Milk is a nutritive beverage mostly obtained from dairy cows, although it is obtained from camels, goats, water buffalo, and reindeer in different parts of the world. In the United States and many other industrialized countries, raw milk is processed before human consumption. These processes include adjusting fat content, adding vitamins, and killing potentially harmful bacteria using the pasteurization process. There are many different types of milk and classification is based on different factors, such as the amount of milk fat, type of processing, and the type of dairy cow that produced the milk. Prior to 1998, FDA standards required that milk sold as whole milk must have no less a 3.25% milk fat, low-fat milk must have 0.5% to 2.0% milk fat and skim milk must have less than 0.5% milk fat. Updated standards require 2% milk fat to be labeled as “reduced-fat” and 1% milk as “low-fat”. Milk fat is the richest energy component of milk, containing a high proportion of saturated fatty acids. Processing-based classifications include pasteurized (heated to kill any harmful bacteria), homogenized (fat particles reduced in size and blended to prevent cream formation), and vitamin-fortified (various vitamins added during manufacturing). Most milk produced in the United States fits all three of these classifications. Grade A milk is produced under sufficiently sanitary conditions to be sold as fluid milk and about 90% of milk made in the U.S. meets these standards. Grade B milk is acceptable for manufactured products like cheese where further processing is required. Certified milk is produced under highly sanitary standards and is more expensive than Grade A milk.
The average composition of cow’s milk is 87.2% water, 3.7% milk fat, 3.5% protein, 4.9% lactose, and 0.7% ash. This composition can vary based on cow breed, season, animal feed content, and many other factors. Cows are milked twice a day using mechanical vacuum milking machines, which are filled, cooled, and then pump the milk into a refrigerated tank truck where the milk is transported to the processing plant and again pumped into refrigerated tanks in the plant. The cold raw milk passes through a clarifier to remove debris, bacteria, and sediment. A separator can be also be used to perform these tasks. The separator can also separate heavier milk fat from the lighter milk to produce both cream and skim milk. Excess fat can also be drawn off and processed into cream or butter. Vitamins A and D can be added to the milk after separating and before pasteurization. Pasteurization kills any bacteria and the standard requirement for whole milk, skim milk, and standardized milk is heating to 161°F for 15 seconds. The requirements vary for other products and a temperature sensor redirects the milk for another pass through the pasteurization tube if the temperature has fallen below the required level. After pasteurization, homogenization is performed by pressurizing the hot milk to 2500 psi to 3000 psi by a piston pump and forcing the milk through small passages in an adjustable valve. The shearing effect breaks down the fat particles to proper size, which ensures even distribution and prevents the milk fat from separating and floating to the surface as cream. After homogenization, the milk is quickly cooled to 40°F to avoid any harm to the taste. The cooled milk is packaged in either coated paper cartons or plastic bottles, sealed, and shipped to distribution warehouses in refrigerated trailers.
NIR spectroscopy has emerged as a tool for rapid, non-invasive, and cost-effective analysis of parameters of interest in milk that could potentially replace traditional reference methods. Fat is considered the most important parameter in milk because it is the richest energy component and its content is highly regulated. Fat has been successfully measured using NIR spectroscopy and calibration models in numerous studies. Protein, lactose, and urea are considered important parameters as well and these have also been successfully measured using NIR spectroscopy. Other parameters of interest including freezing point, solids content, and acidity measurements. Adulteration is a tremendous problem in the food and beverage industry and milk is no exception. Liquid milk adulteration can occur by mixing a cheaper brand of milk with a more expensive brand. Milk powder is the second most adulterated food behind olive oil according to the USP database on food fraud and economic adulteration. All of these parameters and measurements have been studied using NIR spectroscopy with results showing the potential to replace traditional reference methods.
Milk: How Products Are Made
What Are The Steps In Milk Processing?
Accuracy of In-Line Milk Composition Analysis with Diffuse Reflectance Near-Infrared Spectroscopy – Melfsen, Hartung, Haeusserman, Journal of Dairy Science 95: 6465-6476 https://www.sciencedirect.com/science/article/pii/S0022030212006509
Accuracy of the FT-NIR Method in Evaluating the Fat Content of Milk Using Calibration Models Developed for the Reference Methods According to Rose-Gottlieb and Gerber – Mlcek, Dvorak, Sustova, Szwedziak, Journal of AOAC International Volume 99, No. 5, 2016
Non-Targeted NIR Spectroscopy and SIMCA Classification for Commercial Milk Powder Authentication: A Study Using Eleven Potential Adulterants – Karunathilaka, Yakes, He, et al., Heliyon 4 (2018) e00806. https://www.heliyon.com/article/e00806/pdf