Fats & Oils
Fruits & Vegetables
Hydrocolloids, Vegetable Gums
Meat, Fish, Poultry
Frequently Asked Questions
- use the Science of Foods terminology in relation to discussing foods
or food products.
- use the chemistry and composition of foods to explain how it relates
to the quality of a food product.
- discuss and problem-solve how processing and preparation procedures
impact food quality.
- utilize value-added products to develop unique quality products.
Modified from "USDA Egg Grading Manual" 1969. Agriculture Handbook
|% Composition Whole Egg
The diverse roles of egg in the recipes occur due to their particularly
high water solubility, excellent foaming and emulsification capacities,
heat coagulability, and high nutritive values. They contribute structure
to a product that contains egg.
The quality of eggs is dependent upon the egg itself. For example, in
hard cooked eggs the size of the air cell will impact whether it is oval
egg or has an indentation on the end, a "less than perfect" egg for "hard
boiled", deviled eggs. In custard sauces, the chalazae removal is thought,
by the true chef, to improve the quality of the product. For these reasons,
it is important to know the egg and factors which impact the quality of
the egg and resulting products.
Below is an image of the basic egg structure. It is made up of approximately
11% shell and 89% interior. The composition of the shell is important
from the viewpoint of food safety, sanitation, and esthetics. It contains
calcium carbonate (94%), magnesium carbonate (1%), calcium phosphate (1%),
and 4% organic matter. It is important to recognize that there has been
considerable information that the hen's diet can impact the composition
of the egg.
The composition of the egg yolk and white is of primary importance and
the chief factor affecting the way that eggs function in a baked product.
Eggs and egg whites have been used as a clarifier. The process is as
it sounds, the clearing of a liquid. Master chefs use it to clarify stock
to produce clear consommes.
|| In the old "campfire coffee", egg white was added to
coagulate around the coffee grounds and clarify it. Personally, when
I have tried it, the results have left something to be desired.
It is of interest that clarification is used for other items.
For example, many years ago we had a relative who would give us
gallons of clarified butter. This was butter that had been carefully
warmed to melt the butter fat. This fat was then drawn off and the
milk solids removed. The hypothesis was that it would keep "fresh"
longer. Actually, theory would support this antedoctal idea. With
drawing off the fat, the water and salts, syngeristic components,
would no longer be available to take part in the hydrolytic rancidity
Physicochemical Constants of Albumen and Yolk
Used with permission from John Morris
of the Donut Shop
| Egg whites, egg yolks, whole eggs, and egg washes
brushed on breads and other baked products add a rich, shiny glaze.
The glaze is caused primarily by the protein and fat interaction.
The simple glaze is generally made by adding a "little water" to the
egg and brushing it on. In this image, John Morris is brushing an
egg glaze on pigs (hot dogs) in cresants before baking.
|Electrical conductance(mho-cm-1 x 10-3)
|Heat of combustion(cal/gm
|Solubility coefficient for CO2
|Specific heat (cal/gmC)
|Specific resistance (ohm-cm)
|Surface tension (dyn/cm)
|Vapor pressure (in % of NaCl)
|Viscosity (poise at 0C)
|Latent heat (But/lb)
1 Vadehra, D.V. and K.R. Nath. 1973. Eggs as a source of protein. CRC
Critical Reviews in Food Technology 4: 193-309.
2 Cotterill, O.J. 1973. Egg Science and Technology Stadelman,
W.J. and O.J. Cotterill (eds), AVI Publishing, Westport, Conn.
Certainly the grade of the egg impacts its "quality"; however, there are
many other conditions. Eggs are graded as a freshly laid egg. This freshly
laid egg may be a AA or even a C graded egg to start with. However, all
eggs will begin to deteriorate upon storage. The question and problem is
to minimize storage losses. Storage in a room where temperature is maintained
slightly above the freezing point [-2C; 28F) of eggs and humidity of 90%
will maintain quality of eggs for several weeks. As temperature and/or humidity
increases, the storage time will decrease. The following table indicates
changes with storage and possible reasons for the change.
|Change in Egg
||Reason For This Change
|increase in pH
||Carbon dioxide diffuses out of the egg. This may cause
a rise from the fresh egg pH of 7.9 to as much as 9.3 in the white.
The pH of the yolk is initially around 6.2 with little rise in pH.
The carbon dioxide, a product of the metabolic pathways in the chicken,
forms carbonic acid and bicarbonate buffers. These no longer exist
when it diffuses out.
|thinning of "thick" membrane
|increase in air cell size
||This occurs between the two membranes at the shell interface.
Part of this is due to the natural cooling of the egg, with concurrent
shrinking of contents. Air is physically drawn in during this shrinkage.
|increase in yolk size
|thinning of thick white
|weakening of vitellenin membrane
|deterioration of odor and flavor
||This deterioration depends upon the storage conditions
and the reasons for storage changes. Storage does bring about slight
changes in the protein and fat of the egg which may contribute in
small part to the changes.
Images Showing Egg Quality
Quality of eggs has been defined and quantitated by the
USDA. Although any egg of any quality can be laid, a high quality egg
generally shows the same interior egg changes with storage as the decrease
in grade reflects. These grades give an indication of the quality of eggs
and, depending upon the product, may reflect the final quality of the
food product for which they are used. At this time, the grade is generally
assigned by a method called candling. This can be done as a automated
system where light is shined through the rotating egg to reveal condition
of the shell, the size of the air cell, and size, distinctness, color
and mobility of the yolk. Additionally abnormalities such as blood spots,
embryonic development, and spoilage are identified. The importance of
these conditions in a food product depends on the specific role and function
of the egg in the food product.
|| Shell: clean; unbroken, practically normal
Air cell: 1/8 inch or less in depth; practically regular
White: clear, firm, "upright"
Yolk: well centered;
outline slightly defined; free from defects
|| Shell: clean; unbroken, practically normal
Air cell: 2/8 inch or less in depth; practically regular
White: clear, may be reasonably firm
Yolk: may be
fairly well centered; outline fairly well defined; practically free
|| Shell: clean to slightly stained; unbroken, may
be slightly abnormal
Air cell: 3/8 inch or less in depth,
may be free but not bubbly
White: clear, may be slightly
Yolk: may be off center, outline well defined, may
be slightly enlarged and fattened, may show definite but not serious
||Shell: clean to moderately stained, unbroken, may be abnormal
Air cell: may be over 3/8 inch in depth, may be free or bubbly
White: may be weak and watery, small blood clots or spots may
Yolk: may be off center, enlarged and flattened,
may show clearly visible germ development but no blood; may show other
serious defects; outline may be plainly visible.
|| Modified from Hauver, W.E. Jr. and J.A. Hamann. 1961.
Egg grading manual. Agricultural Marketing Service No. 75. 52 pp.,
U.S. Department of Agriculture.
These differences can be seen by the images below:
| Broken Out Eggs Profile of Cut USDA Grade AA, A,
||Hard Cooked Eggs Profile of Cut USDA Grade AA, A,
|| US Grade AA
|| US Grade A
|| US Grade B
|| US Grade C
What are the grades and weight of eggs? Are they the same or different?
The weight of eggs has no relationship to grade and vice versa. The
major weight classes are:
Typical Egg Sizes
|| Grams Per Egg
When I was growing up, in the spring we would have pullet eggs. These
were eggs from hens, or pullets, that were just starting to lay. Typically,
they were small and of variable size.
The grades of eggs are: AA, A, B, C. Most of the eggs on the market
are AA, or A. The B and C eggs go in for pet foods and other egg products.
During storage, the grade of the egg may deteriorate, although with the
current knowledge of storage requirements, this is not necessarily an
absolute. Actually, the major deterioration may be due to improper handling
at home. It is important to know that not all eggs being laid are
grade AA. Some chickens are failures and lay poor quality eggs to start
with. This may be due to genetics or to the environment or feed.
The weight of the egg is important in that most recipes have been developed
on the basis of a 48 gram egg. Certainly, in a fried egg, scrambled eggs
or such, the size just means more. However, if one is looking at a product
with a balanced formula, such as cream puffs, muffins, an angel food cake,
the difference between a small egg versus a jumbo in the final product
could be dynamic. As far as selection of grade, as with weight, in some
products it makes a difference. In others there is none. Assuming a safe
egg, it makes no difference whether an AA or a B is used in a scrambled
egg. However, if being used in a sponge or angelfood cake one would have
a different yield and foam stability with the two grades. Certainly, with
a fried egg, sunny side up, the egg would be more attractive if a AA or
A egg was used.
The coagulation of eggs is critical to many food products. Coagulation
is simply the solidifying of the egg by the application of heat. In many
instances, the egg in a formula will serve to glue the product together.
For example, it is sometimes added to meat loaf or on the surface of okra,
in both instances serving as a glue. Certainly, the coagulation of the
white and yolk permits the "structure" of the product for deviled eggs.
A large portion of the discussion of stirred and baked custard hinges
on egg coagulation. It is of interest in the many products to know the
coagulation mechanism of the yolk and the white.
Following are the general temperatures at which various egg parts and
egg products will coagulate:
-egg white 60-65C
-egg yolk 65-70C
Generally, whole egg begins to become opaque at around 60C and increases
in viscosity to 72C. At 75C it is a soft curd and increases in firmness
up to 87C. Certainly, heat is the critical factor in bringing about
the denaturation of the egg protein and forming structure. However,
heat has many other causative factors. It should be remembered that
heating the egg product itself, if (liquid or dried form) has been done
during the pasteurization of eggs. This heat will decrease the functional
properties of these eggs.
Again, one needs to be reminded that the rate of heating, the added
ingredients, the concentration of the egg and other subtle factors will
impact the range of temperature and the optimum endpoint. The work on
model systems of eggs by Pawayal et al.  essentially appears to
indicate that there is a relationship between time and rate of heating.
The faster the mixture heats the lower the point of increased viscosity.
Practically speaking, what are the implications of the differing
coagulation points of the various egg parts?
When most consumers talk about egg products they likely are thinking
about the whole shell eggs and the many different foods. Neff [Neff,
J. 1998. Foods of Tomorrow. The Great Egg Breakthrough. Food Processing
59(1): 25.] indicates that in 1997 there were 6.443 million dozen eggs
in the shell and 1.621 million dozen breaker eggs consumed. The per
capita egg consumption was 171.5 shell eggs and 66.5 breaker eggs. However,
there are other egg products used by the consumer, foodservice industry,
and value-added portion of the food industry. Most consumers are well
aware of the market for egg substitutes and egg replacers.
Although one should review egg selling sites to get the latest on
available products, some of the different egg products are as follows:
Dried Eggs: (blends of whole egg and/or yolk with sugar or
corn syrup) added, flake albumen, free-flowing whole egg or yolk solids,
instant egg white solids, spray-dried egg white solids, stabilized whole
egg yolk solids (glucose free), whole egg or yolk solids
Frozen Eggs: egg white, egg yolks, salted whole egg, salted
yolks, sugared yolks, whole eggs, whole eggs with com syrup, whole eggs
with added yolk (fortified), whole eggs with yolks and corn syrup, yolk
and white blends w/wo sweeteners or salts
Refrigerated Liquid Egg Products: egg whites, egg yolks and
various mixtures of these.
Specialty Products come in a wide variety of forms. Some of
my favorites are the precooked egg yolks/whites and cooked egg white
with a center tube of cooked yolk. There are boiled eggs available for
purchase by institutions. One can order the "real egg" in about any
There are many products that serve as egg substitutes or replacers.
Eggs have also been used as substitutes for other ingredients. Neff
reported the work of Dan Neumeister in developing a heat-denatured egg
yolk to substitute for potassium bromate to improve the texture and
outward appearance of bread.
What is the effect of the egg product in a custard? Funk et al. 1969
has plotted the heat penetration of custards as essentially rise steadily
to 74C and than slowing down in rate until approximately 82C and rising
again to above 87C. They observed that the type of egg processing [foam-spray,
frozen, freeze, spray-dried] did make a slight difference in the heating
rate. Workers have hypothesized that the actual coagulation occurs as
a two step process. The first step is the actual denaturation of the
protein and the second step is the protein-protein interaction and the
formation of a gel or increased viscosity. Interesting, Lowe 
observed that forewarming of the milk may decrease curdling and shortening
cooking time. However, it is important to remember that 0.75% of the
milk protein is heat coagulable.
Which of the following are egg custards?
A custard is made of egg, milk, sugar and usually salt and
flavoring. Through application of heat and manipulation, custards can
become a viscous sauce or a semi-rigid gel. In each case, denaturation
of the egg protein, ovalbumin, conalbumin, and ovoglobulin is primarily
responsible for the thickening of the custard. Stirred custards are
cooked on top of the range while gel type custards are usually baked.
Custards may be altered by manipulation or ingredient variation. Because
egg is the primary structural ingredient of a custard, some differences
are seen when fresh, frozen, old, dried or egg substitutes are used.
The egg source, for example duck or turkey, also influences custard
quality. Milk, although not as structurally important as egg, contributes
to the viscosity or gel strength of the finished product. Calcium ions
present in the milk are needed in the formation of a thicker custard,
as custards made with water will not gel or thicken. Differences in
processing will also influence the custard quality. Nonhomogenized milk
produces a baked custard with a thinner crust, more delicate browning
and better sheen than those custards made with homogenized milk. For
example, stirred custard usually is considered to have "more body".
Sugar is also important to the viscosity and gel strength of custards.
Sugar tends to increase the denaturation temperature of the egg proteins
resulting in a less stiff product. Salt and flavoring have no appreciable
affect on custard quality other than for taste.
The proportion of ingredients is important to custard quality. The
concentration of egg protein is proportional to the viscosity or gel
strength of the custard. With increasing concentration, a custard sauce
becomes more viscous and the gel strength of a baked custard firmer.
Also, with increased egg protein, the product becomes more sensitive
to end point temperature. End point temperature is the point at which
optimum denaturation has occurred without curdling or syneresis. Milk
serves to dilute the egg protein so less viscosity and gel strength
are observed with increasing proportions of milk. Because sugar increases
the denaturation temperature of egg proteins, increased sugar concentrations
results in softer custards. At a 30% sucrose concentration, a custard
will not gel at all.
Producing a good quality custard depends on a number of factors. In
addition to the proportion and ingredient variation, temperature and
rate of cooking are important. A custard heated slowly begins to thicken
at a lower temperature, thickens gradually over a wider temperature
range and reaches doneness at a lower temperature. On the other hand,
a custard heated rapidly must be heated to a higher temperature before
thickening begins and overcooking, resulting in curd formation or porosity,
occurs easily. Syneresis or weeping results as the curds separate from
the serum. Slow cooking can be achieved by placing the baked custard
mixture in a waterbath in the oven or by using a double boiler with
a stirred custard. Endpoint is indicated when the stirred custard "coats
the spoon", or when a knife inserted into the baked custard "comes out
| Which of the following custards was cooked the
Which of the following custards could
represent the custard having the greatest amount of sugar?
Which of the following custard could likely represent
the custard having egg white substituted for the whole egg?
When one turns to a recipe or formula and it lists eggs as an ingredient,
it is usually understood to be whole chicken eggs. The egg itself and
the method of processing of the food product it is in will make a difference
in the reaction of cooking. The effect of ingredients in products will
make a difference. Certainly it will vary depending upon the product
and the function of the egg. Following are some affects.
|| Milk's contribution in egg products such as
custards is underestimated. As early as 1942, Carr and Trout reported
research that indicated the cooking time for custards made with
homogenized milk took 15 to 20 minutes longer than custards made
with nonhomogenized milk. Heat penetration was slower. Additionally,
viscosity and gel strength was greater with homogenized milk. However,
this webber has observed that milk's effect on egg gelation is not
well documented. Milk contains electrolytes which will enhance viscosity
and gel strength.
|| Sugar will inhibit and delay denaturation of
the egg proteins, depending upon the product. In a custard, it delays
coagulation so that equivalent thickening (from the standard) will
occur only at a higher temperature, if at all. In the production
of foams, it has been shown that adding sugar will delay the formation
of an egg white foam. However, once a foam is formed, it is more
|| A variety of factors may influence the pH of
eggs. Shimada and Matsushita (1980) observed that pH and protein
concentration affected coagulation of egg albumin. The net charge
of egg albumin increased up to pH 8, plateauing up to pH 11 at which
time no coagulation occurred. The charge would increase as concentration
increased. This is due to the change of the proteins themselves.
Certainly if one adds acid to a egg containing product it will coagulate
faster. This effect is often utilized. For example, angelfood cake
foam has lemon juice or cream of tartar added as a stabilizer.
| The amount, how it is processed and type of eggs
makes a difference. The part of the egg will impact the functional
properties within an egg product. Researchers have determined that
egg yolk will also coagulate, beginning at ~65C, losing fluidity
at 70C and coagulating at 85C. Woodward and Cotterill (1987) felt
that the major denaturation occurred in egg yolk at 68C to 70C.
Egg white will coagulate approximately 5C higher. As with egg white,
there was significant pH-temperature interdependence. Other workers
[Shimada and Matsushita, 1980] have observed that coagulation increased
with increased protein level and concentration. This is of particular
importance when using selected fluid or dried egg products.
The method of processing the egg makes a difference. Research
done by Funk et al. in1969 plotted the heat penetration of custards,
rising steadily to 74C and then slowing down in rate until approximately
82C and rising again to about 87C. They observed that the type
of egg [foam-spray, frozen, freeze, spray-dried] did make a slight
difference in the heating rate. Another study conducted by Downs
et al  observed that spray-dried eggs tended to coagulate
faster than frozen, freeze-dried and foam-spray-dried eggs.
Powrie, W.D., H. Little, and A. Lopez. 196 . Gelation of egg yolk.
Journal Food Science :38.
Egg safety is critical to minimize the problems of food infection
and poisoning. There are a number of truisms necessary:
Start with washed and sanitized eggs.
Keep the eggs refrigerated
Do not place eggs on a surface or in a container unless it is washed
before further food use
albumen: Variant spelling of albumin, used generally to mean white of egg.
albumin: A group of simple proteins composed of nitrogen, carbon, hydrogen, oxygen, and sulfur that are soluble in water. Albumin is usually derived from egg white and employed as an emulsifier in foods and cosmetics. May cause a reaction to those allergic to eggs, and in large quantities can produce symptoms of lack of biotin, a growth factor in the lining of the cells. Often used as a non-specific term for protein (e.g. albuminuria means protein present in urine), but strictly refers to simple proteins soluble in water and coagulated by heat, such as ovalbumin in egg, serum albumin in blood, lactalbumin in milk.
beading: the appearance of tiny droplets of syrup on the surface of a baked meringue as it stands
chalazae: is the thick "cord" that anchors the yolk to the egg shell.
clarify : to make clear a cloudy liquid such as heated soup stock by adding raw egg white and/or egg shell; as the proteins coagulate, they trap tiny particles from the liquid that can then be strained out. Is, broadly speaking, a process by which some harmless matter is added to, or some harmless process acts upon, a liquid body in such a manner that a fine insoluble network is formed throughout, which either settles down or rises as scum, entangling the opaque particles, however minute, that are floating about and constituting the cloudiness; these the network bears with it in its upward or downward progress. The process of clarifying is therefore purely mechanical.
coagulate: to form a clot, a semisolid mass, or a gel, after initial denaturation of a protein; to produce a firm mass or gel by denaturation of protein molecules followed by formation of new crosslinks
of tartar: is a white, crystalline, acidic compound obtained as a by-product of wine fermentation and used chiefly in baking powder. is the acid salt of tartaric acid which occurs as crystals or powder. It has a solubility of one gram in 165 mL of water at 25C and one gram in 16 mL of boiling water. The acidulant is used in chemical leavening to release carbon dioxide which produces the loaf volume. It has limited reactivity in the cold so when used in reduced temperature, batters has little gas evolution during the initial mixing. At room temperatures, it has a relatively fast reaction rate. It is used in baked goods, crackers, and candy. The chemical name is potassium acid tartrate, potassium bitartrate.
potassium acid tartrate, the partial salt of tartaric acid, an organic acid; the acid salt of tartaric acid; a weak acid substance commonly added to fondant to produce variable amounts of invert sugar from the hydrolysis of sucrose.
custard: May refer to custard powder, or to egg custard. Egg custard is composed of milk and egg cooked together. Is a cooked or baked mixture consisting mainly of eggs, milk, and sugar. A custard is made of egg, milk, sugar and usually salt and flavoring. Through application of heat and manipulation, custards can become a viscous sauce or a semi-rigid gel. In each case denaturation of the egg protein, ovalbumin, conalbumin and ovoglobulin, is primarily responsible for the thickening of the custard. Stirred custards are cooked on top of the range while gel type custards are usually baked.
Custards may be altered by manipulation or ingredient variation. Because egg is the primary structural ingredient of a custard, some differences are seen when fresh, frozen, old, dried or egg substitutes are used. Eggs other than form a chicken, for Example duck or turkey, also influence custard quality. Milk, although not as structurally important as egg, does contribute to the viscosity or gel strength of the finished product. Calcium ions present in the milk are needed in the formation of a thicker custard, as custards made with water will not gel or thicken. Different processed milks will also influence the custard quality. Nonhomogenized milk produces a baked custard with a thinner crust, more delicate browning and better sheen than those custards made with homogenized milk. The stirred custard usually is considered to have "more body". Sugar is also important to the viscosity and gel strength of custards in that sugar tends to increase the denaturation temperature of the egg proteins resulting in a less stiff product. Salt and flavoring have no appreciable affect on custard quality other than for taste.
denaturation: a change in the molecular structure from the native structure of a protein. Is a change in a protein molecule, usually by unfolding of the amino acid chains, with a decrease in solubility.
linkages: bonding through two sulfur atoms (-s-s-)
eggs: generally from fowl.
grades: For chickens, AA, A, B, C.
egg wash: egg yolk and/or white mixed with a small amount of water or milk and brushed over a bread product prior to baking. An egg wash gives color and gloss to the product.
egg white: 87.8% water, 10.8% protein, 0.6% ash. Composed of outer layer of thin white, layer of thick white, richer in ovomucin, and inner layer of thin white surrounding the yolk. Eggs vary in ratio of thick to thin white, depending on the individual hen. Higher percentage of thick white desirable for frying and poaching (helps the egg to coagulate into small firm mass instead of spreading); thin white produces larger volume of froth when beaten than does thick. Proteins are ovomucin, ovalbumin, ovomucoid, ovoglobulin and conalbumin.
egg yolk: is a yellow portion of the egg, representing approximately 35% of the edible egg. It is composed of approximately 49% water, 16% protein, 32% fat and trace carbohydrate. It is used as an emulsifier in mayonnaise, salad dressing, and cream puffs. It is also used as a source of color.
emulsion: the dispersion of one liquid in another with which it is usually immiscible, for example, oil in water. An emulsion is a mixture of two or more normally nonmixable liquids shaken so thoroughly together as to appear homogenized, for example, as is done with salad dressing. Most oils form emulsions with water.
flocculated: is separated into small woolly or fluffy masses.
foam: is the dispersion of a gas in a liquid such as a beaten egg-white mixture.
gel: A sol or colloidal suspension that has set to a jelly.
Acolloidal dispersion that shows some rigidity and will, when unmolded, keep the shape of the container in which it had been placed; a semi-rigid structure at room temperature. This dispersion does show some rigidity or moldability.
lipoproteins: are proteins combined with lipid or fatty material such as phospholipids.
efficiency ratio: A measure of the nutritive value of proteins carried out on young growing animals. Is defined as the gain in weight per gram of protein eaten. The maximum values, e.g. egg protein, are about 4.4. Zero values are obtained for those proteins which, when fed alone, do not permit growth, but may still have some limited value.
sol: A colloidal solution, i.e. a suspension of particles intermediate in size between ordinary molecules (as in a solution) and coarse particles (as in a suspension). A jelly-like sol is a gel.
souffle: is a cooked heated egg foam with whitesauce as a stabilizing. Generally has meat, cheese, or some other flavoring as well.
activity: is the lowering of the surface tension of a liquid because of agents that tend to concentrate at the surface.
tension: is the tension or force at the surface of a liquid that produces a resistance to spreading or dispersing; due to the attraction of the liquid molecules for each other.
syneresis: is when two or more factors act co-operatively, so that their combined effects when acting toghether exceed the sum of their effects when each acts alone.
viscosity: Term used of liquids to define their resistance to flow (i.e. the internal friction).
Viscosity is a critical factor in use of vegetable gums. The readings below only reflect the actually viscosity indicated by the ability of stress and strain from an instrument. Hydrocolloids may contribute a sliminess to a product which changes their viscosity. For example, carboxy methyl celluose, locust bean gum, methyl cellulose, pectin, and sodium alginate are considered to be slimy hydrocolloids. Carrageenan, guar, karaya, and tragacanth are less slim and starch is nonslimy.
vitelline: generally refers to the membrane or proteins of concern to the yolk.
Updated: Wednesday, May 23, 2012.