A number of interesting controversies have revolved around the differences in milk from various species (especially as to the relative appropriateness for use as milk for humans, notably babies). There are some notable differences in composition of milks according to the USDA’s Handbook of the Nutritional Contents of Foods, by Watt and Merrill, United States Department of Agriculture, Dover Publications, Inc, New York (1973). (See Table I, pp 38 & 39.) Data for sheep, water buffalo and reindeer came from Food Values, Pennington and Church, Perennial Library, (1985)
I have picked out nutrients from their table which have the greatest variation and compiled them in the table below. (100 mL is a little less than 1/3rd cup):
Additional Milk Nutrients from Food Values, Pennington and Church, Perennial Library, 1985
One could write a whole paper on these differences. I think the most interesting are:
Human milk has about 1/3rd as much protein as either cow or goat.
Human milk has twice the carbohydrate as either cow or goat
Human milk has 1/4th the phosphorous as either cow or goat
Human has five times the of vitamin C. This is not surprising, since of these three species, only human are unable to synthesize their own vitamin C, and thus must get it through the diet.
It may be notable that the fat content is about the same for all three species. Also, there are relatively few differences between cow’s and goat’s milk in this table, and yet our experience is that goat’s milk is more easily digested than cow’s. Persons with peptic ulcers have said that it helped considerably to consume goat’s milk.
We do not know, but we know that goat milk has smaller fat globules than cow milk. Besides making the fat content more easily digested, this explains why cream separates much faster from cow than from goat milk.
Also, over many years, we have had parents of young infants come to us for goat’s milk because their infants could not digest either cow’s milk formula, or soy milk formula, and the mother had dried up (or been dried up by the pediatrician!). These parents have been emphatic that their infants thrived on goat’s milk as opposed to cow’s or soy “milk.” The differences in digestibility of goat’s versus cow’s is apparently in some category not listed in this table.
We know that goat’s milk is naturally homogenized, since the cream does not separate nearly as readily as cow’s milk. We have interpreted this as the reason for goat’s milk digestibility. However, we have received email correspondence indicating that human milk does separate overnight, so there is probably more to the story.
The following stations should be set up in a logical arrangement in the lab to expedite the class testing of bacterial counts in meat and milk.
Before the exercise, each student should collect the following dilution tubes and empty plates and label as indicated:
FIRST: CONSTRUCT A TABLE in your lab book listing:
the five plates you will pour, the specimen,s the dilutions, the aliqouts, the CFUs/plate, and the final CFUs/g or mL.
SECOND: COLLECT AND LABEL TUBES AND PLATES:
five 16 x 150 mm tubes in rack (see ‘tube marking’ column in table)
five empty sterile plates (see last three columns in table)
THIRD: ALIQUOT OUT STERILE dH2O IN THE DILUENT IN TUBES:
plate #, specimen
CFU/g or mL
dilution: milk 102
dilution: meat 101
dilution: meat 102
dilution: meat 103
STATIONS FOR MEAT AND MILK ASSAY
I. A single blank station: Aliquot out Dilution Blanks:
Repipet with sterile dH2O, set on 9.9 mL (one for dilution of milk)
Repipet with sterile dH2O, set on 9.0 mL (three for dilution of meat suspension)
II. Two weighing stations: Meat Weighing (in pairs, side by side): One for each ground meat:
two 16 x 150 mm test tube ½ full of 95% EtOH
two sets of four microspatulas in the EtOH tubes
hot soapy water in nearest sink to wash off meat and fat from spatulas
III. One suspension Station: Meat Suspension Station on a bench near the weighing stations
Repipet with sterile dH2O, set on 3 mL
Repipet with sterile dH2O, set on 7 mL
IV. Four sample dilution, aliquoting Stations: Dilution of Samples, Aliquoting to Plates. (in pairs, side by side):
2 mL pipets
1 mL pipets
V. Two agar pouring Stations: Pouring agar into the prepared plates:
deep hot block, 45 C, with liquid 45 C Plate Count Agar (covered with a folded towel to prevent solidification)
tub of warm water to receive emptied test tubes
The most important consideration in good flavored milk and milk products is the proper handling of the milk from the time it is milked out to the time that it is consumed or made into cheese. It makes me wonder about commercially available goat’s milk, which in my experience has a strong “goaty” flavor. The goat’s milk I produce only has had such flavor problems if the steps given below were not followed. Many people who say they hate the taste of goat’s milk are usually referring to ‘store-bought’ goat’s milk, and find mine mild, sweet and rich. Likewise, many commercial goat cheeses taste like they were cured in the billy pen… NOT to my liking.
Here are the critical factors I have discovered over a couple of decades of keeping goats and making cheese. All of these are aimed at keeping bacterial contamination as low as possible. Undesirable bacteria are what make milk products have an off flavor. The goal in cheese-making is to add beneficial bacteria which produces good flavor while avoiding the rest. Undesirable bacteria abound on goat hair and dander. Removal of these is the goal of careful filtration.
Never try to make cheese out of “turned” or spoiled milk–the unpleasant flavor will linger. Feed it to your pets if they will drink it. Otherwise, put it on the compost pile.
Note that repeated reference is made to complete drying of thoroughly cleansed equipment. The reason is that most bacteria do not survive well on clean dry surfaces exposed to the air.
Avoiding Bacterial Contamination in your Milk Products
Cleanliness/Sterility of Milking Cans and Storage Bottles:
Immediately after milking, rinse equipment in lukewarm water to remove the majority of milk. If you let the equipment sit, the drying milk will glue itself in the cracks and crevices, and will be come a breeding ground for bacteria.
You should carefully wash the rinsed milking cans in very hot soapy water, rinse well, and air dry COMPLETELY. (Do not dry with a towel, it is easy to introduce bacteria this way.) If you have no problems with odor or taste in your milk, actually sterilization of the cans may not be required. But if you are having problems, your implements should be boiled and air dried. I avoid chlorine because of its poisonousness, but in the worst cases, may have to be resorted to.
Essentials of Recommended Cleansing:
Wash implements well in very hot water and soap
Rinse thoroughly in very hot fresh water
Ensure that they are thoroughly air dried before using
If you must use chlorine for sterilization, use as little as possible, and avoid any trace in your milk.
Stages of Milk Handling
I. Setting up Milking Equipment
II. Setting up Goat to be Milked
III. Cleansing Udders
IV. Milking and Feeding
V. Filtering and Recording
VII. Cleansing Equipment after Milking
Keep the milk chilled at 4ºC until ready for use. Do not add warm milk to previously chilled milk. It will encourage any bacteria in the older milk to grow. However, once thoroughly chilled, milk from sequential milkings can be pooled.
Follow these steps and maintaining a temperature of no more than 4ºC in your refrigerator and your milk should keep easily for more than a week without pasteurization. If goat’s milk is kept this long, cream can be skimmed off when making cheese. Freeze this cream immediately after skimming to produce delicious ice cream.
If you don’t follow these steps closely, you risk a number of bacterial contaminations including those of Salmonella, Escherichia coli and reportedly, Listeria.
Milk is extremely perishable and many means have been developed to preserve it. The earliest one which has been used for many thousands of years is fermentation. Milk can be fermented by inoculating fresh milk with the appropriate bacteria and keeping it at a temperature which favors bacterial growth. As the bacteria grow, they convert milk sugar (lactose) to lactic acid. You can detect its presence by the tart or sour taste (sour is how we taste acid). The lowered pH caused by lactic acid preserves the milk by preventing the growth of putrefactive and/or pathogenic bacteria which do not grow well in acid conditions.
Fermentation is a means by which cells growing anaerobically can still generate a little ATP. Fermentation is defined biochemically as the catabolism of glucose (or other sugars) in which the terminal hydrogen acceptor is an organic molecule (carbon containing). During the breakdown of sugar, known as glycolysis, excess hydrogen atoms are generated and must be deposited somewhere. In lactic acid bacteria, they “dump” excess hydrogens on to pyruvic acid, the end product of glucose. This turns pyruvic acid into lactic acid.
Our muscles do the same thing, which causes the sting in over exercised muscles. In all fermentation, NADH gives up its hydrogen to produce NAD, which is required for further glycolysis. Yeast too performs fermentation, but with different terminal hydrogen acceptors (acetaldehyde) and products (CO2 and ethanol). You will note that alcoholic fermentation is also an anaerobic process. Since the terminal hydrogen acceptor in each of these microbiological processes is an organic molecule, they are, by definition, fermentation.
In contrast, respiration uses an inorganic terminal hydrogen acceptor (such as oxygen). If oxygen is the acceptor, then water is produced.
Casein, the predominant protein in milk, is soluble at a neutral pH, but insoluble in acid. Thus when milk sours, casein precipitates which thickens the product. Numerous strains of bacteria are capable of converting lactose to lactic acid. We will look at several fermented milk products to study their morphology and staining characteristics.
Make a thin smear of each milk product well spaced on the same slide, labeling with a wax pencil Y, B and S. (see protocol Smear and Staining of Bacterial Specimens)
Stain them according to the procedure for the Gram stain (see related protocol Gram Stain Protocol), or any simple stain such as methylene blue, should you only be interested in seeing bacterial morphology.
View the stained smear at 400x to determine the characteristic features, select a field which is well spread and typically stained. Then switch to 1000x with oil. (The oil immersion lens is challenging to novices. Do not use this lens unless you have been instructed in its use.)
Illustrate typical fields for each milk product showing all observed morphologies of bacteria. Label the morphologies and their probable identities according to the following type of bacteria expected in these fermented milk products:
Yogurt is produced by a mixed culture of two types of bacteria. Imbedded in particles of the protein casein, you will see chains of cocci or diplococci (Streptococcus thermophilus) and big rod-shaped bacilli (either Lactobacillus acidophilus or L. bulgaricus). If you do a Gram stain, the bacteria will be Gram positive (purple) and the protein will be pink. The illustrations at the top of the page are micrographs I took of a Gram stain of yogurt. The purple rods are Lactobacillus, the purple spheres are Streptococcus. The pink globs are casein, milk protein. BUTTERMILK
Buttermilk is the fermentation of milk by a culture lactic acid-producing Streptococcus lactis plus Leuconostoc citrovorum which converts lactic acid to aldehydes and ketones which gives it its flavor and aroma.
Sour cream is produced by the same bacteria as buttermilk, but the starting milk product is pasteurized light cream. Bacteria are less numerous than in buttermilk.
Pour Plate Technique for Bacterial Enumeration Dilutions
Fresh food will typically have very low bacterial content, but as it is handled and stored, the bacterial concentration may increase dramatically. Pasteurized Grade A milk is required to have less than 20,000 bacteria/mL by standard plate count. Ground beef may contain up to 50 million bacteria/gm. Since the number of bacteria may vary by several orders of magnitude, samples of these foodstuffs must be diluted and several dilutions plated out in order to achieve the desired range of colonies per plate (50-500). Typically, for pour plate technique, 1.0 mL of dilutions ranging from undiluted to 103 (or higher) should be plated. (Greater dilution is necessary for highly contaminated samples.) The following procedure is for that purpose:
Milk to be tested. Have date of origin, if possible. Calculate age of material.
Standard Plate count agar*
Sterile dH2O in 4 repipets
Clean sterile petri dishes
EQUIPMENT: 15 mL melted Plate Count Agar in:
sterile capped 16x150mm test tubes
45o C bath (deep enough to = agar
depth. Hot Block, or water bath)
stainless steel spatula in test tube with 95% EtOH
sterile 16x150mm test tubes
0.1, 1.0, 2.0 & 10 mL pipets, sterile
colony counter with magnifying glass
DILUTIONS, AND ALIQUOT ADDITION TO PLATES:
Label two empty plates with your initials, the date, specimen (milk), aliquot volume (0.1 or 1.0 mL) and dilution factor (10^2).Prepare dilution blank: Add 9.9 mL sterile dH2O to a sterile 16x150mm test tube.
Dilute the milk: pipet 0.1 mL milk into above dilution tube (10^2 dilution), vortex to mix
Add the aliquots: Using a 2.0 mL pipet, pipet 0.1 mL into first plate, 1.0 into the second.
Pick up a tube of 15 mL melted plate count agar, cooled to 45C, (here in a hot block, a 45C water bath can be used.)
ADD MELTED AGAR TO MAKE POUR PLATE:
Add melted agar (dry off if maintained in hot water) to each plate in turn, swirl to mix completely. Plunge the emptied tube immediately into warm water before agar solidifies to ease cleansing.
When the agar is solid, invert the plate and incubate 35C for 48 hr.
Count colonies on the plates and calculate CFU per mL:
CFU on plate x dil’n factor (102) x aliquot factor (either 1/1.0 ml or 1/0.1 mL) = CFU/ mL milk
Example plate: 40 colonies on plate x 10^2 x 1/1.0 mL = 4000 CFU/mL of milk
Enter your results into the class table ( your initials, the milk manufacturer, its expiration date, CFU/mL)
* Standard Plate Count Agar: 5 g tryptone, 2.5 g yeast ext..1 g dextrose, 15 g agar, 1 L water
Suggested stations (at least two each? If enough repipets):
Empty plates station, wax pencil sterile bags of empty plates
16×150 mm sterile capped test tubes (5 per student)
test tube racks to hold 16×150 tubes (one per student)
Milk Dilution Station:
sterile capped 16×150 mm test tubes
repipet with sterile dH2O, set for 9.9 mL
0.1, one per student
2.0, one per student
Pour Plate Station:
hot block with styrofoam cage, 45 C
wash tub with warm water in it.
16x150mm test tubes with 15 mL 45 C melted Standard Plate Count Agar (5 per student)
Be sure to have enough tubes of SPCA (at least 3 tubes/person if they work in pairs on both experiments).
advanced testing laboratories
Standard methods agar
Plate count agar:
5 g tryptone
2.5 g Yeast est
1 g dextrose
15 g agar
1 L water (pH 7
suggested limit total viable
basic food microbiology book, Banwart, OSU Microbiology