operating fields. As formerly mentioned the development of the fermentation must be checked although there is little danger of undesired transformations particularly so at a low temperature. After the fermentation process the egg white is after-treated, filtered, and pasteurized.

Ammonia Treatment/Pasteurization

As mentioned the serum proteins in albumin are more sensitive to heat than those contained in whole egg and yolk, while on the other hand salmonellae and coliform are more easily killed in the albumin. Despite this, albumin cannot be heated to a temperature which is high enough to guarantee a safe or satisfactory product in respect of bacteria without a coagulation of the serum proteins.

Albumin must undergo some initial treatment prior to its pasteurization in order to ensure that there will be no survival of salmonellae and coliform bacteria. The purpose of the processing methods used is either to weaken the bacteria to such an extent that they will be killed at some lower temperature - or to stabilize the serum proteins so as to allow a raising of the temperature during pasteurization.

The ammonia treatment belongs to the former category. This treatment is given to fermented albumin before drying and takes place as soon as the fermentation process is completed. The albumin is transferred to a tank; ammonia water is added until the pH-value has risen to at least 10.3. The ammonia must be poured in with the agitator in motion. The admixing could be performed automatically by a pH-plant. The ammonia treatment is carried out at a temperature of 15°C min. and takes about 24 hours. This treatment is to be followed by pasteurization at 51-52°C for 3 minutes. Because of the low temperature there will be no coagulation of the serum proteins. This method will give by far the best end product - an egg albumin powder with unreduced whipping qualities. It is to be emphasized, however, that the method is applicable only if the albumin is going to be dried. The ammonia will evaporate in the drying process and is undetectable in the finished product.

Drying of Albumin

The drying plant for albumin has a drying chamber with conical bottom, a bag filter for separation of the fines from the drying air, and a pneumatic cooling/conveying system, as can be seen in fig. 2.

Contrary to the plant for whole egg and egg yolk, it is necessary to use a bag filter here to separate powder from the drying air due to the very small particle size of the albumin powder. Also in order to reduce powder loss from the cooling system, the air from the cyclone is conveyed back to the bag filter. The drying conditions of the pulp are approx. 200°C inlet and 80°C outlet temperature. The feed temperature will normally be about 10°C. After the drying, the powder will be packed in bags or cartons. For albumin powder dried on our plant we can give the following guarantees:

The bacterial quality depends on the pretreatment but will always be within the normal, commercial guarantee.

Pretreatment of Whole Egg and Yolk

We shall now deal with the pretreatment of whole egg and egg yolk pulp. The pretreatment of these products is fairly uncomplicated compared with the pretreatment of albumin.

After breaking/separation, the pulp is filtered and pasteurized as already dealt with in connection with pretreatment of fluid products.

In some cases depending on the final product specification, the pulp may, however, also be homogenized and fermented.

For whole egg and yolk the enzymatic process seems the most desirable. The other two processes may give disagreeable odours and flavours in the finished product, presumably because of lipase activity.

The pump and filter treatment ensures that the product is homogenous when it enters the fermentation tanks. For some uses, however, it appears necessary to homogenize whole egg and yolk prior to their fermentation. For this purpose a conventional dairy type homogenizer can be used.

The amount of enzyme and hydrogen peroxide added is adjusted according to the amount of glucose contained in whole egg and yolk, respectively. On adding the hydrogen peroxide it should be remembered that the foaming-up should be rather less than with fermentation of albumin.

Fermentation time is approximately 6 hours for whole egg and approximately 4 hours for yolk. Constant agitation is necessary during the process. When the glucose content has been reduced to the desired level, the product is pasteurized and dried.

The fermentation tanks must be insulated so as to ensure a constant temperature throughout the processing period. The agitator must be designed so as to handle the product gently. A pH unit for automatic checking and adjustment of the pH value is preferable also.

Drying of Whole Egg and Egg Yolk

The drying plant for whole egg and egg yolk consists of a drying chamber with a conical bottom, one main cyclone, and pneumatic conveying system as can be seen on fig. 1. The pulp will be dried at an inlet temperature of approximately 200°C and an outlet temperature of 80°C. The feed temperature will normally be about 10°C.

After the drying, the powder will be packed in bags or cartons. For powder dried on our plant we can give the following guarantees:

The bacterial quality depends on the pretreatment, but will always be within the normal commercial guarantees.

We have described the plants for whole egg/egg yolk and albumin as two separate plants. However, these two plants can be combined in one so that it will be possible to dry all 3 products in one plant. For bigger capacities, we recommend, however, to have two separate plants.

New Developments

Due to the low solids content of whole egg and especially albumin it has always been desirable to try to increase the solids content of the pulp. This problem has been tackled in different ways and recently a new development has come up. Whereas it has previously proved possible to increase the solids content of whole egg by more or less conventional evaporators where the evaporation takes place by means of the heat exchange system, this has not been possible for albumin, which is highly sensitive to the heat treatment. By means of ultrafiltration and reverse osmosis it is possible to double the solids content of albumin, and thereby obtain a saving in the cost of the spray drying plant, the capacity of which can be reduced as a consequence.

Within the field of after-treatment of the powder we are doing constant research and are in the progress of developing methods whereby it is possible to make a more free-flowing and instant product.

by Ib Haugaard Sørensen Niro A/S, Copenhagen, Denmark