Pretreatment of Albumin in Egg Powders
By: Ib Haugaard Sørensen Niro A/S, Copenhagen, Denmark
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.
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.
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.
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.
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:
|Residual moisture, max. g/%:||8|
|Bulk density, g/ccm tapped 100 x:||0.30-0.35|
|Whippability, mm:||min. 150|
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:
|Whole egg||Egg Yolk|
|Residual moisture, max. g/%:||4||4|
|Bulk density, g/ccm tapped 100 x:||0.30-0.35||0.30-0.35|
|Baking volume, ml:||min. 1000|
|Free fatty acid,%:||max. 3.5|
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.
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