Making Buttermilk, Illustrated

Cultured buttermilk is probably the easiest and most fool proof fermented milk product to make. (Note that cultured is different than “old fashioned buttermilk.”) All you need is active cultured buttermilk for the starter, and fresh milk for it to act on (store bought is fine). The formation of buttermilk is based on the fermentation by the starter bacteria which turns milk sugar (lactose) into lactic acid. As lactic acid is formed, the pH of the milk drops and it gets tart. Milk proteins, most notably casein, are no longer as soluble under acid conditions and they precipitate out, causing what we recognize as clabbering. Thus the two marked characteristics of buttermilk, its tartness and its thickened nature, are both explained by the presence or the action of lactic acid. Additional by-products of fermentation give subtle variations in buttermilk flavor.

The acidity of buttermilk also explains its long refrigerator shelf life. Acid is a natural preservative because it inhibits the growth of pathogenic bacteria. Thus buttermilk keeps easily for weeks in your refrigerator. If you keep it longer, it may develop mold on the inner walls of the jar. This mold belongs to the same group of fungi which grow on cheese and is not dangerous. Remove it and the buttermilk can still be used for baking. However, because the desired bacteria may have died in older samples, buttermilk older than three to four weeks may not work as an inoculum to make buttermilk.

Ingredients & Equipment

  • 6-8 ounces active cultured buttermilk
  • Check the label: it needs to say cultured buttermilk, and is not out of date. (The bacteria die down over time)
  • 3 cups whole milk (store bought works. 2% or skimmed too, but less rich.) very clean 1 quart container with secure lid (I prefer Mason jars).


I have used this recipe for years to make buttermilk in large quantities. I like to use it for baking as well as drinking. It makes pancakes, waffles, and cakes rise very well. You can make any volume of buttermilk you like, so long as you hold to the proportions of 1 part buttermilk plus 4 parts whole milk.

Every year for years, I have prepared a gallon of buttermilk (an ingredient in my cornmeal waffles) for an annual waffle breakfast I serve at Clermont College, serving about 120 people. To make a gallon of buttermilk, I add 1 quart buttermilk to 1 gallon of fresh whole milk in a large container, mix, and pour back into the original containers. The next day, the whole five quarts are nicely thickened.

It works because Streptococcus lactis (or a mixed culture of S. lactis plus Leuconostoc citrovorum) ferments the lactose in milk to lactic acid. The acidic pH causes the protein in milk (most prominently casein, pink in the picture below) to precipitate, thickening the liquid. Because much of the lactose has been broken down to lactic acid, buttermilk should cause less of a problem for those who are lactose intolerant.

It may be that buttermilk could be made with a lower proportion of starter (i.e. 1+6 or 1+8. Anyone have experience with this?) However, the 1+4 ratio has worked so well that I have not wanted to mess with the proportions.


Basic Cheese Making, Illustrated

Cultured Buttermilk from Scratch

  1. Allow a cup of filtered fresh raw milk to sit covered at room temperature until it has clabbered (usually several days).
  2. Place 1/4 cup of the clabbered milk in a pint mason jar, add a cup of fresh milk (does not have to be raw at this point), cover, shake to mix, allow to sit at room temperature until clabbered.
  3. Repeat this transfer of sub-culturing several more times until the milk dependably clabbers in 24 hours. Taste a small amount to confirm that it is tart, thickened, and has no off flavors. It should taste tart not bitter, for instance.
  4. To then make a quart of buttermilk with this culture, add 6 ounces of the buttermilk to a quart jar, fill with fresh milk, cover, shake to mix, allow to sit at room temperature until clabbered.
  5. Refrigerate.

Sour Cream Recipe

Sour Cream can be made with the same procedure as buttermilk, using one cup of cream mixed thoroughly with 2 Tbl fresh active buttermilk and letting it sit for 12-24 hours at room temperature. The higher butterfat in the cream, the thicker the finished sour cream.

Cultured versus Old Fashioned Buttermilk

“Cultured buttermilk,” commonly available in United States’ supermarkets, is not the same as “old fashioned buttermilk,” about which I get many questions. The latter is the liquid which remains after churned butter is removed. The two buttermilks bear few traits in common. See the following description of churning butter for the differences.

Churning Butter

In “olden times,” farm families would let freshly milked milk sit for half a day and skim off the cream which had risen.  This cream would be set aside in a cool place, around 50-60 F.  Each milking’s cream would be added until several gallons had accumulated. 

In the meantime, naturally occurring bacteria in the cream would cause it to slightly sour.  This souring increases the efficiency of churning.  The accumulated, slightly sour, cream would be churned at the optimum temperature (approximately 58 F) such that the butter was firm enough to separate out, but soft enough to stick together into a mass.  The butter was removed, washed in very cold water to remove the remaining milk, and salt worked in to preserve it.  The remaining liquid after the butter was removed was called buttermilk.  I call it “old fashioned buttermilk,”  which is slightly sour, has the consistency of  milk, but is slightly paler.  It has flakes of butter floating in it. 

Commercial manufacturers sometimes add colored “butter flakes” to imitate the old fashioned buttermilk.  However, the two products are very different, cultured buttermilk being thick and tart, old fashioned being thin, and slightly acid, depending on how sour the cream got before it was churned.

Microbiology of Buttermilk

See the page on Smearing and Staining of Bacteria to learn how to see these bacteria with a microscope, and the page on Milk Fermenting Bacteria for a demonstration and discussion of Streptococcus lactis, which is the bacterium which performs this fermentation. Below is a photomicrograph of buttermilk which has been smeared and gram stained. Cells of Streptococcus lactis can be seen as purple spots in a row. Casein is the pink mass covering most of the image.

Smearing and Staining of Bacteria, Bacteriological Smear and Staining Protocol




Milk Fermenters

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.

  1. 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)
  2. 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.
  3. 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.)
  4. 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 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.