Butter varies in flavor, both good and bad. Flavors can be: absorbed, bacterial, or chemical. Left in the open air, butter turns rancid quickly. This is why a butter crock (which keeps the butter away from air) is valuable. Due to its high water content, the size of the water droplets in the water-in-oil emulsion being less than 6 µm, butter is subject to rapid microbiological spoilage at normal temperatures.
It used to be that one farmer’s butter tastes different from another’s because of the pasture. You could taste the pasture. Most dry feeds (like hay or concentrates), silage, green alfalfa, and various grasses produce feed flavors in butter. Butter would taste different if the cow was eating feed (alfalfa, sweet clover) or weeds (onion grass, dandelions). Today, most cows eat pretty standardized diets, but the smell of the barn will affect flavors, as does the amount of time since the cows last ate. Cows impart an odor and taste within 30 minutes of eating or breathing grass or corn silage, legume hay, or brewer's grains. Similarly, butter will readily absorb flavors and odors from your refrigerator.
Bacterial
Butter can taste a bit malty (like “Grape Nuts”), or sour if bacterial had a chance to grow in the milk. The cause is usually due to Streptococcus lactis in poorly cooled milk. Bacterial degradation results from bacteria that get into the milk upon contact with improperly washed or sanitized equipment, from external contamination, and is made worse by improper cooling. Milk is an excellent growth medium for bacteria. It provides the nutrients and moisture and has a near neutral pH. Off-flavors are the results of bacterial growth psychrotrophs).
Chemical flavors
Chemical flavors can be cowy (ketosis), rancid, oxidized, sunlight, and medicinal. The cowy or ketone flavor is the result of the animal suffering from ketosis. A foreign flavor can be caused by medications, a reaction to pesticides, disinfectants, or any number of contaminants. Rancidity and oxidation result from the degradation of milkfat. This is the most common. Many of the oxidation pathways are not entirely understood. Salted butter was developed to prevent spoilage, and to mask the taste of rancid butter.
Rancidity
A sour-bitter taste is identifiable with rancidity (i.e. soapy, baby-vomit, blue cheese). Rancid butter becomes yellow to brown and the flavor becomes harsh. There appears to be a seasonal effect, with the months between July and September having the highest occurrences, and is also caused by stressed cows, and plumbing issued with the processing tanks. Rancidity is caused by a chemical development, which continues until the milk is pasteurized. It often occurs if the membranes around milkfat globules are weakened or broken. When butter becomes rancid, the enzyme lipase breaks it down into glycerol and fatty acids. Hydrolytic rancidity results in the formation of free fatty acids and soaps (salts of free fatty acids) and is caused by either the reaction of lipid and water in the presence of a catalyst or by the action of lipase enzymes. Low levels of free fatty acids are not objectionable if they are sixteen or eighteen carbon fatty acids as commonly found in soybeans, corn or animal fat. However, in butter fat (and coconut oil), low levels of shorter carbon chain fatty acids may be quite objectionable. The worst offender being butyric acid (butanoic acid).
Oxidation
A cardboardy or metallic taste is caused by oxidation. It is more common in milk from the winter and early spring because the cows eat less vitamin E, an antioxidant, in stored forages. It can also be caused by excessive copper or iron in the water supply used to wash equipment or compensate for dirty milking equipment The off-flavor can sometimes be detected in raw milk, but more often is noticed in high fat products such as butter or vanilla ice cream. Oxidative rancidity results from more complex lipid oxidation processes. The processes are generally considered to occur in three phases: an initiation or induction phase, a propagation phase, and a termination phase. In complex systems, the products of each of these phases will increase and decrease over time, making it difficult to quantitatively measure lipid oxidation. During the initiation phase, molecular oxygen combines with unsaturated fatty acids to produce hydroperoxides and free radicals, both of which are very reactive. For this phase to occur at any meaningful rate, some type of oxidative initiators must also be present, such as chemical oxidizers, transition metals (i.e., iron or copper), or enzymes (i.e., lipoxygenases). Heat and light also increase the rate of this and other phases of lipid oxidation. The reactive products of this initiation phase will, in turn, react with additional lipid molecules to form other reactive chemical species. The propagation of further oxidation by lipid oxidation products gives rise to the term "auto-oxidation" that is often used to refer to this process. In the final, termination phase of lipid oxidation, relatively unreactive compounds are formed including hydrocarbons, aldehydes, and ketones.
Other flavors
The following are other flavor characteristics common to butter:
| Acid | Associated with moderate acid development in the milk or cream, or excessive ripening of the cream. |
| Aged | Associated with short or extended holding periods of butter. The holding temperature will affect the rate of development of this flavor. May also occur if high quality raw material is not properly handled and promptly processed so that the flavor loses its freshness. |
| Bitter | Attributable to the action of certain microorganisms or enzymes in the cream before churning, certain types of feeds and late lactation. |
| Cooked | Associated with using high temperatures in pasteurization of sweet cream. |
| Coarse | Associated with using high temperatures in pasteurization of cream with slight acid development. |
| Feed | Attributable to feed eaten by cows and the flavors being absorbed in the milk and carried through into the butter. Most dry feeds (like hay or concentrates), silage, green alfalfa, and various grasses produce feed flavors in butter. Silage flavor may vary in degree and character depending on the time of feeding, extent of fermentation and kind of silage. |
| Flat | Attributable to excessive washing of the butter or to a low percentage of fats or volatile acids and other volatile products that help to produce a pleasing butter flavor. |
| Malty | Attributable to the growth of the organism Streptococcus lactic var. maltigenes in milk or cream. It is often traced to improperly washed and sanitized utensils in which this organism has developed. |
| Musty | Attributable to cream from cows grazing on slough grass, eating musty or moldy feed or drinking stagnant water. |
| Neutralizer | Attributable to excessive or improper use of alkaline products to reduce the acidity of the cream before pasteurization. |
| Old Cream | Attributable to aged cream, or inadequate or improper cooling of the cream. This flavor may be accentuated by unclean utensils and processing equipment. |
| Scorched | Associated with using excessively high temperatures in pasteurization of cream with developed acidity, prolonged holding times in forewarming vats or when using vat pasteurization. Also associated with vat pasteurization without adequate agitation. |
| Smothered | Attributable generally to improper handling and delayed cooling of the cream. |
| Storage | Associated with extended holding periods of butter for several months or longer. |
| Utensil | Attributable to handling or storing milk or cream in equipment which is in poor condition or improperly sanitized. |
| Weed | Attributable to milk or cream from cows which have been fed on weed infested pastures or weedy hay. |
| Whey | Attributable to the use of whey cream or the blending of cream and whey cream for buttermaking. |