1. INTRODUCTION
2. WHOLE EGG versus EGG YOLK
3. MICROENCAPSULATED DIETS MADE FROM WHOLE EGG
1/ This text was prepared after the Training CourseK. W. Chow
Food and Agriculture Organization
Rome, Italy
Egg is Unquestionably one of the most nutritionally balanced foods known for man and animals. Together with the facility with which it could be prepared and fed, and its general availability, egg was viewed with high expectations by early aquaculturists as a promising replacement for expensive live food such as Artemia in the feeding of very young fish. The ability of yolk and white to remain separate during cooking of unprocessed egg plus the fine granular texture of boiled egg yolk have indeed provided aquaculturists with a practical artificial diet that is superior to most other artificial feeds for this purpose.
Egg yolk per se is highly nutritious - perhaps more so than egg white. However, as a diet for very young fish its high energy-protein ratio could result in inadequate intake of protein necessary for maximum growth. At the same time, there are seemingly difficult problems concerning the preparation and feeding of whole egg to fish.
The ramifications of feeding egg to fish larvae will be explored in this chapter. Simple processing procedures for solving problems encountered in preparing a fish diet base on whole' egg will also be described.
2.1 Composition of Egg Components
2.2 Whole Egg versus Egg Yolk as Diet for Fish Larvae
Table 1 shows the composition of egg on a dry basis.
Table 1 Composition of (chicken) Egg
|
|
Whole egg |
egg white |
egg yolk | |
|
Protein, % |
48.8 |
76.9 |
32.8 | |
|
Fat, % |
43.2 |
- |
62.2 | |
|
Gross energy, kcal/kg |
5 830 |
3 070 |
6 910 | |
|
Metabolizable energy (ME), kcal/kg |
4 810 |
2 533 |
5 700 | |
|
ME: protein ratio |
9.8 |
3.3 |
17.3 | |
|
Calcium, % |
0.2063 |
0.0427 |
0.2653 | |
|
Phosphorus, % |
0.873 |
0.282 |
1.020 | |
|
Amino Acids, % | ||||
|
|
Arginine |
2.968 |
4.179 |
2.369 |
|
|
Cystine |
0.837 |
1.282 |
0.526 |
|
|
Isoleucine |
2.734 |
4.307 |
1.896 |
|
|
Leucine |
4.063 |
6.273 |
2.790 |
|
|
Lysine |
3.047 |
4.427 |
2.369 |
|
|
Methionine |
1.563 |
2.700 |
1.663 |
|
|
Phenylalanine |
2.500 |
4.427 |
1.316 |
|
|
Threonine |
2.500 |
3.692 |
1.843 |
|
|
Tryptophan |
0.837 |
1.350 |
0.577 |
|
|
Tyrosine |
1.952 |
3.076 |
1.316 |
|
|
Valine |
3.674 |
6.025 |
2.263 |
It is clear that egg yolk is the more energy dense food although its protein content is 33 percent below that of whole egg. This disparity is exemplified by the very high energy-protein ratio of egg yolk. Animals, fish not excepted, eat to satisfy their energy requirements (see Chapter 2). A fish's food intake increases or decreases corresponding to a reduction or an increase in the diet's energy density. For diets of fixed protein composition, the requirement for this particular nutrient may not be sufficiently met if the diet has too high an energy-protein ratio.
The following calculations compare whole egg with egg yolk as an artificial diet for fish larvae.
Assume that a diet containing metabolizable energy (ME) at 4.0 kcal per kg represents a practical reference diet, with respect to energy density, for fish larvae.
(a) Egg Yolk
Amount of egg yolk required to provide the same amount of ME as 1 kg reference diet
0.702 kg egg yolk contains
0.702 × 0.328 = 0.230 kg protein
Therefore a diet consisting of 0.702 kg egg yolk plus 0.298 kg inert filler will have the same energy density as the reference diet and will contain 23.0 percent protein. This dietary protein level, we know, is inadequate for rapid growth of young fish.
(b) Whole Egg
Amount of whole egg required to provide the same amount of ME as 1 kg reference diet
0.832 kg whole egg contains
0.832 × 0.488 = 0.406 kg protein
Therefore a diet consisting of 0.832 kg whole egg plus 0.168 kg inert filler will have the same energy density as the reference diet but will contain 40.6 percent protein.
Clearly, whole egg more adequately meets the protein level required in the diet of fish larvae.
Computations involving other nutrient parameters may similarly be made. Thus the whole egg diet will provide, per kg feed on an isocaloric basis, 15.47 mg and 2.30 mg of riboflavin and pantothenic acid, respectively, as opposed to 7.72 and 0.29, respectively by egg yolk.
3.1 Physical Properties of Eggs
3.2 Processing Techniques
3.3 Advantages of the Novel Diets
3.4 Formulating Encapsulated Egg Diets
Raw egg contains the growth inhibitor avidin which must be de-activated before the egg can be fed to fish. This is achieved by the application of heat. Cooking unprocessed egg, however, causes irreversible separation of the two egg fractions, yolk and white, due to denaturation of protein components in both fractions. Blending the two fractions, after cooking, info a homogenous water stable diet, requires the addition of suitable binders which add to cost.
The ideal product would be one that will possess the following characteristics:
(i) Wholesomeness- the product should be of high nutritive value and be free from the harmful effects of avidin(ii) Consistency
- size and texture of the product should favour ready acceptance by fish consuming the diet(iii) Water stability
- the product components, i.e. yolk and white, should not separate during feeding; feed particulates should have some buoyancy and remain in the water column for a sufficiently long period of time for the fish to eat it; the product should have minimum solubility in water but undergo some leaching of flavour to attract fish to the food(iv) Low biological oxygen demand (BOD)
- the product should not undergo rapid microbial degradation in water(v) Good shelf life
- the product should keep well under ordinary storage conditions(vi) Ease of preparation
- the product should be easy to prepare.
A hard boiled egg is a macroscopic representation of a product with such desirable properties. The egg eaten whole is highly nutritious and is free from avidin. The hardened albumin casing of hard boiled eggs prevents the separation of yolk. Heat denatured egg albumin is insoluble in water but some leaching of flavour occurs when the shelled, boiled egg is immersed in water for a period of time. Heat denatured protein is far more resistant to microbial action than is raw egg albumin. An intact boiled egg will stay 'fresh' for days under ambient conditions. Preparation of a hard boiled egg only involves the immersion of the unbroken raw egg in near boiling water for a short time period. In essence a hard boiled egg is a form of encapsulated food product in which the relatively unstable yolk is protected by a rather resistant albumin casing forming the capsule. If this can be brought down in scale, a similarly wholesome diet that is suitable for feeding fish larvae would be obtained.
When shelled eggs are placed in a glass container - taking special precaution not to damage the yolks - we have an intimate mixture of two substances, yolk and white (Figure 1).
Fig. 1. Unhomogenized whole eggs in container. Shaded spheres denote yolks.
The mixture is, in fact, a macro-colloidal system. The yolk constitutes the colloid, or dispersed phase, and, the white is the dispersing medium. This system also qualifies as an emulsion consisting of the stable dispersion of one liquid (fat) in a second immiscible liquid (water).
Egg yolk is more than 60 percent fat on a dry weight basis (see Table 1). This fat is interspersed with phospholipids, nucleoprotein and lipoproteins which impart physical stability to the yolk. When boiled egg yolk is examined closely, it can be seen that it consists of aggregates of tiny spherical droplets of fat encased in heat coagulated protein capsules. This thin coating is insufficient to protect the lipid from oxidation by the atmosphere thus leading to rapid onset of rancidity and mould formation on the exposed product.
When raw whole egg is homogenized, the yolk is broken up and an ordinary emulsion is produced. This emulsion consists of countless microscopic-sized fat droplets (yolk) dispersed in the same immiscible watery medium (white), Figure 2. Coagulation of the dispersing medium will permanently coat the tiny lipid spheres to produce a microencapsulated whole egg product. The procedures for this process are simple and will be described in the following section.
Fig. 2. Homogenized whole eggs. Inset shows globules of egg yolk magnified.
Micro-encapsulated whole egg feed may be made by any of the following three methods.
|
Method A |
|
|
Step |
Procedure |
|
1 |
Crack egg into heat resistant container. |
|
2 |
Beat egg vigorously with fork or paddle; egg may also be homogenized with a mechanical blender. |
|
3 |
Pour rapidly (approximately 150 cc for each egg) boiling water into homogenate, with constant stirring. A fine, opalescent suspension is obtained. |
|
4 |
Make up to desired volume with cold water. A 50 g egg contains about 12 g dry matter. |
|
5 |
Feed by the spoonful or scoopful directly to fish. The feed may also be applied as a spray using a 'knapsack' type sprayer. |
|
6 |
Store unused feed in a tight container in a refrigerator. |
|
Method B |
|
|
Step |
Procedure |
|
1 |
Same as Method A |
|
2 |
Same as Method A |
|
3 |
Pour homogenate directly into boiling water. Amount of stirring depends on feed particle size desired. |
|
Initially strands of cooked egg are formed. Dispersal of egg yolk remains the same. Yolk is still encased in denatured (cooked) albumin. |
|
|
4 |
Same as Method A |
|
5 |
Feed resuspended material directly to fish. Do not apply with sprayer as this will result in feed particles of undesirably small size for large-size fry. |
|
6 |
Same as Method A |
|
Method C |
|
|
Step |
Procedure |
|
1 |
Same as Method A |
|
2 |
Add 50 cc water and proceed as in Method A. |
|
3 |
Steam homogenate in same container or after transfer to appropriate dish or bowl. |
|
A custard made up of the solidified egg emulsion is formed. |
|
|
4 |
Feed custard directly to fish after breaking up into desired size. |
|
5 |
Store unused custard in tight container under refrigeration. |
Whole egg contains all the necessary nutrients needed during the first ten days of life of most species of fish. The diet prepared by Method A described in Section 3.2 possesses all the previously listed characteristics desired in feed for very young fish: i.e., for wholesomeness, consistency, water stability, low BOD, long shelf life, and ease of preparation. The opalescent protein coat of the micro capsule reflects enough light to attract fish to it. This also enables close monitoring of the feeding, process leading to reduction in feed wastage and maintenance of water quality.
The product from Method B is more suitable for larger fry and custard made with Method C is more suited to elvers which can be fed from a platform placed at the water surface.
Shelled egg is lacking in the water soluble vitamins, especially ascorbic acid. An egg-based diet fed to fry for an extended period should be supplemented with vitamins and perhaps also calcium during Step 2 of the preparative procedure of each method. Similarly, a product of lower protein content may be obtained by adding high carbohydrate ingredients such as wheat flour or cassava flour. These ingredients must be finely ground, however.
The product from Method A can be freeze-dried, after removal of the water by centrifugation or filtration, to obtain an amorphous, dry, encapsulated feed.
