MILK AND ICE CREAM
3.2 bIOLOgY AND CHEMISTRY OF MILK: SUgAR, PROTEIN, AND FATS
3.2.11 butter
3.2.11.1 Making Butter Butter can be made from sweet cream or cultured cream
Sweet cream is a term used to distinguish untreated cream from the cream separated from milk after bacteria have been introduced and grown to acidify and partially TAbLE 3.2 Fat Composition in buttera
Fatty Acid Structure
% Total Fatty Acid in Butter
Oleic acid CH3(CH2)7CH═CH(CH2)7COOH 31.9
Myristic acid CH3(CH2)12COOH 19.8
Palmitic acid CH3(CH2)14COOH 15.2
Stearic acid CH3(CH2)16COOH 14.9
Lauric acid CH3(CH2)10COOH 5.8
Butyric acid CH3CH2CH2COOH 2.9
Caproic acid CH3(CH2)4COOH 1.9
Capric acid CH3(CH2)8COOH 1.6
Caprylic acid CH3(CH2)6COOH 0.8
Linoleic acid CH3(CH2)4CH═CHCH2CH═CH(CH2)7COOH 0.2 Linolenic acid CH3CH2CH═CHCH2CH═CHCH2CH═CH(CH2)7COOH 0.1 From http://antoine.frostburg.edu/chem/senese/101/consumer/faq/butter‐composition.shtml Good link on evolution and lactose intolerance http://evolution.berkeley.edu/evolibrary/news/070401_lactose and http://www.sciencedaily.com/releases/2005/06/050602012109.htm
a Reproduced with permission of 1997–2010 by Fred Senese.
metabolize milk providing a sour or cultured taste. The cream is often concentrated by heating. To make cultured butter, the cream is inoculated with acid‐producing bacteria, such as Streptococcus cremoris, Lactococcus lactis, and Leuconostoc.
These microorganisms use the fats and sugars found in food to produce lactic acid and another flavorful compound, diacetyl, which give milk its slightly sour (lactic acid) and rich, buttery taste (diacetyl). Historically, the cream would be allowed to stand and sour from environmental bacteria. Now it is easy to introduce the correct living bacteria by adding cultured yogurt to fresh cream as cultured yogurt contains the proper living bacteria. Fermentation by these same bacteria also produces sauerkraut from cabbage and the sour in sourdough breads.
The cream is then cooled and aged to allow some of the fat to crystallize. Once churning begins, crystals seed the growth of additional crystals and help to tear the milk fat globule membranes. As the cream is churned, the milk fat globules are broken, and the fats coalesce first into granules and then into larger solid masses.
Finally, the residual buttermilk liquid is separated, and the fat is kneaded to remove pockets of remaining buttermilk and to evenly distribute the fat. At this time (or sometimes during the churning process), salt is added as a 10% solution to end up with a 1–2% final concentration. In addition to enhancing the flavor, many harmful bacteria and other microorganisms that can use butterfat as a food source are not able to grow in salty conditions; in this way, the added salt is a preservative, increasing the shelf life of butter as it inhibits bacterial growth.
The buttermilk is the low‐fat (0.5%), high‐protein liquid remaining after the milk fat has solidified. Modern buttermilk is made by adding Streptococcus lactis bacteria to low‐fat milk. As before, the bacteria will acidify the milk by producing lactic acid, which causes curdling of some of the proteins and produces the tangy flavor associated with buttermilk. Sometimes citric acid is added to aid in the process, and a second bacterium is added to create diacetyl to give the buttermilk a more buttery taste.
Butter adds a rich flavor to baked goods. Much of the flavor of butter comes from over 120 different compounds including the familiar fatty acids in triglycerides, lactones, methyl ketones, and diacetyl (Fig. 3.20). Short‐chain fatty acids with four to six carbon chains contribute to flavor, while longer‐chain fatty acids have little taste or aroma. While some of these fats are from the diet of the cow from which the milk came, many are produced when enzymes called lipases in the milk itself and from the bacteria used to culture the cream use water to cleave the free fatty acid from the glycerol backbone. One fatty acid, butyric acid, makes up only a small fraction (2.9%) of the total fat but is responsible for the rancid flavor and smell of rotten milk and butter. Depending on the time of year and what the cow was eating when milked, short‐chain fatty acids can be more abundant in the triglycerides of the butter. Over time, the enzyme lipase can remove butyric acid from the glycerol backbone, causing the dairy product to smell and taste rancid.
While the triglycerides found in milk and therefore cream and butter are primarily comprised of fatty acids bonded to a glycerol backbone, a small percentage of triglycerides contain hydroxy acids (~0.3%) and keto acids (~0.85–1.3%) bonded to the glycerol. While the fraction of hydroxy acid and keto acid containing
triglycerides is very small, these acids can produce powerful flavor molecules. When heat or a lipase cleaves a hydroxy acid from the triglyceride, the product hydroxy acid can lose a water molecule and cyclize into a lactone. When a keto acid is enzy-matically or thermally freed from the glycerol backbone, the product keto acid loses carbon dioxide and forms a methyl ketone. Only a very small amount of these mole-cules is required to be noticed by the human senses. For example, the flavor threshold of small molecules containing methyl ketones is 0.02 ppm when in oil. When milk or butter is heated in making baked goods or ghee (clarified butter), lactones provide the nutty and fruity aromas and flavors of butter sauces. Lactones and methyl ketones are produced readily when butter is heated in cooking. The enzymes in bacteria used to culture cream can also catalyze these reactions and impart flavor to the butter. In the same way, blue cheeses owe much of their flavor and aroma to methyl ketones pro-duced as fats are consumed by the mold Penicillium roqueforti.
CH2
CH2 CH2 C
H2 H2C
HC
H2C OC
CH2 C H2 C
H2 C H2 H2
C H2 C
H2 C
H2 C H2 C H2
C H2 C O
OC C H2
CH2 CH2 C
H2 O
OC O
OC O
OC O
O C
O HC
H H O
OC CH2
CH2 CH2 CH2 C
H2 C H2
CH2
CH2
CH2
CH2 C H2 C
H2 C H2 CH2 C
H2 CH2 C
H2
CH2 C H2 C
H2 C H2 H2
C H2
C H2 C H2 C
H2 C
H2 C
H2 C
H2 C
H2 C
H2
C H2
C H2 C O
O C O
O C CH2 CH2 O
C CH
O
O C O
O C O
H O H
OH
O C O H O
H
O C C
O
O C O
OC O
O
O C O
O C O H
O C O O
C O Triglyceride
Glycerol backbone
Lipase
H2C H2C
H2C
CH3
CH3
CH3 HC
H2O
Glycerol backbone
Hydroxy group
H2C H2C O
HC
H2C
CH OH
CH3
CH3
CH3
CH3
CH3
CH3 CH3
CH3 CH3
CH3
Heat or lipase
H2O
H2O HC
H2C
A free hydroxy acid
Butyric acid
This group of atoms is called a “carboxylic acid”
Diglyceride
H2C CH2
CH2 CH2
CH2 CH2
CH2
CH2 CH2 H2
C H2 C H2
C
H2 C H2 C H2
C H2 C CH3
CH3
CH3
A lactone (“cyclic ester”) Lactones are powerful flavor molecules H3C
H2 C H2
C
H2 C H2
C H2 C CH CH2
CH2 Glycerol
backbone
Glycerol backbone Hydroxy acid containing triglyceride
These make up 0.3% of the fatty acid found in milk triglycerides
ketone group
Heat or lipase
Keto acid containing triglyceride These make up another 0.3% of the fatty acid found in milk triglycerides
Glycerol backbone
H2 C H2 C
H2 C H2C
HC
H2C CH2
CH2 CH2 C
H2 C H2 CH2 C
H2 C H2 H2 C H2 C H2
C H2 C
H2 C H2
C CH3
CH3 CH3
A free keto acid
A methyl ketone Methyl ketones are powerful flavor molecules
CH2 C H2 C
H2 H3C
H2 C H2
C H2 C
C C
O
O H
O
FIgURE 3.20 generation of some of the aroma and flavor compounds of butter.
In the production of buttermilk by bacterial fermentation and in the fermentation of “cultured” butters, bacteria metabolize citric acid to diacetyl. Diacetyl can be smelled at very low concentrations (1–2 ppm) and has a characteristic nutty and but-tery aroma. The presence of diacetyl is the major difference between cultured and sweet cream butters. Diacetyl is also a primary component of artificial butter flavors.
Butter also has other important uses in cooking and baking. Many of the organic flavor and aroma molecules of herbs and spices are hydrophobic and, as such, will not be easily released into a watery environment. The fat in butter is also hydro-phobic, and in liquid form as an oil, butterfat, is an effective solvent for hydrophobic flavor molecules; butter acts as a flavor carrier. The crumbling of baked cookies, bread, and other foods is due to dried and crystallized starch. Amylose, one of two complex carbohydrates found in starch granules, can form a gel during baking and eventually crystallize after cooling. Over time these crystals dry and the result is a crusty, crumbly food. Butter helps trap the moisture surrounding the sugar crystals maintaining a softer crumb. Cakes, breads, and biscuits often use butter as a key ingredient. Butter coats gluten, a wheat protein responsible for forming the proteina-ceous matrix that gives baked goods their structure. If mixed with the flour before adding the liquids, butter helps to coat the starch granules of the flour, limiting the penetration of water and the formation of the elastic gluten. It is this property that makes butter critical to the flakiness of pastry dough. In addition to limiting gluten formation, the incorporated butter forms thin layers separating the dough, and when baked, it melts and keeps the pastry flaky. Butter can also reduce the length of the protein strands that form the gluten matrix, resulting in a more tender baked good.
However, a note of caution should be considered. Salt has the opposite effect on gluten and tightens the protein strands to make a more dense baked good. Using unsweetened cultured or sweet butter is a way to avoid the impact of salt on gluten.
In addition to fat, butter will still have remnants of water, whey protein, and lactose sugars. The protein and sugar biomolecules are considered milk solids, which, when heated, fall out of solution and are easily burned. The sugar also provides a source of energy for microbes that shorten the shelf life of butter. One way around this problem is to remove the milk solids and water to produce clarified butter. The smoke point for butter is around 65°C (150°F), but removal of the sugars and proteins raises that smoke point to 450°F/230°C for clarified butter. Clarified butter is made by warming the butter gently to boil away the water, and the resulting protein floats to the top of the butter where it can be skimmed off, while the sugar and some of the proteins precipitate to the bottom of the melted butter, where they can be separated by pouring.
The resulting yellow liquid is pure butterfat and can be used for dipping lobster, coating steak, and forming the base of a sauce (roux) and can take the high tempera-tures of sautéing. Without the sugars and proteins of butter, clarified butter is a more efficient storage of butterfat without refrigeration. Ghee, a clarified butter tradition-ally made from cow or buffalo curdled milk (yogurt), is widely used in Indian foods where it has been utilized for its storability.
I can’t believe it’s not… When margarine first came to market as a butter substi-tute, it was made with buttermilk and animal fat or tallow; modern margarine as a
butter substitute is primarily produced from vegetable oil. Margarine is a water‐in‐fat emulsion of solidified vegetable fat, water, and in some cases skimmed milk for protein and cookability. The solidified vegetable fat begins as vegetable oil, produced from the seeds of corn, sunflower, flax, canola, and other plants. These oil‐containing seeds are pressed for the oil and further processed by extraction with a volatile solvent such as hexane. The solvent is removed and oil refined by distillation (see Chapter 12 for more information on distillation). The resulting liquid is filled with mono‐ and polyunsaturated fats (Fig. 3.21).
These kinked fatty acids do not stack well and are easily melted at room tempera-ture. In order to convert the liquid vegetable oil into a solid fat (margarine), the double bonds of the fatty acids have to be converted to saturated single‐bonded carbon. The process of “partial hydrogenation” is the addition of hydrogen atoms to the double‐bonded carbons. The conversion of liquid oil to solid fat is accomplished by bubbling hydrogen gas at high temperatures in the presence of a nickel catalyst.
The result is an incomplete conversion of triglycerides containing unsaturated fatty acids that are liquid at room temperature to the solid forming saturated fatty acids (Fig. 3.22). This partial hydrogenation is an inexpensive way to produce a butter substitute. The partially hydrogenated oil is then mixed with water, salts, and emulsifiers for taste and cooking, in addition to gums that serve as thickening agents.
Some of the controversy in the consumption of margarine is due to a side product of the reaction. The double bond found in most plants and animals is arranged in what is called a cis configuration. This causes the carbon atoms on either side of the double bond to be on the same side (Figs. 3.22 and 3.23). The prefix cis is based on the Latin
CH2 C H2 C
O O
C C
H
H
C O
O
C C
CH2 H
H C
O O
CH2 C H2
CH2
CH2 C H2
CH2 CH2
CH2 CH2 C
H2 CH2 C
H2 C
H2 C
H2 H2
C H2
C
H2
C H2
C
H2 C
H2
C H2
C H2 C
H2
C H2
C H2
C H2
C H2
C H2
H2 C C These “fatty
acids” are part of a triglyceride
Saturated carbon chain
Unsaturated fatty acids have
kinks or folds in their chains due to the double bonds
cis Double bond trans Double bond
CH3
CH3
H3C
FIgURE 3.21 Unsaturated and saturated fatty acids.
O C C H2
CH2
CH2
CH2 CH2
C2
H2
C C2
H2
C
C C H + H2
C
O C
C H H
O C O
C C H
H A fatty acid that is part of a plant triglyceride.
Plant triglycerides are comprised mostly of triglycerides made of cis fatty acids
Partial hydrogenation A metal catalyst
50% Saturated fat (very unreactive to oxygen)
50% trans Monounsaturated fat (less reactive to oxygen than cis fat)
H3C CH2
CH2 C
H2 C
H2
CH2 C
H2 C
H2 C
H2 CH2
CH2 C H2
C
H2
C H2
C H2
C H2
C H2
C
C C C C
CH3
CH3
FIgURE 3.22 Partial hydrogenation. Heat and a metal catalyst help to add hydrogen atoms across the double bond of a plant saturated fatty acid. The result is a partial mixture of solid at room temperature of saturated fatty acid and a monounsaturated (trans) product.
“cisalpine,” meaning “on the near side of.” This bond creates a bend or kink in the carbon chain and alters the physical character and melting point of the fat. The high temperatures used in hydrogenation create a handful of fats that are in the trans configuration. Here the carbons are still in a double bond, but instead of the bend, the trans double bond containing carbon chains is straighter—more like a saturated fatty acid. Trans fats are potent antimicrobials and, unlike cis fatty acids, they resist reac-tion with oxygen, which results in rancid fats and spoiled food. However, an increase in trans fatty acid consumption has been associated with a number of health risks. In particular, a diet high in trans fat increases the LDL or “bad cholesterol” responsible for forming plaques and heart disease. Modern hydrogenation processes use different pressures, times of reaction, and temperatures to give rise to fats free of trans fats.
Artificial or low‐fat butter is another butter substitute that works well for spreading on food but not for cooking. These spreads are a mixture of vegetable oil emulsified with whey protein, buttermilk, and water. Depending on the product, many are very high in starches, gums, and milk proteins, all of which burn easily, making cooking with these butter substitutes difficult.