Home Milk Processing
Dairy products such as yogurt, kefir, and cheese result from similar processes. Outcomes that range from soft and spreadable quark to hard grana-style cheeses like Parmigiano–Reggiano DOP (Protected Designation of Origin in Italian) result from variations in process time, process temperature, and the selection of microbiological cultures. While the constancy of dairy products made by large cheese plants and skilled artisans is difficult to achieve at home, it is possible to make good versions of most dairy products with simple tools. After all, fermented dairy products predate the complex equipment and manipulations now common in milk processing.
Cheese, yogurt, and kefir all begin as milk, yet they finish with dramatically different appearances, textures, aromas, and flavors. Each variation represents a particular decision made along the way during processing. Home processors will find that while recipes for making yogurt, kefir, and cheese are helpful guides, the best way to develop reliable methods is over successive attempts. It helps to keep records of each batch so that the next effort can be just slightly tweaked to influence the outcome.
Much depends on milk quality. Artisan and industrial dairy processors constantly adjust their processes to account for changes in this basic raw ingredient. As the seasons change and animals move through their lactation cycles, the solid components of milk such as protein and milkfat change. At the industrial level, processors will often standardize milk by adding or removing fat so that the ratio of protein to milkfat is consistent in every lot. Artisan cheese makers may also standardize milk but, because of their simpler processing equipment, some adjust their process or even change the product altogether. For example, milk from a seasonal herd of cows in late lactation will often not set a curd as firmly as that made from early-lactation milk. This difference would favor making a softer cheese late in the season, such as a soft-ripened washed-rind cheese or a fresh spreadable cheese. Home dairy processors often adjust their process in response to gradual changes in milk performance. For the more advanced, some may separate cream from the milk using a small hand-cranked machine, and some may apply heat treatments to alter the microorganism populations of raw milk.
Raw milk as it comes from the animal is the best starting point for home dairy processing. Raw cow’s milk is available in most northeastern states. Check with your NOFA chapter for local laws about buying it and finding sources. If raw milk is not available, the second-best option is unhomogenized pasteurized milk, which is sometimes called cream-top milk. Vat-pasteurized milk, which is processed at 145˚F, is preferable to milks pasteurized at higher temperatures. Pasteurized homogenized milk, such as that commonly available in stores, has been heated to high temperatures, and the structure of the milkfat has been upset so that the cream does not separate. While standard store-bought milk will work well in certain applications like making whole milk ricotta and other fresh cheeses, it will not perform as well in others, such as making firm cheeses.
Home processors making cheese may wish to heat-treat or alter the fat content of the cheesemilk in order to control for certain microorganisms. Thermization, which is less common in the United States than in Europe, involves briefly heating the milk to temperatures as low as 135˚F, which has the effect of killing many undesirable bacteria without altering the performance of the milk. This can be a useful step when milk must be held for a few days before processing, or when the intention is to exactly control the microbiological community in the milk.
Pasteurization entails holding the milk at 145˚F for 30 minutes, which kills a broader range of microorganisms but can have the effect of causing milkfat to separate from the milk and can potentially affect cheese yields. Small countertop cream separators help home processors separate cream either to make a cream product or adjust the milkfat in cheese milk. Alternatively, cow’s milk cream will rise to the top on its own and may be skimmed off.
The home processor will usually not manipulate the milk, but will begin with fresh raw milk. In general, it is best to work with at least a gallon at a time when making cheese because much of the milk’s volume is lost as whey during processing. Firm cheese will yield 10%–12% of the weight of the milk. Twenty pounds of milk (2.33 gallons) will yield two pounds of tomme-style cheese, which is a minimum size for proper ripening of firm cheeses. Yogurt and kefir, however, have a 100% yield; one gallon of milk yields one gallon of yogurt. Strained yogurt products such as labneh and “Greek” yogurt lose some weight as the whey drains from the yogurt.
Milk processing basics
Most dairy processes rely on the development of lactic acid in the product as a result of temperature manipulation over time. Lactic acid bacteria digest the milk sugar lactose and excrete lactic acid. These bacteria also produce other products of fermentation, such as buttery-tasting diacetyl, that are responsible for flavors characteristic of dairy products. Mesophilic bacteria are most active and produce the most lactic acid at temperatures close to body temperatures, and die at temperatures above about 105˚F. Thermophilic bacteria are less active at lower temperatures, but thrive in the range of 110˚F to 135˚F. Cheese types made at lower temperatures, such as tomme-style cheeses like Cricket Creek Farm Maggie’s Round, rely on mesophilic bacterial species. Cheeses made at higher temperatures, such as Alpine-style cheeses like Robinson Farm’s A Barndance, rely on thermophilic bacteria.
In addition to time and temperature controls, the home dairy processor will manipulate the product mechanically to achieve the desired result. This includes straining yogurt to produce a thicker product, agitating cut curd during the scalding process of cheese making, or the hooping and pressing of cheese curd to form wheels.
The first step in almost any process, save kefir- and possibly butter-making, is to heat the milk to the initial process temperature. When heating milk on the stove, always work in heavy-bottomed stainless steel pots, which are non-reactive and easy to clean. Gas or induction ranges are easier to control compared with electric coil ranges. The home processor should gently yet constantly stir the milk as it warms. The milk should be in motion any time it is heating.
Yogurt is a lactic-set curd facilitated by thermophilic bacteria. As the milk incubates at 110˚F, the resulting acid from fermentation denatures milk proteins and results in the matrix that constitutes yogurt’s texture. An initial high-heat process facilitates this matrix: heat the milk to 180˚F, cover the pot and turn off the heat, and wait 10 minutes. After 10 minutes, put the pot in a sink filled with cold water and stir the milk to rapidly cool it to 110˚F. Once the milk is at the incubation temperature, inoculate it with a desired bacterial culture.
There are different yogurt cultures available that yield slightly different acidities and consistencies. Most cultures feature multiple species; ABY-2C from Danisco includes Streptococcus thermophiles, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus delbrueckii subsp. lactis, Lactobacillus acidophilus, and Bifidobacterium lactis, where each species contributes in part to the thick body, lactic aroma, and mild acidity found in the finished product. Follow the dosage directions on the package when using a commercial direct-set culture such as this. Alternatively, inoculate the yogurt with existing yogurt. If using this method, choose a locally made yogurt free of pectin, nonfat dry milk powder, or gelatin. Add about 2% of the weight of the total milk in prepared yogurt. For example, if working with 8.6 pounds of milk (one gallon), add about three ounces of finished yogurt.
After incorporating the culture, fill clean jars and incubate them at 110˚F for six to eight hours. To keep the jars warm, place them in a plastic cooler and fill the cooler with approximately 130˚F water. Add enough water to cover the jars about half way, and then close the cooler. Alternatively, use a box-type food dehydrator, such as those made by Excalibur, to maintain the incubation temperature. Set the thermostat to 115˚F. After the incubation, set the jars in the refrigerator until they are cold.
If the goal is to make raw milk yogurt, forgo the high-heat processing and heat the milk to only 110˚F. The resulting yogurt may not set as firmly as milk that has been treated with high heat. To strain yogurt, line a colander with fine wet cheesecloth and drain the yogurt overnight.
Sour cream and cultured butter
Sour cream is a mesophilic fermentation of cream. To make it, separate cream from cow’s milk, inoculate it with a mesophilic culture blend such as FloraDanica, and incubate the cream in a jar for about eight hours at 75˚F–80˚F. Once cooled, it will be rich and spoonable, but free of the gums and stabilizers common in commercial sour cream.
To make cultured butter, follow the same steps as those above for sour cream using a half-gallon mason jar as the fermentation vessel. Fill the jar ¾ full, which leaves a significant amount of space in the jar. After fermentation, chill the cream to refrigeration temperature, and then allow it to warm to about 55˚F. With the lid on, shake the cultured cream. The mass will liquefy and the butterfat will begin to congeal. After the butterfat is clearly separated from the buttermilk and floating on its surface, strain the butter from the buttermilk with a colander lined with wet cheesecloth. Save the buttermilk for use in baking or as a refreshing drink. Place the butter in a pre-chilled metal or ceramic bowl and work it with two wooden spoons to press out more buttermilk. Salt the butter, if desired, and shape it into a squat round before wrapping in paper or plastic and refrigerating.
Kefir relies on a symbiotic mass of organisms commonly called kefir grains. This fermented milk product is liquid and thin, and will vary in terms of acidity and carbonation depending on the length of the fermentation and storage. Sandor Katz, in his excellent 2012 book The Art of Fermentation, reports that only half of the many microorganisms resident in kefir grains are known or even named. The microorganisms in the grains rapidly sour milk while producing other fermentative byproducts, including ethanol and carbon dioxide.
To make kefir, add kefir grains at 5% of the weight of the milk to milk at ambient temperature. It works well to use a glass jar with a lid that seals. Ferment the milk at room temperature for 24 hours to three days. Agitate the jar over the course of the fermentation to redistribute the grains and make more nutrients available to the fermenting microorganisms.
Once the desired level of acidity is reached, fish out or strain out the grains and, if desired, seal the jar. Keep the sealed jar in the refrigerator for one or two days, and the kefir will carbonate in the sealed environment, resulting in a sparkling beverage unique among dairy products.
Cheese is the result of time, temperature, culture, and mechanical process decisions. In all cases, cheese is defined as the concentration of milk solids. There are three common ways to isolate the protein and fat from milk to yield cheese: the addition of acid, lactic acid fermentation, or the addition of enzymes. Most cheese made in the United States is of the last sort. Many fresh and young goat’s milk cheeses are of the second sort. Acid-set cheeses include paneer, ricotta, and mascarpone.
Acid-set cheeses are made at high temperatures and without the aid of bacterial fermentation. Instead of lactic acid from these organisms, the home cheese maker relies on citric, acetic, or tartaric acid.
To make an acid-set cheese from whole milk, heat one gallon of milk to 180°F, stirring gently but constantly. Once the milk reaches the process temperature, turn off the heat and stop stirring. Pour one cup of white vinegar into the milk and wait one minute. Observe the formation of the curd, which will collect like fluffy white pillows. If the milk is still white and opaque, add more vinegar, up to one additional cup. Ideally, the liquid surrounding the curd should be translucent and have a green-yellow color. Allow the curd to form for another five minutes, without stirring, and then strain the curd through a cheesecloth-lined colander.
This approach can be varied to use lemon juice. Classic ricotta involves the same process, but is made from whey or a mixture of whey and milk. The resulting cheese has a light texture and should be lightly salted. Add about 1.5% salt in reference to the weight of the resulting cheese. For example, add about a quarter of an ounce, or one and a half teaspoons, of kosher flake salt to one pound of ricotta cheese. To make a firmer slicing cheese, shape the loose curd into a package using cheesecloth and press it with a moderate weight for a few hours in the refrigerator.
Lactic-set cheeses rely on a very different and cooler process. Classic lactic-set cheeses and semi-lactic-set cheeses such as Rawson Brook Farm Monterey Chèvre don’t reach above 80˚F. This cooler process temperature requires a much longer process time that usually runs overnight and can last up to 18 hours. Over that long fermentation period, mesophilic lactic acid bacteria acidify the milk to the point that it curdles. Many cheese makers will add a small amount of enzymatic coagulant to help set the curd. Rennet is the catch-all term for the enzymes used to set curd. Due to its unique property of maintaining a relatively homogenous milkfat, goat’s milk is especially suited to lactic-set processing.
To set a lactic-set curd, heat milk to a mild 75˚F and inoculate it with mesophlic bacteria, such as Danisco Choozit MM-series. To ensure a good curd set, add double-strength rennet at a rate of one or two drops per gallon of milk. Dilute the rennet in five times its volume of non-chlorinated water before adding it to the milk and gently stirring it under. Cover the vessel and set the milk in an area that will hold a consistent 75˚F for the duration of the long fermentation. This could be a warm mechanical room in the winter, or a shaded corner of a house in the summer. At the end of the fermentation, the curd will have a lactic yogurt aroma and will have contracted from the walls of the vessel. If these characteristics don’t present themselves, the curd likely needs to acidify further before draining. Insufficiently acidified curd will not drain properly.
Work gently with the curd when draining it. Either scoop the curd into a wet cloth or nylon draining sack (a clean thin cotton pillowcase can also work), or gently ladle the curd into perforated cup-like cheese molds. Allow the curd to drain for another six to 12 hours. If working with cheese molds, flip the cheeses two or three times during the draining period to result in an evenly shaped and drained cheese.
Salt the resulting cheese with kosher flake salt such as Diamond Crystal. Salt fresh spreadable cheese at a rate of 1.5%–2% of the cheese’s total weight. For molded cheeses intended for ripening, sprinkle salt on both the top and bottom of the cheeses to cover them evenly and lightly. Ripen cheese with the aid of Penicillium candidum (white) mold and Geotrichum yeast—mix a bit of each inoculant in a quart of water and spray the cheeses’ surfaces to inoculate them evenly. Age the cheeses in an area with minor air movement, about 90% relative humidity, and 60˚F–70˚F, such as a cellar. Turn them twice or three times a week for three to four weeks. Mold will bloom and then subside, and the cheeses will slowly dry and begin to mature under the influence of the mold.
Rennet is often the word used to refer to a number of enzymatic coagulants that set curd. The primary enzyme, chymosin, may be extracted from the abomasa of kids, calves, or lambs. Microbial rennet is manufactured by microorganisms. Some plant-derived coagulants are used in specific traditions, but these plant-derived coagulants are not as versatile as animal or microbial rennet. Rennet is available in single and double-strength concentrations and liquid, paste, and tablet forms. Refer to the package for dosage rates. Because it is higher in solids, sheep’s milk requires about 25% less rennet than cow’s or goat’s milk.
To make a tomme-style cheese, warm milk within the range of 88˚F–96˚F and inoculate it with choice mesophilic cultures such as Danico Choozit 4000-series and surface-ripening cultures such as Penicillium candidum (white mold). Ripen the milk for 40–60 minutes at the same warm temperature, and then stir in the measured and diluted amount of rennet. Once gently stirred in, still the milk and allow it to set until the curd is resilient. Test the curd by poking a finger in under the surface and lifting up: if the curd splits open cleanly, it is ready to cut. If it is watery or does not cohere, it needs more time to set.
Cut the curd into evenly sized pieces. If working in a pot, it can be easiest but not the most precise to use a whisk to very gently and slowly achieve curd pieces the size of a large pea. Once cut, allow the curd to settle for a few minutes. Then turn on the heat and slowly warm the curd in the whey while stirring gently with a submerged hand, increasing the temperature by one degree every two minutes up to 102˚F. At this point, turn off the heat and continue stirring gently. The curd should cohere: when gently squeezed in a hand, it sticks together.
Scoop off the whey and fill the moist curd into a perforated hoop mold. Flip the cheese over ten minutes after filling, and then flip again after two hours and once more after four hours.
The next day, generously cover the surface of the cheese with kosher flake salt, but remove any excess. Allow the cheese to dry for a day, and then begin ripening it in an area with high (95%) relative humidity and a temperature of 55˚F. Flip the cheese twice a week and clean its surface with a soft brush or damp cloth as it ages for up to eight weeks.
All cheese approaches are open to variations in temperature, time, moisture in the curd, and mechanical manipulations. It is good to experiment with changes as long as the bacterial cultures are not killed in the process and salting rates are kept in the appropriate range.
Fermented dairy products are the result of natural processes that happen as time passes and microorganisms act. The attention the home dairy processor can afford a particular project is often greater than what a busy professional cheese maker can afford. The home processor has the luxury to study and care for cheese as it ripens or explore different yogurt cultures with curiosity.
Sanitation is very important when working with milk. Boil equipment prior to use to help yield more consistent outcomes. Always boil cheesecloth or other fabrics before use. Keep ripening areas clean by protecting cheese from dust and rodents. Ripening cheese will often host native molds. While these are not necessarily harmful, they can upset the intended outcome. It is up to the processor to determine what is or is not acceptable in terms of appearance and taste. With freedom comes responsibility.
- 1 gallon of milk = 8.6 pounds
- Thermization temperature: 135˚F for 15 seconds
- Vat pasteurization temperature: 145˚F for 30 minutes
- Cheese yield: 10% of weight on tomme-style cheese; slightly higher on fresh cheese
- Yogurt process temperatures: 180˚F for 10 minutes, then incubate at 110˚F for six to eight hours
- Inoculate kefir at 5% of total milk weight
- Acid-set cheese process temperature: 180˚F
- Lactic-set cheese incubation temperature: 75˚F
- Age individual lactic-set cheeses at 90% relative humidity and 60˚F–70˚F
- Age tomme-style cheese at 95% relative humidity and a temperature of 55˚F
NOFA Mass Raw Milk Network
Dairy Connection/Get Culture
Brent Wasser manages the Sustainable Food & Agriculture Program at Williams College. He made cheese professionally at Sprout Creek Farm in Poughkeepsie for four years and made goat’s milk cheeses in Austria and Belgium. His forthcoming book, The Cheese Professional: A Guide to Understanding, Selecting, and Serving Cheese, explains the relationships between cheese qualities and cheese making processes.