Visual scoring of milk mixed with blood

Sorting of normal and abnormal milk at time of milking is done visually for conventional milking systems, but more concrete standards are needed when milking is done in automatic milking systems (AMS). Several panel tests were carried out to find out how different consumer groups, milkers and advisors look at and respond to the visual appearance of milk mixed with blood, in order to set a limit for what they think is acceptable. It is concluded from the test panel results that milk samples with 0.4% or more of blood all will be scored as pink and samples with 0.1% blood (about 6 microM-haemoglobin or 100 mg/l) can be visually detected if they are compared with milk samples without blood. The consumer group scored fewer of the samples with 0-1% blood as normal than did the professional groups. The test panel scored 65% of the samples with 1% blood as normal when milk was presented in a black strip cup, which is the reference method when foremilking takes place in a conventional parlour. Only 2% of the milk samples with 2% blood (about 120 microM-haemoglobin or 2000 mg/l) were scored as normal in a black strip cup and should consequently be detected by conventional as well as automatic systems. One model of AMS was tested for its ability to detect and separate milk coloured by blood. The model separated milk with > or = 6 microM-haemoglobin. Milk mixed with blood injected into the milk stream for a short time at the beginning of milking was not separated. We lack data on how blood is naturally expelled into milk and in what amount. We propose that cow composite milk with > 6 microM-haemoglobin should be separated because at this level milk will have a red tinge.