Introduction
Cheese is a dairy product derived from milk and is produced in a variety of flavors and textures. It comprises proteins and fat from milk and forms by coagulation of the milk fat casein. The milk can be either acidified or the enzyme rennet can be added to cause the coagulation. Most cheeses are made from whole cow milk, with worldwide production of 18.7 million metric tons in 2014. The United States accounts for approximately 29% of this production, with various European countries accounting for most of the rest. Other types of milk used for cheese are skimmed cow, goat, sheep, and buffalo milk. There are approximately five hundred varieties of cheese recognized by the International Dairy Federation. There are many different criteria that can be used for classification, such as length of aging, texture, method of making, fat content, milk, and country or region of origin. There are some broad categories for cheese classification, such as acid or rennet cheese and natural or process cheese. Natural cheese is made directly from milk while process cheese uses natural cheese and other ingredients that are cooked together, affecting texture, melting properties, and shelf life. Another common category is moisture content, which directly affects hardness and can be designated as soft, semi-soft, semi-hard, and hard. However, there is no universal standard for most methods of cheese classification.
Cheese Manufacturing
Cheese manufacturing is a highly varied process depending on the exact type of cheese being manufactured. Temperature, time, target pH, sequence of processing steps, the use of salting or brining, block formation, and aging can vary considerably between cheese types and even in the intended end use of the same type of cheese (such as Cheddar manufactured for shredding, melting, or for cheese meant to be aged for several years). It is the job of a cheesemaker to control the spoiling of milk into cheese in a consistent manner with specific characteristics such as appearance, aroma, taste, and texture. The process described below generally refers to the manufacture of Cheddar, one of the most produced and popular cheeses. However, potential variations in the manufacturing process for other types of cheeses are referred to as well.
The first step in cheese manufacturing is often standardization of the milk. Standardization is the practice of adjusting the composition of cheese milk, usually by optimizing the protein to fat ratio. This can maximize economic return from the milk components while maintaining both quality and composition specifications. These specifications can be defined by the manufacturer or may be imposed by government regulations (often varying from country to country). Milk can be pasteurized or mildly heat-treated to reduce spoilage organisms and optimize the environment for starter cultures to grow. Some varieties of cheese are made from raw milk. Such cheeses must be aged for a minimum of sixty days to reduce the possibility of exposure to disease-causing pathogens that may be present in the milk. The milk must be cooled to a temperature of 90°F (32°C) if pasteurized or heated to the same temperature if the milk is raw for the starter bacteria to grow. Such cultures are called lactic acid bacteria (LAB) because their primary source of energy is milk lactose and their primary metabolic product is lactic acid. Starter cultures assist with coagulation by lowering pH prior to rennet addition while adjunct cultures provide and enhance characteristic flavors and textures. Bacteria which only produces lactic acid during fermentation are homofermentative while bacteria which produces other compounds as well are heterofermentative. Cheeses with a clean, acidic flavor like Cheddar require a homofermentative bacteria while fruity cheeses and cheese with bubbles in it require heterofermentative bacteria. Starter cultures and any adjunct cultures are added to the milk and held at 90°F for thirty minutes to ripen, which allows the bacteria to grow and begin fermentation as well as lowering pH and developing flavor.
Once the cheesemaker has determined that sufficient lactic acid has been developed during fermentation, the coagulation process begins. Rennet contains the enzyme chymosin which coagulates the milk protein and forms curds. As a general rule, 3 oz to 4 oz of rennet is added per 1000 lb. of a mix. It converts κ-casein to para-κ-caseinate, which is the main component of cheese curd. The vat must be thoroughly mixed to ensure a uniform mixture and the mixture must set for a minimum of 30 minutes to allow the curd to set. The mixture is kept between 84°F and 88°F. Temperature is controlled by flowing warm water through the jacket of the vat. Once the curd is ready, the liquid whey must be separated from it. Some soft cheeses are simply drained, salted, and packaged at this stage. For cheddar, the curd is cut into small pieces, cooked, and stirred until the desired temperature and firmness of the curd is reached. The cube size after cutting ranges from 0.25 to 0.63 cubic inches and the moisture level of the cheese will increase with larger cubes. After cutting, the curd is allowed to set for ten to fifteen minutes and is handled gently to prevent fat and protein loss. Hot water is added to the jacket of the vat for cooking. Stirring is constant during cooking and the time ranges from twenty minutes to sixty minutes. By the time cooking is complete, the pH will be around 6.4. The whey is then drained from the vat. There are different ways to separate the curd and liquid whey, but this is considered a standard method.
Depending on the type of cheese, various types of curd processing usually occur after initial separation of the curd and whey. Cheddaring is a unique process for cheddar cheese that involves stacking loaves of curd on top of one another to squeeze additional whey out of the loaves below. It also allows fermentation to continue to lower the acidity. Six-inch-wide loaves are cut along each side of the vat. After ten minutes, the loaves are turned over and stacking begins. The weight of the stacked loaves allows additional moisture to be expelled. This process continues using larger stacks (usually up to 4) and the loaves are turned every ten minutes until the pH of the whey reaches 5.1 to 5.5. The loaves are then cut into smaller pieces, returned to the vat, and sprinkled with dry salt. Salt is generally added at an amount between 1% to 3% of the weight and helps remove additional whey, prevents spoiling, adds flavor, prevents the cheese from becoming too bitter and acidic, and hardens texture by interacting with proteins. After salting, the curd pieces are placed in cheese hoops and pressed into blocks to form the cheese. The blocks are then stored in coolers and aged from anywhere to several months to several years, depending on the variety of cheese. There are many variations that can take place in this process for different types of cheeses. Moist and creamy cheeses like Brie and Camembert require a more gentle treatment of the curd. They are drained after coagulation by being hung from cheese hoops, immersed in a salt solution, and are either immersed in or sprayed with mold spores. Other cheeses can be ripened internally during some point in the process or ripened on the surface with yeast. Mozzarella and Provolone are both stretched in curd form and kneaded in hot water, contributing to the stringy texture and fibrous body. Lower acidity cheeses like Gouda and Colby are washed in warm water to reduce acidity. Cheesemaking is a complex manufacturing process with many variations and much skill required to ensure proper product quality.
Conclusion
NIR spectroscopy has emerged as a tool for rapid, non-invasive, and cost-effective analysis of parameters of interest in cheese that could potentially replace traditional reference methods. The rapid analysis is especially important as the different stages of cheese manufacturing often happen quickly, and the cheesemaker is reliant on sensory skills to make the cheese at the desired quality. Fat, protein, and moisture are critical parameters in nearly all types of food manufacturing and these parameters have been successfully measured in cheese using NIR spectroscopy. Acidity is important as well and although salt is not directly measurable using NIR spectroscopy it does change other compounds that are measurable, and the feasibility of this indirect measurement has been proven in studies. When manufacturing is complete, the aging process starts and is important for determining many physical parameters and these have been examined as well by NIR spectroscopy. In the case of fresh cheese, shelf-life is short and critical parameters will begin to change as the cheese ages further and eventually becomes unfit for human consumption. Adulteration is an issue in all food manufacturing and cheese is no exception, especially inexpensive cheeses produced in a particular region under strict standards and guidelines. All of these parameters and measurements have been studied using NIR spectroscopy with results showing the potential to replace traditional reference methods.
References
Fundamentals of Cheese Science – Fox, Guinee, Cogan, McSweeney, Springer 2000 https://www.amazon.com/Fundamentals-Cheese-Science-Patrick-Fox/dp/1489976795
Milk Facts: Cheese Production
http://www.milkfacts.info/Milk%20Processing/Cheese%20Production.htm
FAOSTAT: Food and Agriculture Organization of the United Nations
http://www.fao.org/faostat/en/#data/QP
Standardization of Milk for Cheese Making
https://www.uoguelph.ca/foodscience/book-page/standardization-milk-cheese-making
Determination of Fat, Protein, and Moisture in Ricotta Cheese By Near-Infrared Spectroscopy and Multivariate Calibration – Madalozzo, Sauer, Nagata, Journal of Food Science & Technology, March 2015 52 (3): 1649-1655 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4348305/
NIR Spectroscopy: A Useful Tool for Rapid Monitoring of Processed Cheeses Manufacture – Curda, Kukackova, Journal of Food Engineering 61 (2004) 557-560
https://www.sciencedirect.com/science/article/abs/pii/S0260877403002152
Using Near-Infrared Spectroscopy for the Determination of Total Solids and Protein Content in Cheese Curd – Sultaneh, Rohm, International Journal of Dairy Technology, Volume 60, No.4, November 2007 https://onlinelibrary.wiley.com/doi/full/10.1111/j.1471-0307.2007.00347.x