When managing a farm, certain nutritional programs or management practices will cause certain milk flavour problems. It is essential that the farmers prevent this from happening as the quality of milk cannot be improved after it leaves the farm. Having milk flavour problems will definitely affect the farm's bottom line as customers will take notice and lose confidence in the quality of the product.
Classification of Off-Flavours
Off-flavours can generally be classified into 3 categories:
Absorbed - feedy, barny, cowy, unclean, weedy and musty. Usually occur when milk is left uncovered or kept in close proximity with other odour-producing food.
Bacterial - acid, malty, unclean, fruity, and putrid. This happens when bacteria get into the milk through contact with improperly washed or sanitised equipment or external contamination.
Chemical - cowy (ketosis), rancid, oxidized, sunlight, and medicinal. Cowy or ketone flavour occurs due to animals suffering from ketosis. Rancidity and oxidation usually results from the degradation of milk fat.
Soapy-bitter taste is usually related to rancidity. Highest occurrences seems to be between July and September. Main cause of rancidity is chemical development, which usually continues until the milk is pasteurised. This will involve lipase or other enzymes which reacts with the milkfat and form free fatty acids. In order to prevent this, ensure that intact membranes around milkfat globules is available as the causes for rancid flavour are those that result in weakened or broken milkfat globule membranes.
Farm Oxidised Flavour
This is also a reaction of milkfat. Normally, the off-flavour can be detected in raw milk. However, there are times where it can only be detected two days after collection. It can also occur in any pasteurised milk or dairy product that has not been flavoured.
Main reason there is a higher chance for chemical development of oxidation is due to the lack of antioxidant in the milk. The main cause being decreased amount of vitamin E, an antioxidant, in stored forages, which reduces the amount found in milk.
As milkfat begins to oxidise, it will only further intensify overtime. Taste may not be so apparent in milk but it is easily detectable in high fat products such as butter or vanilla ice cream.
All feed flavours are absorbed directly through the cows' system instead of the milk. The animals will impart an odour and taste within 30 minutes of eating or breathing silage, especially after about one hour.
The main ingredients causing this problem are usually grass or corn silage, brewer's grains or legume hay. Here is how the odour or taste can transfer to the milk:
Nose or mouth -> lungs -> blood -> milk
Mouth -> digestive tract -> blood -> milk
There are instances where the milk have unpleasant or dirty after taste. This usually occurs during winter. It can happen through absorption where cows breathe air with a barny odour and transfer it to the milk.
Dust, dirt as well as manure can cause the milk to taste unclean. It is essential to ensure that the cows' surrounding is kept clean. Other possible factors include unclean milking equipment or washing the cows' udder with water but failing to dry them properly. It is also possible for animals with ketosis to cause this problem.
Milk is a great medium for bacteria to grow as they provide the nutrients and suitable conditions for bacteria growth. The off-flavours of milk are usually caused by bacterial growth. In this case, type of bacteria is more important than the number. Psychrotrophs bacteria are capable of surviving in extreme cold temperatures and they have the ability to cause spoilage. They are usually the type of bacteria that will cause off-flavour in milk.
Malty flavour usually occur due to Streptococcus lactis in poorly cooled milk. Milk usually not cooled for 12 hours will have high count of bacteria. Malty flavour is also an indication of high acid flavour. It is rarely developed in pasteurised milk. With that being said, the characteristic flavour will remain after processing even though the flavour is developed in raw milk. Malty flavour will become high acid if not stopped by pasteurisation.
The key to prevent high acid flavours is proper sanitation. Spoilage is usually due to bacterial action on lactose (milk sugar).
All equipment used to handle milk should be clean thoroughly before reuse. Bulk tanks must be washed each time they are emptied. The milk from the bulk tanks should be picked up at least every other day as any milk left for more than 3 days will have potential for high bacteria counts and this will cause off flavours to the milk products.
Putrid flavours are mainly due to bacterial contamination, a storage temperature above 5°C and age. The spoilage is by bacterial action on the protein rather than lactose. If left for few days, putrid milk will curdle, separate, and smell rotten.
The most common source for this problem is contamination in the processing plants following pasteurisation. In general, psychrotrophs do not survive pasteurisation unless there is a large number of bacteria in the milk. Generally, the contamination occurs due to some part of the pumping, holding and filling system not being cleaned properly.
It is also possible that putrid flavours may originate from raw milk caused by contamination and holding the raw milk for more than 3 days after collection from farm. This is evident as retail samples of milk are found with putrid flavours and high bacteria counts. The easiest way to solve this is through proper pasteurisation and packaging without contamination. Even though bacteria are destroyed, the spoiled flavour remains in the milk. No amount of heating or vacuum treatment can lessen the flavour.