Cheese
& Yogurt
CE 457/527
Adam Cummings
David Hess
Bryan Patel
Hongwang Zhang
INTRODUCTION
Cheeses and yogurts are complex colloidal suspensions made from animal's milk. The colloidal suspension of milk can be described an oil in water emulsion, with insoluble proteins forming complicated micelles in a water solution. For cheeses, the insoluble proteins and fats, called curd, are taken out of the water phase, also known as whey. The coagulated insoluble protein becomes what we call cheese. Much like cheese, yogurt is also formed from the coagulation of the water-insoluble proteins into a gel matrix.
History/Origins of Cheese
Cheese was probably first discovered in the Middle East by nomads. There is a story that a nomad stored milk inside of a saddlebag that was made of an animal’s stomach. The stomach in turn contained rennet, which in conjunction with the heat of the desert sun and agitation from the galloping horse coagulated the milk into a primitive cheese. Greeks contend that Aristaeus son of Apollo and Cyrene first produced cheese. At any rate, cheese first truly became an art with the Romans. Cheese making was done with precision and knowledge from experience, thus allowing for different flavor and texture characteristics in the cheese.[1]
Anne Pickett of Wisconsin founded the first true cheese business in America in 1841. Pickett got her start by using the milk of a neighbor's cow. Following her lead, by 1880, 3293 cheese producing facilities sprang up around the United States.[2]
Types of Cheese
Since there are a multitude of different cultures in the world, it only seems appropriate that there be many different types of cheeses. Cheeses range from cottage cheese all the way to everyone's favorite: Cheddar. These cheeses are classified by the processes used to make them, as well as by their actual composition and physical appearance. For our purposes we will only look at natural cheese. Natural cheeses are cheeses that are formed through the use of natural processes and do not involve any synthetic ingredients. However, it should be noted that some of the proteins and agents used to form the cheese are not always 100% natural, and so those may be synthesized.
Hard Cheeses
Hard cheeses are those that are not easily spread. They include the almost brick-like Parmesan. These cheeses have a longer shelf life than softer cheeses. Some harder cheeses, such as Parmesan or Romano will have almost infinite shelf lives.
Semi-Hard Cheeses
These cheeses are not easy to spread, but are not nearly as bricklike as hard cheeses. Semi-hard cheeses include the likes of cheddar. Cheddar cheese has a typical shelf life of about two to three months.
Soft Cheeses
Soft cheeses are relatively easy to spread. These cheeses include Brie. Brie should be consumed as soon after ripening as possible due to its short shelf life. In general, this group is very perishable and will age rapidly during production.
v Cheese can further be classified due to the processes that form it.
Curd Particles Matted Together
This type of cheese includes cheddar. Basically, milk is heated and constituents are added in order to separate the curd and whey. The individual curd particles are then removed from the watery whey. This curd contains the insoluble proteins of the milk, and the fat particles. Next, the curd particles are spread onto a platform or table and are matted together into sheets. The larger slabs are then cut into smaller slabs and are ready for aging and then finally shipping to vendors.[3]
Curd Particles Kept Separate
Colby cheese is the general example
of a cheese that is formed by keeping curd particles separate. The whey and curd are formed as usual using
the necessary separating constituents.
Curd is removed from the whey, however this time the curd is constantly
stirred in order to prevent matting between individual particles. From this process comes a cheese that is
very similar in taste to that of Cheddar, but does not taste as strong.3
Bacteria-ripened Throughout Interior with Eye Formation
Everyone knows that Swiss cheese
has holes in it. The holes aren't there
because some mouse ate through the cheese, as cartoons would have us
believe. The holes come from the
anaerobic respiration of bacteria.
Bacteria are added to the milk at the right time in coagulation. The curd formed is then cut into small
pieces and is constantly stirred in order to hinder the matting effects of the
cheeses. Once the curd has become firm
the cheese is allowed to set and ripening starts to take place. Thus, the
bacteria still inside of the cheese form the well-known holes in the cheese.3
Prolonged Curing Period
Parmigiano
Reggiano (Parmesan) is the most famous cheese of this type. It is a grana (hard grating) cheese that
most of us will put on our pasta. It is
made usually by whole and nonfat milk.
After some time bacterial starter and coagulating enzyme is added. Curd starts to form. These particles are then cut into tiny
pieces and are stirred for about forty-five minutes. The curd is then drained and salted in brine for about fourteen
days. Once the cheese is removed from
the brine, it is dried for roughly ten days.
Finally this cheese is ripened in a cool room for months. The rind formed on the cheese is typically
treated with olive or vegetable oil.3
Pasta Filata
Both Provolone and Mozzarella fall
into this category. Both cheeses are
formed in a process similar to Cheddar’s.
Milk is separated using the proper constituents and the curd is removed
from the whey. At this point, cheddar
would be matted into a large brick. Provolone and Mozzarella take a different
route. Instead of being matted, these
cheeses are stretched, kind of like stretching pizza dough. They are then formed into desirable shapes,
most commonly a ball, and the cheese is brined. After brining the cheese is then left to properly age.3
Mold-ripened Throughout Interior
Mold spores are added to milk used
in the production of these cheeses. The
classic mold used is Penicillium roqueforti. This mold results in a blue to blue-green colored veining
throughout the cheese. The curd that is
formed after coagulation is then treated in much the same way that cheddar
is. It is not matted, but instead is
put into metal hoops for draining. The
cheese is then salted over the period of about seven days. Then the cheese is punctured with needles
and allowed to aerate. As aging is
taking place, the cheese is periodically scraped to remove any harmful or
unwanted microorganisms. Cheeses of
this type are typically called blue cheeses.3
Surface-ripened Principally by Bacteria and Yeast
Limburger cheese is an example of this type of cheese. These cheeses are usually formed by whole milk that is coagulated by an enzyme that forms a jelly-like curd. The curd is cut into pieces and then stirred as to prevent any matting that may take place. The cheese is then sent to be ripened. The ripening takes place in a room that exposes the cheese to Bacterium linens. Within time, the yeast will make the cheese favorable for bacteria development. Thus, the bacterium in the room deposits itself onto the cheese and allows for the cheese to form a nice reddish-brown rind.3
Surface-ripened Principally Through Mold
Camembert and Brie cheeses are
formed in this manner. Mold, bacteria,
and even yeasts ripen Brie, separating the curd from the whey to forms the
cheese. The curd then is dried for a
long time. During the drying process
the mold is either rubbed or sprinkled with salt. The mold can also be introduced through the milk itself. If mold is added to the milk, then there can
be added flavor. A rind is then formed
due to the addition of mold to the surface of the cheese.3
Curd Coagulated Primarily by Acid
It is obvious that cottage cheese
and cream cheese are very different from cheddar or Munster. Cottage and cream cheese are coagulated
using acids. Acids are added to milk
and the curd is coagulated in a process described later in the report. The curd is separated from the whey and is
then cut and washed. While the curd is
firm, it may be salted. These types of
cheeses need to be refrigerated and typically do not have as long shelf
life. Their ripening periods generally
take only a few hours, in comparison to the days or even months that are
necessary for other types of cheeses.3
Whey Cheeses
Ricotta and similar cheeses are
made by concentrating the whey. The
whey is then coagulated using heat, in a process described later in the
report. From this coagulation a cheese
is formed that is used wet or is dried for grating.3
Cheese Facts of America
Cheese is a large, booming industry in the United States. Cheddar is the king of all cheeses in our country. It is produced more than any other cheeses, resulting in 1,195,705 metric tons in 1997. This total is roughly one third of the entire cheese producing market within the United States. The second-most produced cheese is Mozzarella. Again, nearly one third of American industry produces the pizza topping.[4]
America is the world's largest manufacturer of cheese, with France a distant second. American production accounts for about a quarter of the world's supply. A large reason is because of the huge technological advances that the United States has made the dairy industry. It is because of these advances that America is not only the world's largest producer but also its most efficient. Cheese produced in America is by far the safest product in the world. Due to the extremely rigorous health regulations, production conditions in America are by far the most sanitary in the world.
A
recent major innovation is that of IQF cheese.
IQF, or Individually Quick Frozen, is cheese that has been shredded and
the individual shavings have been frozen.
This technique allows for better preservation as well as excellent
quality once the product is defrosted.
IQF cheeses can be utilized in the pizza industry.2
America has been a haven for creating new cheeses. Among the popular cheeses that were found in the United States are cream cheese, Monterey Jack, Colby, and Brick cheeses. Most of these cheeses are formed from cow’s milk and natural processes.
Nutritional Information
Cheeses and yogurts are one of the best sources of protein and calcium. It is obvious, since it is a part of the dairy section of the food pyramid, below. The food pyramid calls for two to three servings of dairy products per day. A single serving of cheese is approximately 1.5 to 2 ounces, while one serving of yogurt is 8 ounces.
Figure 1 – Food Pyramid5
CNN posted a study on cheese nutritional information. A sample of this nutritional information for semi-hard cheeses is as follows.[5]
|
ITEM |
SERVING |
||||||
|
AMERICAN, PASTEURIZED*
|
1 oz. |
106.5 |
9g |
5.5g |
6.5g |
405.5mg |
174.5mg |
|
BLUE |
1 oz. |
100 |
8g |
5.5g |
6g |
395.5mg |
149.5mg |
|
BRICK |
1 oz. |
105 |
8.5g |
5.5g |
6.5g |
158.5mg |
191mg |
|
BRIE |
1 oz. |
94.5 |
8g |
5g |
6g |
178.5mg |
52mg |
|
CAMEMBERT |
1 oz. |
85 |
7g |
4.5g |
5.5g |
238.5mg |
110mg |
|
CARAWAY |
1 oz. |
106.5 |
8.5g |
5.5g |
7g |
195.5mg |
191mg |
|
CHEDDAR |
1 oz. |
114 |
9.5g |
6g |
7g |
176mg |
204.5mg |
|
CHESHIRE |
1 oz. |
110 |
8.5g |
5.5g |
6.5g |
198.5mg |
182.5mg |
Table 1 – Hard Cheese
Nutritional Data5
From this data, one can see that Cheddar is high in sodium and calories, but is also high in protein and calcium. More nutritional information can be found in Table 2 in the Appendix.
Yogurt has much of the same characteristic nutritional value as cheese. YoplaitTM gives nutritional information for yogurt showing that it is low in sodium and high in calcium. A sample of YoplaitTM’s nutritional information follows. More complete nutritional information can be found in Figure 2 in the Appendix.
|
|
Serving |
* |
Cal |
Fat |
Total |
Sat. |
Cholest. |
Sodium |
Potas- |
Total |
Dietary |
Total |
Protein |
|
99% Fat Free Fruit Flavors(1) |
6
oz. (170.1g) |
|
170 |
15 |
1.5 |
1 |
10 |
80 |
260 |
33 |
0 |
27 |
5 |
|
%DV |
|
|
3 |
5 |
3 |
3 |
7 |
11 |
0 |
|
11 |
||
|
Coconut Cream Pie |
6
oz. (170.1g) |
|
190 |
25 |
3 |
2 |
10 |
85 |
260 |
34 |
0 |
27 |
5 |
|
%DV |
|
|
5 |
11 |
4 |
3 |
7 |
11 |
0 |
|
11 |
||
|
Lemon |
6
oz. (170.1g) |
|
180 |
15 |
1.5 |
1 |
10 |
80 |
250 |
36 |
0 |
31 |
5 |
|
%DV |
|
|
3 |
5 |
3 |
3 |
7 |
12 |
0 |
|
11 |
||
|
Pina Colada |
6
oz. (170.1g) |
|
170 |
15 |
2 |
1.5 |
10 |
95 |
250 |
33 |
0 |
28 |
5 |
|
%DV |
|
|
3 |
6 |
3 |
4 |
7 |
11 |
0 |
|
11 |
||
|
99% Fat Free Fruit Flavors(2) |
4
oz. (113.4g) |
|
110 |
10 |
1 |
0.5 |
5 |
55 |
170 |
22 |
0 |
18 |
4 |
|
%DV |
|
|
2 |
4 |
2 |
2 |
5 |
7 |
0 |
|
7 |
Table 2 – YoplaitTM Yogurt Nutritional Information66 (See Appendix)
Chemical and Biological Components
Cheese and yogurt get their properties from the microorganisms, fats, proteins, and carbohydrates that are present in milk. They can be made from bovine, water buffalo, and even goat milks. Each of these milks contains the required protein and fat composition necessary to generate the needed environment for cheese and yogurt fermentation. These milks tend to be opaque due to tiny water insoluble proteins called caseins (roughly 100 nm in diameter) and fat globules. Caseins and globules scatter light passing through the liquid, giving milk its “solid white” appearance. Milks that have a high fat content tend to have a slight yellow hue from the larger fat globules. These globules usually average 3 to 5 microns in diameter. During fermentation, it is the fat and casein that coagulate to form the curd. What is left is a watery liquid called whey. Whey is very high in water-soluble proteins: the main constituents being a-lactalbumin and b-lactoglobulin. Besides proteins and fats, milk is high in sugar (lactose), minerals, and vitamins.
Fats/Lipids[6]
Milk is most commonly sold on the basis of fat content (most commonly seen in the grocery store as 2%, whole, etc.). Fatty acids of milk are formed from microbial activity in the rumen. These acids are transported through the blood and lymphatic systems of the cow by the secretory cells. Most milk lipids are triglycerides. A triglyceride is made of a glycerol backbone with three individual fatty acids that are bound to it. Each of these fatty acids contains a hydrocarbon chain and a carboxyl group. Most milk fatty acids fall into the following categories. The first category is long chain acids, which consists of Myristic (C14), Palmitic (C16), Stearic (C18), and Oleic (C18). The next category is short chain, which contains Butyric (C4), Caproic (C6), Caprylic (C8), and Capric (C10). The fatty acids that do not fall into either of these categories fall are classified as either phospholipids or lipoproteins.
Since milk has a low acidity, it shows that mono- and di-glycerides are almost absent. This shows that the tri-glycerides are the most common fats found within the milk system. Milk can be centrifuged to determine its components. When put through such a device, the milk will separate into a slimy substance that is primarily fat. This substance is water-insoluble and works with the proteins to form curd that can be processed into cheese.
Approximately 0.1568 percent of the slime, or about 27 percent of the total solid content, is cholesterol. Cholesterol is not soluble in water, but is extremely soluble in hot alcohol or various other organic solvents. These types of fats have a melting point of about 146oC.
Phospholipids are fats that will add stability to the final cheese product. They tend to contain lecithin acids that occur naturally within the milk.[7]
Proteins7
All proteins have a basic structure that consists of a polypeptide chain of amino acids that is joined to other chains though peptide linkages. This basic amino acid sequence forms the protein’s primary structure. Each protein can be cross-linked through disulphide bridges.
The three-dimensional conformation of proteins is divided into three structural levels, secondary, tertiary, and quaternary. The secondary structure refers to amino acids that are very close to each other and arranged in a linear order. “The a-helix and b-pleated sheet are examples of secondary structures arising from regular and periodic steric relationships.” The tertiary structure refers to the distanced amino acids in the linear sequence of the proteins. The proteins go through coiling and folding. “If the protein is tightly coiled and folded into a somewhat spherical shape, it is called a globular protein.” Long polypeptide chains linked together are called fibrous proteins. The quaternary structure occurs when multiple polypeptide chains form the amino acid. “The nitrogen content of milk is distributed among caseins (76%), whey proteins (18%), and non-protein nitrogen (NPN) (6%).”
Ø Casein[8]
The most abundant protein in milk is the casein protein. It comprises about 80% of all the proteins present in milk. There are four main fractions of casein: alpha1 (as1), alpha2 (as2), beta (b), and kappa (k) caseins. The distinguishing property of all caseins is their low solubility at pH 4.6. Caseins are primarily conjugated proteins containing phosphate groups. The phosphate groups are vital to the conformation of the micelle formed by casein. Calcium binding by the individual caseins is proportional to the phosphate content.9 The conformation of casein micelles is similar to the globular proteins. The secondary structure contains a high number of proline residues, which causes particular bending on the protein chain9. These residues inhibit the formation of close-packed, ordered structures. In fact, “this lack of a tendency to form typical globular structures in spite of their predominantly hydrophobic character is what marks out the caseins as a unique group of proteins.”[9] The lack of tertiary structure accounts for the stability of caseins against heat denaturation because there is little structure to unfold. The absence of a tertiary structure leads to the formation of hydrophobic accumulations.9 This creates a stronger interaction between the caseins and promotes insolubility in water.[10]
|
Description
of the four fractions of caseins:7 |
||||
|
|
|
|
|
|
|
Casein |
Molecular Weight |
Residues |
Proline Residues |
Description |
|
as2 |
23,000 |
199 |
17 |
- Two
hydrophobic regions which contain the proline residues - polar
region with phosphate groups - can be precipitated at low
calcium levels |
|
as1 |
25,000 |
207 |
10 |
- can be precipitated at low levels of
calcium |
|
b |
24,000 |
209 |
35 |
-
amphiphilic protein acts like detergent molecule - less
sensitive to calcium precipitation |
|
k |
19,000 |
169 |
20 |
- rennet
cleavage leaves hydrophobic part, para-kappa casein, hydrophilic part (casein
glycomacropeptide) -
resistant to calcium precipitation |
Table 3 – Casein monomer
properties7
Primary structures of the different caseins are located in the Appendix.
Whey Proteins
Whey proteins are those proteins that remain in the fluid milk after sufficient acidification and coagulation. Typically the neutral nature of milk is altered to be roughly a pH of 4.6. These proteins are globular in shape and are very resistant to heat denaturation, thus giving them a more water-soluble characteristic than that of casein. Whey is largely made up of b-lactoglobulin, a-lactalbumin, bovine serum albumin (BSA), and immunoglobulins.
|
Description
of Whey Proteins:7 |
|||
|
|
|
|
|
|
|
Molecular |
|
|
|
Protein |
Weight |
Residues |
Description |
|
|
|
|
-half of
total whey protein |
|
|
|
|
-2
internal disulphide bonds, 1 free thiol group |
|
b-Lactoglobulins |
18,000 |
162 |
-secondary
structure |
|
|
|
|
-exists
naturally as noncovalent dimer |
|
|
|
|
-formed
in mammary gland |
|
|
|
|
-about
13% of total whey |
|
|
|
|
-highly
ordered secondary structure |
|
|
|
|
-can be
found in spherical tertiary structure |
|
a-Lactalbumins |
14,000 |
123 |
-thermal
denaturation results in release |
|
|
|
|
of bound
calcium |
|
|
|
|
-4 disulphide
linkages and no phosphate groups |
|
|
|
|
-identical
to blood serum molecules |
|
BSA |
69,000 |
- |
-contains
no phosphorus |
|
|
|
|
-has 17
disulphides, 1 free sulfhydryl group |
|
|
|
|
-may use
albumin to carry free fatty acids |
|
|
light |
|
-4
classes lgG1, lgG2, lgA,lgM |
|
|
20,000- |
|
2 are
light chains, 2 are heavy chains |
|
Immunoglobulins |
25,000 |
|
|
|
|
heavy |
|
-not
produced in mammary gland |
|
|
50,000- |
|
|
|
|
75,000 |
- |
-provide
passive immunity to calf |
Table 4 – Whey Protein
properties7
Ø Structures
Both lactoglobulins and lactalbumins are most commonly found in their secondary protein structures. Typically b-lactoglobulins form a barrel type structure. The a-helix portion of the molecule is only on its surface, thus producing a hydrophobic center that can bind the protein with hydrophobic molecules. This is very useful in the speculated binding with Vitamin A in the mammary gland. This binding is thought to have a regulatory effect within the mammary gland itself.
Figure 2 - Three-dimensional structure of bovine b-lactoglobulin. Original structure by Sawyer, reproduced from Swaisgood, 1996.7
All mammalian milk that contains lactose must also contain a-lactalbumin.
The a-lactalbumin has a similar structure to b-lactoglobulin. Like b-lactoglobulin, a-lactalbumin is produced in the mammary gland. This structure enables the molecule to behave in a very peculiar way. When a-lactalbumin is heated, it is much more stable that when the protein is in an environment free of calcium. Calcium typically will promote ion intermolecular cross-links with other proteins. Cross-links of this nature will hold strongly in heating, thus, making thermal unfolding of the molecule fairly unlikely.
Figure 3 - Three-dimensional structure of baboon a-lactalbumin for Swaisgood, 19967
Primary structures of both b-lactoglobulin and a-lactalbumin can be found in the Appendix.
Starter Cultures, Molds, and Yeasts
When producing cheeses, milk is heated to high temperatures to kill any undesired or existing microorganisms that are already living in the milk itself. In order to induce the processes necessary to make cheeses and yogurts, cheese producers add bacteria. The bacterium is primarily used to create lactic acids from already existing lactose. Within their natural processes, the bacteria add flavor and aroma to the cheeses and yogurts. Along with culinary attributes, cultures can inhibit the growth of other organisms, helping to prevent spoilage.[11]
The formation of acid within the milk provides for an atmosphere that is suitable for coagulation. Many cheese starter cultures are mesophilic. Most yogurt starter cultures fall under the thermophilic category. Thermophilic cultures can also be used for hard and soft cheeses. Hard and soft cheeses typically use single species cultures, whereas yogurts tend to use multiple species cultures.[12]
American yogurt producers use a blend of cultures including, Streptococcus salivarius, subsp. thermophilus (ST), and Lactobacillus delbrueckii subsp. bulgaricus (LB). Each can grow independently, however, for most efficient acid production, the blend is preferred. ST will grow the fastest, producing acids and carbon dioxide. It forms an environment that is perfect for the growth of LB. Thus, LB produces stimulatory peptides and amino acids that allow ST to thrive. Streptococci are credited with lower the initial pH of yogurt to roughly 5.0. It is the added lactobacilli that are necessary for the further pH drop to about 4.0. From the addition of these two cultures, products including lactic acid, acetaldehyde, acetic acid, and diacetyl are produced. Each of these products adds to the familiar taste of the yogurt.[13] The final state of the yogurt as it is presented to the customer is ultimately dependent upon the starter culture. Different cultures will give different final viscosities to the yogurt. This can be found in the mass flux of the fluid as it flows past a medium.[14]
Streptococcus lactis subsp. lactis is the most common starter culture for milk. As stated before, this bacterium generates lactic acid that ferments the cheese and gives cheese its familiar attributes. Besides bacteria, molds play a very important role in cheese making. Many types of cheese rely on molds to give them robust, full flavors. Danish blue cheese is an example of such a cheese. Producers form the cheese and then actually puncture the cheese in order to allow for air to reach the inside parts to promote mold growth. The mold will spread and perform hydrolysis on fats and proteins, giving the cheese its characteristic “blue streak” appearance.[15] Mold cultures work along side enzymes to give cheese its characteristics. The enzymes will break down large molecule strands to smaller ones that the mold can exploit. These processes, proteolysis and lipolysis, will give the cheese its distinct flavor and consistency.7 Cheese flavor development is discussed in greater detail later in the report.
Yeasts are typically used for surface ripening of cheese. They will neutralize the surface of the cheese by breaking down the lactic acid, thus forming alkali catabolites. The yeast will also add flavor to the cheese through lipolytic and proteolytic reactions that occur through its natural behavior of living. Along with flavor, yeasts also will help to provide for the proper environment for cheeses to develop desired microorganisms to give color, flavor, and consistency.7
Coloring Agents
Coloring of cheeses can be from molds as described before, but also, other ingredients can add color to the cheese. Many cheeses come in yellowish or orange hues in the grocery stores. These colors can come from dies added to the cheese before final ripening. A common orange coloring for this color is beta-carotene. Cheese makers also tend to use annatto in their cheeses to add appealing color. Most of these colorings are made from natural sources and are resistive to heat and light.[16]
Colloidal
Properties of Milk, Yogurt, and Cheese
Physical Structure of Milk
In order to understand the physical structure of cheese and yogurt, one must first understand the physical structure their raw material, milk. Milk is a complex system of the proteins, fats, lactose, and other components. These components assemble into three distinct physical systems. On the largest scale, milk is a stabilized oil-in-water emulsion with fat globules dispersed in the continuous serum phase. On a smaller scale, it is a colloidal suspension of casein micelles, globular proteins and such, inside the serum phase. Finally, on the smallest scale, it is a solution of lactose, vitamins, minerals, and soluble proteins in water.[17],[18]
The fat globules in milk are microscopic, and vary in diameter from 0.1 mm to 22 mm. The average diameter is 3 mm.[19] Fat globules tend to separate if left alone, because of the buoyancy forces caused by the density difference between the fat and serum phases. This phenomenon, known as creaming, can be simply modeled by Stokes’ Law.[20] As the fat clusters cream, the larger globules rise faster and carrying along with them the smaller globules.18 Homogenization prevents this by breaking up fat globules, and ensures that the majority of globules are the same size.
The fat globules are encased in a membrane of phospholipids and proteins, which mainly come from the cell that secretes the milk. This membrane keeps the surface tension of the lipid-serum interface down around 1 to 2 mN/m.18 Homogenization destroys much of the original fat globule membrane,[21] and causes the surface tension to spike to 15 mN/m. After this the serum proteins and casein micelles also adsorb to the lipid droplets.18 Many studies have been done to determine the exact structure of the proteins adsorbed at the oil-water interface, but all that is known there is some complex interaction between casein, the whey proteins, and the phospholipids that all serve to stabilize the emulsion.[22],[23]
Despite the lack of knowledge about emulsion stabilization, a few features of the membrane are known. The whey protein b-lactoglobulin is known to adsorb strongly at the oil-water interface in numerous tests, but its exact contributions in milk are uncertain.[24] Also, the proteolytic enzyme trypsin has been used to study the structure of b-casein at the interface. From these studies it is inferred that b-casein projects out its most hydrophilic N-terminal region into the aqueous phase from the oil-water interface as a loop or tail.[25]
Caseins form the vast majority of the proteins in milk, and form an extremely complex micelle structure that remains in question. The old theory, which is well established, is the “casein sub-micelle model”. It holds that a- and b-casein form small groups of 10 to 20 molecules called sub-micelles. Some of these sub-micelles contain k-casein, while others do not. The sub-micelle core is hydrophobic, while the hydrophilic coat is where the k-casein resides. The hydrophilic portion of k-casein (called caseinomacropeptide, or CMP) is a polyelectrolyte chain that sticks out into the serum as a hair-like protrusion.18,[26]
These sub-micelles join together as hundreds or thousands to form the micelles visible to electron microscopy. These micelles range from 20 to 600 nm in diameter, and have a mean diameter of 140 nm.[27] These micelles are bound by colloidal calcium phosphate (as opposed to the calcium phosphate in solution) either covalently or electrostatically. The k-casein sub-micelles remain near the surface, and the k-casein acts as a polyelectrolyte brush or an external “hairy” layer of at least 7 nm in length. This layer causes steric (or polymeric) repulsion between different micelles and prevents micelle aggregation.18,27 The following picture illustrates the basics of this model.[28]
Figure 4 – Casein Micelle and
Submicelle Model28
The following shaded picture shows how it might look in 3-D. The lighter surfaces represent a- and b- caseins, while the darker shaded circles on the surface represent k-casein.[29]
Figure 5 – 3-D Representation of
Casein Micelle from the Sub-Micelle Model29
The newest theory, known as the open model, postulates that there is no lesser order of casein submicelles. Instead, the micelle is still spherical, but the internal protein chains are linked together by colloidal calcium phosphate. There is also a hairy layer visible in this model. The advantage of this model is that it accounts for a more even distribution of Ca9(PO4)6 .29
Figure 6 – Micelle Structure
According to the Open Model29
In either model, these micelles are porous, and contain citrate, enzymes, lesser ions, and serum. Because of these entrained components, the micelles are rather large, and compose 6 to 12% of the total volume in milk.18
The rest of the milk serum is mostly water, but contains many important vitamins and minerals in solution. Whey proteins, or the non-casein proteins, are also dissolved in this phase. They acquired the name whey proteins because they come out in the whey during curd formation.18
Structural Transformations from Milk to
Cheese
The first step in the transformation of milk to cheese is the addition of a bacterial starter culture to the milk. Commonly known as LAB, or lactic acid bacteria, these cultures are added to the milk after most of the native milk bacteria are killed by pasteurization and heat treatment of milk. These bacteria (discussed earlier in detail) are present to convert lactose to lactic acid, and thereby lower the pH in later steps. This quickens coagulation, helps syneresis (or whey separation), influences flavor and texture, and also inhibits development of other bacteria.[30]
Coagulation
The main structural transformation of milk to cheese occurs during curd formation. Curds are formed by the coagulation of the casein micelles and the creation of a gel that traps the fat. Coagulation is initiated with one of three methods: enzymes, acid treatment, or heat acid treatment.[31] The following picture shows an electron micrograph of coagulated casein micelles in cheese.[32]
Picture 1 – Electron Micrograph
of Coagulated Casein Micelles in Cheese32
The enzyme rennet is the traditional catalyst used in cheese making. The active ingredient of rennet is chymosin, and the variety regularly used in cheese production is secreted in the fourth stomach of newborn calves. Rennet starts the coagulation process by breaking the Phenylalanine (105)-Methionine (106) amino acid bond on k-casein. k-casein separates into a hydrophobic portion, para-k-casein, and a hydrophilic part, CMP.31,[33],[34]
Loss of the CMP creates two problems. The steric repulsions from the hair-like protrusions of the k-casein brush kept them from aggregating are now gone, eliminating one barrier to coagulation.27 In addition, the micelle surface is now partially covered by the hydrophobic and positively charged para-k-casein.31 When these micelles collide in solution, the para-k-casein is strongly attracted to the negatively charged a-casein, b-casein, and k-casein on other micelles,35 which keeps the aggregated micelles together so that it might avoid the serum phase. In the following diagram, the pathway on the right shows an example of this coagulation mechanism.[35]
Figure 7 – Gelation Mechanism of
Cheese35
In earlier studies by Dalgliesh suggested that 90% of k-casein must be hydrolyzed before aggregation begins.[36] However, current theory holds that enzymic hydrolysis and aggregation are not two separate and distinct steps, but instead occur concurrently after a small initial amount of k-casein is hydrolyzed. Since successful collisions between two micelles with surfaces reactive to coagulation are unlikely, the initial rate of aggregation will be very low, but it will still proceed.35
The
total calcium content of milk has a dramatic effect on coagulation and the time
required for gelation. Calcium not only
binds micelles internally, but also neutralizes the net negative charges on the
casein proteins. This not only reduces
the electrostatic repulsion between micelles but also increases the
hydrophobicity of these protein chains.
In this way, coagulation-reactive sites on micelles are produced faster
and become more functional sooner. This
has the overall effect of reducing gelation time.35 More specific information on the casein
gel network is available in the literature.[37]
In acid coagulation, there is no enzyme catalyst such as rennet, and coagulation is initiated by the lowering of the pH from the activity of the LAB. It can also be initiated by the addition of an acidifying agent such as glucono-d-lactone.[38] Acidification has two effects that lead to instability. First, colloidal calcium phosphate becomes more soluble in the serum, and its loss further destabilizes the micelles. Second and most important, the isoelectric point of the caseins is at a pH of 4.6, and at this pH caseins are still insoluble in water. Thus, aggregation will occur as a combination of both of these effects.[39] Cottage cheese is an example of a cheese coagulated in this way.18, 31
Heat acid coagulation is also initiated by acid, but without the aid of LAB creating lactic acid. Instead, by heating the milk, the whey proteins are denatured and associate with the casein micelles. At the same time, the buffering ability of milk salts is reduced, carbon dioxide is released, and organic acids are produced. In this way, casein and whey proteins coagulate together and form curds. This method of coagulation is very sensitive to the initial volume fraction of casein micelles,[40] and thus some forms of skim milk are unable to coagulate by this mechanism. Ricotta is manufactured by this method.18,31
Curd Treatment
After curd formation is complete, various steps are taken to treat the curds and control the moisture content of the final cheese product. The first such step is the cutting of the curds. Cutting is a quick process, and usually takes less than five to ten minutes. There is a direct correlation between curd size and moisture content. Low moisture cheeses require the finest-cut curds, at about the size of rice grains. High moisture cheeses like the soft-ripened varieties are merely broken up to 2-cm curds. The smaller curd cheeses have a greater fat content, mainly because the larger curds lose the finer fat globules when crushed during processing.[41]
The next step in curd treatment is cooking of the curds. Heat and continuing acidification (from the bacteria) furthers syneresis and expels more moisture, acids, lactose, and water-soluble minerals. Care must be taken to cook the cheeses properly, because quick cooking creates an external layer that inhibits further syneresis. Here the smaller curd cheeses become drier mainly because of the increased surface area for syneresis to occur.42
Draining and then washing of the curds effectively rinses the whey and lactose out of the cheese for good. Washing the curds can be used to adjust the final lactose content, since the wash water can be used to leach lactose out of the water retained in the cheese and keep the pH above 4.6 (the natural level that occurs during coagulation). In Muenster, the cheese is still high moisture, but maintains a pH of about 5 because of this leaching. The temperature of the wash water can also be used to affect the texture, such as using hot water to dry Gouda and develop its texture.42
Various other closing steps occur during curd treatment such as dipping of the curds in brine, pressing of the curds, and salting of the curds. These steps allow for some structural fine-tuning for different cheeses. For example, both cheddar and American cheese are salted before pressing, while others are salted after pressing. Salting and brine dipping checks the growth of spoilage bacteria because the LAB are much more tolerant to salt than pathogens.42 A flow chart of manufacturing techniques is presented on page 37 of this report.
Ripening
The final major step in the process of cheese making is ripening of the cheese. The ripening process can take as short as a nothing for fresh cheeses like cottage cheese, or years for some very hard cheeses. During ripening, the proteins, fats, and carbohydrates are broken down to compounds that modify flavor and texture.42
Proteolysis, or protein breakdown, is the most intricate and probably the most significant of the ripening transformations taking place. Plasmin is the primary indigenous proteolytic enzyme, and directly attacks b-casein.[42] Excessive protein breakdown from proteinases like plasmin cause the cheese to taste bitter and putrid, and is responsible for many of the undesirable characteristics of overripe cheese. Protein breakdown makes cheeses such as Cheddar more susceptible to melting, and flavor and texture development in Cheddar is mainly based on proteolysis.42,[43]
Proteolysis at the largest scale breaks down proteins into smaller peptide chains (bitter-tasting peptides are common in cheese). Peptidases also break these chains down to their constituent amino acid groups. Further reactions can convert these amino acids into ketones, carboxylic acids, aldehydes, alcohols, esters, and other tasty molecules.42 Some small peptides produced by proteolysis are responsible for the bitter taste of Camembert cheese. Ripening further increases the bitterness of Camembert by increasing the number of small peptides. There is also a minor “matrix” effect from the cheese matrix structure, but the peptides are strong contributors to the overall taste.[44]
Lipolysis, or fat breakdown, creates many of the fats with smooth textures that are associated with dairy products. The fat globules also act to preserve fat-soluble flavor compounds released during proteolysis, and they are stored in the fat until chewing releases them. The fat molecules are also an important component to the melting and softening applications of some cheeses, which causes the viscous substance formed on melting.42
In general, fatty
acids were considered to have only a minor influence on the cheese flavor.[45] In the case of cheddar cheese, short-chain
fatty acids such as the C4, C6, and C8 fatty acids are believed to have an
influence on the “sharp” taste that comes from the ripening of the cheddar.[46]
Data even show that because of
lipolysis, these short-chain fatty acids increase dramatically in concentration
over the ripening period of cheddar, while the long-chain fatty acids they are
derived from drop in concentration.
However, even these high concentrations are considered to be well below
the taste threshold concentrations.46 In spite of this rationale, recent studies have concluded that
the major aroma and flavor components in cheddar cheese include butyric acid
along with acetic acid, methional, 2,3-butanedione, and homofuraneol.47
Fat globule size also plays an important role in the texture of cheeses. This physical property is determined during homogenization, but clear correlations exist between initial milk fat globule size and cheese texture. In fresh cheeses like cream cheese, the “texture may improved by decreasing the milk fat globule size in the initial mixture.”19
Lactose is the only major carbohydrate in cheese. Since most of it is removed during syneresis, any residual lactose is usually quickly converted into lactic acid by remaining bacteria. The most ever found in commercially available cheeses is never more than 0.1%.42
The ripening stage and therefore the final flavor and texture are mainly dependent upon enzyme activity in the ripening cheese. Factors like pH, salt concentration, temperature, and moisture content have dramatic effects on specific bacterial or enzyme activity and thereby influence the end product’s taste. Many of the complex effects of each enzyme that operates in different cheeses have not been thoroughly researched, and therefore much of the current ripening technique is based on experience and trial and error.42
Structural
Transformations from Milk to Yogurt
Yogurt undergoes many changes during production that are similar in principle to those of cheese, but there are major differences in the method and in the end products. Yogurt is solid, but it “has the highest water content of all solid milk products”.[47] This situation occurs because instead of allowing syneresis to occur as in milk, syneresis is actually prevented.
Yogurt production begins using milk that is heated almost to boiling. It is key for the milk to be heated to at least 85oC, and held there for at least 10 minutes. This heat treatment denatures and coagulates whey proteins to increase the system viscosity. The treatment is more drastic than raw milk pasteurization, and also produces a sterile system for the starter culture.48,[48]
The milk is inoculated with thermophilic bacteria to induce coagulation, and they slowly increase lactic acid concentration by breaking down lactose. This changes the pH from about 6.7 to below 4.5.[49] Additionally, the high temperature from the heat treatment leads to the formation of a b-lactoglobulin and k-casein complex on the surface of the casein micelles (Picture 2). This complex imparts on the micelles a limited capacity to aggregate, and they form a looser network of short, branched micelle chains (Picture 3).48,33
Picture 2 – Heat Treated Casein
Micelle Preparing to Coagulate in Yogurt47
Picture 3 – Electron Micrograph
of Casein Micelle Coagulation in Yogurt47
This loose network forms the gel structure that is the basis for yogurt. It is open enough that it may retain large amounts of the water and water-soluble elements, but is firm enough that it may still be considered a solid. Many of the whey proteins and sugars often lost during cheese making are thus retained in yogurt.
Many of the textural properties of yogurt are based on the geometric structure of this casein gel. The concept of texture is itself a complex amalgam of the mechanical properties of yogurt. The gel network is mechanically defined by three major factors: the spatial arrangement of the casein, the strength of the forces binding the network, and the particle structure.[50] Thus, yogurt’s unique consistency is directly related to the nature of the growth of the casein gel network.
The
only major variation on this process is in the fortification of the initial
milk with solids. Increasing the solids
content, especially the protein, increases the gel network density and thereby
decreases the pore size. Often used
fortifiers are whey powder, milk powder, milk protein concentrate, whey protein
concentrate, or sodium caseinate. These
solids strengthen the ability of the pores to hold water and prevent syneresis.48
Syneresis of yogurt is the cause of watery yogurt, or the layer of water that forms on the top of yogurt after it has sat for a while. One possible cause is poor heat treatment of the milk. Other possible causes are the vibrations that occur during transportation and yogurt acidity. However, these concerns are not very crucial, since the water can be added back merely mixing the yogurt and water together.48
Yogurt flavor is most directly influenced by the presence of acetaldehyde, hexanoic acid, 2,3-Butanedione, dimethyl sulfide, and 2,3-Pentanedione.47 “The most important of these is compounds is acetaldehyde which gives yogurt its typical green apple or nutty flavor.”[51] Good-tasting yogurt generally has acetaldehyde in concentrations of 23 to 41 ppm. L. bulgaricus accounts for much of the lactic acid production that gives yogurt its tart, in addition to many aromatic compounds produced as by-products.52
Changes in the raw milk properties have dramatic effects on the final taste. Raw milk used for yogurt production fortified with skim milk powder or other milk solids often leads to higher lactic acid and acetaldehyde content in the product. However, using pectin only increases acetaldehyde concentration. Knowing the result of these factors, the final taste of the yogurt can be fine-tuned by the addition of specific fortifiers before gelation occurs.52
Production/ Marketing
World Production Levels
Cheese is a food used throughout the world to add flavor, texture, or some other desired addition to common foods and recipes. Cheese production has steadily increased in the U.S. and throughout the world from 1991-1999 (see following table). It is obvious by the data that the United States is the largest producer of cheese in the world, owing up to around 24-25% of the total production. Europe, mainly Western, seems to be the next major contributor of cheese in the world. New Zealand is a minor contributor due to cheese manufacturing because of the large population of lactose intolerant people in the region. The same holds true for most of Asia, where 90% of the people are lactose intolerant[52].
|
World Cheese Production[53](1,000 metric tons) Country 1991 1992 1993 1994 1995 1996 1997 1998 1999 New Zealand 125 142 145 192 200 239 267 276 291 Egypt 293 290 300 305 310 371 400 427 430 Russia 394 299 313 285 215 428 378 350 350 Netherlands 610 634 637 648 680 688 693 630 620 Italy 885 890 885 913 922 983 951 951 951 Germany 777 783 821 855 890 1497 1559 1570 1571 France 1500 1489 1509 1541 1576 1605 1622 1653 1655 United 2747 2943 3301 3386 3137 3627 3644 3734 3771 States Rest of 3247 3264 3388 3456 3479 5197 5381 5466 5469 World Total: 10756 10931 11510 11815 11644 14899 15180 15352 15416 |
Table 5 – Word Cheese Production
Levels53
U.S. Cheese Production
The U.S. cheese production follows a trend based on the seasons. Traditionally, the summer months call for the greatest demand in production of cheese. Outdoor picnics and family gatherings are most likely the largest contributor to the increased demand. The smallest demand occurs at the beginning of the year, around January and February. Cheese seems to be a lackluster food in colder temperatures. There is a lack of major holidays at this time of year. Holidays create a significant increase in the production as shown by the increase in November and December, most likely due to major gathering holidays like Christmas and Thanksgiving. The graph showing production trends for the past few years is located in the Appendix.
|
U.S.
Cheese Production[54] |
|
||
|
|
|
|
|
|
|
1999 |
2000 |
2001 |
|
|
|
1000 Pounds |
|
|
Jan |
636,971 |
692,946 |
686,591 |
|
Feb |
594,036 |
649,472 |
632,159 |
|
Mar |
702,937 |
714,816 |
714,057 |
|
Apr |
669,146 |
693,997 |
675,080 |
|
May |
672,883 |
730,387 |
708,832 |
|
Jun |
668,126 |
695,734 |
682,337 |
|
Jul |
646,690 |
687,591 |
679,109 |
|
Aug |
647,252 |
683,776 |
663,390 |
|
Sep |
639,647 |
653,840 |
644,525 |
|
Oct |
670,936 |
688,543 |
682,957 |
|
Nov |
687,918 |
675,004 |
686,246 |
|
Dec |
704,706 |
688,411 |
699,486 |
|
Total |
7,941,248 |
8,254,517 |
8,154,769 |
Table 6 – Seasonal and Annual
Figures for Cheese Production in the United States54
U.S. Yogurt Production
Yogurt production takes a similar trend to cheese production. Early spring and late summer are the prime time for yogurt production as the demand by the consumer reaches its peak. There is a noticeable difference for yogurt in the holiday seasons. Unlike cheese, yogurt seems to be less desired as an integral part of the holiday menu. Production trends are shown on the attached graph in the Appendix.
U.S. Yogurt Production[55]
|
|
|
|
|
|
|
|
|
|
|
|
|
1999 |
2000 |
|
|
1,000
Pounds |
|
|
Jan |
134,616 |
136,776 |
|
Feb |
138,281 |
152,387 |
|
Mar |
161,727 |
172,036 |
|
Apr |
141,300 |
159,734 |
|
May |
145,501 |
163,999 |
|
Jun |
150,446 |
165,544 |
|
Jul |
134,250 |
142,744 |
|
Aug |
148,208 |
156,829 |
|
Sep |
155,376 |
172,085 |
|
Oct |
136,387 |
144,597 |
|
Nov |
125,537 |
131,591 |
|
Dec |
145,552 |
137,059 |
|
Total |
1,717,181 |
1,835,381 |
Figure 8 – Seasonal and Annual
Figures for Yogurt Producton in the United States55
