Wiki: Microorganisms & Cheese Making

This Wiki Article provides an introduction to microorganisms common in cheese making. Microorganisms are organisms that are unicellular or live in a colony of cellular organisms such as bacteria, fungi, archaea, and protists; microscopic plants (green algae); and animals such as plankton and the planarian. Microorganisms were the first forms of life to develop on Earth, approximately 3–4 billion years ago, the study of microorganisms is called microbiology.

Microorganisms live in all parts of the earth’s biosphere where there is liquid water, including soil, hot springs, on the ocean floor, high in the atmosphere and deep inside rocks within the Earth’s crust. Microorganisms are critical to nutrient recycling in ecosystems as they act as decomposers. Microorganisms are present in raw milk and are used in controlling the fermentation process in making cheese, either natively or by additionally applying, and are critical in developing different cheese types flavours and aromas, and in inhibiting undesirable organisms. Microorganisms are also used in brewing, winemaking, baking, pickling and other food making processes. Historically and in many cases currently cheese has been made in very rustic facilities with many ambient/wild microorganisms, which does not necessarily mean unsanitary, but does result in cheese quality that is too inconsistent for modern cheese factories but good for home and artisan cheese making.

The main types of microorganisms used in making cheese are shown below.

Bacteria

Bacteria are a biological kingdom of simple single-celled, prokaryote (lacking a cell nucleus) microorganisms. They are typically a few micrometres in length and have a wide range of shapes ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat, growing in water, soil, acidic hot springs, as well as in organic matter and the live bodies of plants and animals including humans. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water; and in total form a bacteria with a biomass which exceeds that of all plants and animals. There are approximately ten times as many bacterial cells in the human flora as there are cells in the human body, large numbers of bacteria are on the skin and in the intestines. Bacteria are vital in recycling nutrients, however most bacteria have not been characterised, the study of bacteria is known as bacteriology, a branch of microbiology.

Bacteria reproduce asexually by simple division of the cell and its contents, called fission. The doubling time can be as short as 20 min, and as each cell grows and divides at the same rate as the parent cell, this could under favourable conditions translate to an increase from one to 10 million cells in 11 hours. However, bacterial growth in reality is limited by lack of nutrients, accumulation of toxins and metabolic wastes, unfavourable temperatures and dessication. Bacterial populations are expressed as colony forming units (CFU) per gram or millilitre.

Bacterial growth generally has several phases:

  • Lag Phase: Time for bacteria to become accustomed to their new environment. There is little or no growth during this phase.
  • Log Phase: Bacteria exponential growth begins; the rate of multiplication is the most rapid and constant.
  • Stationary Phase: Rate of multiplication slows down due to lack of nutrients and build-up of toxins. At the same time, bacteria are constantly dying so the numbers actually remain constant.
  • Death Phase: Cell numbers decrease as growth stops and existing cells die off.

Milk is sterile at secretion into the udder but is contaminated by bacteria before it leaves the udder. Except in the case of mastitis, the natural microflora of bacteria at this point are harmless and few in number. Further infection of the milk by microorganisms can take place during milking, handling, storage, and other pre-processing activities. If significant lactic acid producing bacteria are in raw milk, it will, with time and warmer temperatures, multiply, acidify and curdle the milk (commonly called clabber) after which whey can be drained. However, depending on bacteria amount and types this can result in either inefficient, uncontrollable, and unpredictable and thus variable results and if lactic acid producing bacteria are minimal, an environment for unwanted/unhealthy bacteria to reproduce. Thus to extend shelf life, milk is commonly pasteurized to kill off most of the bacteria, both good and bad types. Commonly in cheese making, wanted lactic acid producing bacteria are added (inoculation) to the milk, to out-compete unwanted native bacteria, these are called starter cultures.

Common bacteria used in making cheese are:

  • Lactic Acid Producing Bacteria: Commonly called starter cultures, naturally in milk but commonly added to milk to ferment lactose in milk to lactic acid causing coagulation and release of whey to condense remaining milk ingredients into cheese for longer shelf life.
  • Brevibacterium linens which is ubiquitously present on the human skin where it causes foot odor and applied to give cheese rinds an orange colour and a very pungent aroma, examples are Limburger and Port-du-Salut.

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Fungi

In biological terms, Fungi are a Kingdom and thus different from other Kingdoms such as plants, animals, and bacteria. Fungi are made up of yeasts, molds (spelt moulds in Britain) and mushrooms. The discipline of the study of fungi is called mycology, a branch of microbiology. Fungi are abundant worldwide but most are inconspicuous because of the small size of their structures. They may become noticeable when fruiting, either as mushrooms or molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. Fungi have long been used as a direct source of food, such as mushrooms and truffles as a leavening agent for bread, and in fermentation of various food products, such as wine, beer, soy sauce, and cheese.

Little is known of the true biodiversity of the Fungi Kingdom, which has been estimated at around 1.5 million species, with about 5% of these having been formally classified. Most fungi grow in cylindrical, thread-like structures 2–10 µm in diameter and up to several centimeters in length called hyphae. From these they grow by a combination of apical growth and branching/forking resulting in mycelium, an interconnected network of hyphae which is normally more visible to the naked eye, (ie fuzzy mold on damp walls, spoiled food such as bread, or on cheese where used to provide flavour and aroma.

Yeast Fungi

Currently yeasts are thought to be about 1% of all fungal species, about 500 have been described by mycologists. Yeasts ferment carbohydrates to carbon dioxide and alcohols, and are common in baking such as bread making for their carbon dioxide forming attributes and in for their alcohol making attributes in alcoholic beverage making such as beer and wine.

Yeasts are intentionally used in making some cheese types to enable rind development and are also a source of unwanted “infections” resulting in rising bread/beer yeasty smells and swelling. Note that Swiss types cheeses with eyes (Emmenthaler, Leerdammer) are not made with yeast but with Propionibacterium freudenreichii, a bacteria.

Mold Fungi

Molds grow like mushrooms, they have a surface component and send down roots called mycelia into the insides of the cheese where they break down the fats and proteins and create different flavors and textures, some penetrate just cheese rinds, some penetrate the whole paste (for example Brie or Camembert.

Common molds used in making cheese are

  • Geotrichum.
  • Penicillium white strains used in making the fluffy coating on Brie/Camembert.
  • Penicillium blue strains used in giving blue cheeses such as Roquefort and Stilton their distinctive blue to blue-green veins.

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Wiki: Breaking Or Cutting The Curd

Rennet coagulated cow's milk, starting to cut curd with bread knife Curd Cutter.

This article discusses how to break or cut the coagulated milk, a critical process in making cheese. When to break or cut the curd is discussed in the article Wiki: Curds, When to Cut.

When the milk first coagulates, it has a natural tendency to contract and expel whey. This property is called syneresis and it depends on several factors, the largest being surface area of the coagulum or curd. Small surface area versus volume of the coagulum results in slower and less whey expulsion, larger surface area results in faster/more whey expulsion. The purpose of cutting or breaking the curd is to increase it’s surface area and thereby increase it’s expulsion of whey and reduce the water content of the curds and in the final cheese.

Lactic Acid Coagulated Cheeses

Light Cream Cheese primarily lactic acid coagulated cow's milk, ladling in to muslin sock for gravity draining.
Light Cream Cheese primarily lactic acid coagulated cow's milk, ladling in to muslin sock for gravity draining.

Lactic acid coagulated cheeses commonly employ three different methods to break or cut the coagulum depending on cheese type:

  • Some cheeses such as Quark are broken by simply stirring the coagulum.
  • Some cheese such as Cream Cheese are ladled directly from the coagulum in the vat into the cheese cloth or molds without being cut or stirred.
  • Some cheeses such as Cottage Cheese are cut with wire knives similar to rennet coagulated cheeses.

Rennet Coagulated Cheeses

Rennet coagulum’s are normally cut into similar size particles, the size and shape varies depending on the type of cheese being made. Smaller cut curds generally result in drier cheese, larger in moister cheese. The cutting patterns listed below have been refined over centuries of cheese making to allow the right amount of whey to be expelled for that type of cheese. Ideally every curd particle is of the same size to:

  • Create a uniform cheese, large range in cut curd sizes will result in areas of high and low moisture content in the cheese. High moisture can result in sour or fermented areas.
  • Create uniform temperature among the cut curds, especially if cooking the curds. Different sized cut curds will heat at different rates resulting in different internal temperatures which will cause differing whey expulsion and acid production rates.

Method: Cubed Cutting

Rennet coagulated cow's milk, starting to cut curd with bread knife Curd Cutter.
Rennet coagulated cow's milk, starting to cut curd with bread knife Curd Cutter.

Cutting of the coagulum into cubes is the most common system and used for making many cheeses. Cut sizes range from 6-25 mm / 0.25-1 inch cubes, for example Cheddar normally requires 1 cm / 3/8″ cubes. To cut the cubes, commercial cheese makers normally use rectangular shaped Curd Cutters or Harps or Curd Knife with evenly spaced wires. Two of these tools are normally used, the first with horizontal wires to cut the coagulum into horizontal sheets, the second with vertical wires to cut vertically into horizontal square rods and then again vertically but at 90 degrees to create the cubes. Generally a cross-hatched grid curd cutter is only used for large sized cut curds, as for smaller it generally tears the curd into irregular sized pieces as the forces on the curd are too great.

These professional Curd Cutters are normally built to match the size and shape of the large volume vat. Home/hobby small scale cheese makers commonly use a large stockpot for their vat. For them, a common workaround is to use a long thin food grade object as a Curd Knife and:

  • First, slice the curd vertically into parallel even thickness sheets.
  • Second, slice the curd again vertically but perpendicular, at 90 degrees to the first row, resulting in vertical square rods of curd.
  • Third, slice a third row of cuts but at 45 degrees to the first two rows of vertical cuts and with the curd knife tilted at 45 degrees from vertical to form “diamond” shaped cut curds.
  • Fourth, a row of cuts at 90 degrees to the previous 45 degree row and with knife still tilted at 45 degrees, to fully and reasonably evenly form the cut curds.
  • Fifth, circle the knife around the edge of vat to detach cut curd from vat.

Common kitchen examples for a stockpot Curd Knife are: Bread Knife, Cake Knife, or Icing Spatula. Note, if the tool is too thick (like a metal cookie cooling rack) then the curd will be torn rather than cut.

Method: Rice Cutting

Rennet coagulated cow's milk rested 10 minutes after cutting into diamonds.
Rennet coagulated cow's milk rested 10 minutes after cutting into diamonds.

Drier type cheeses like Swiss normally have the coagulum cut into rice sized pieces using a “Spino” harp shaped tool. Often this is improvised in home/hobby cheese making by using a large metal whisk.

Method: Italian Cutting

An egg shaped device for Italian cheeses. Again, this is often improvised in home/hobby cheese making by using a large metal whisk.

Method: Partial Cutting

Camembert cheese making uses only parallel vertical cuts and sometimes 90 degree parallel vertical cuts resulting in long square rods of cut curd. This is normally done with a long single cutting knife or sword. The partially cut curd is then horizontally cut by a tool called a Pelle or with a flat ladle when removing the cut curds from the vat and placing them into the gravity draining hoops.

Wiki: Dry Salting Curds

Four 10 cm diameter Camemberts after dry salting.

This article discusses the five main factors that control the dry salting of curds method of salting cheese, curd size and temperature, and salt type, amount, and application method. Many cheese making recipes or procedures call for dry salting cut curds after cooking or washing and draining whey and before forming the cheese by placing the cut curds in molds to drain or in hoops for pressing.

US Made Morton Brand Non-Iodized Canning & Pickling Salt
US Made Morton Brand Non-Iodized Canning & Pickling Salt

As reviewed in the Wiki: Salt’s Function article the salt % content in the type of cheese being made is critical, thus it is important to understand the factors that control dry salting curds.

For salt to be absorbed by the curd it must first dissolve and form a brine at the cut curd surface after which it then diffuses into the curd.

Curd Size

Ideally you want all the curd pieces, whether cut or milled, to have the same size so that the same amount of salt is absorbed and the same amount of whey expelled, resulting in a uniform cheese.

Large curd pieces are of special concern as they will result in high moisture and low salt content which can result in sour and fermented defects.

Curd Temperature

Curd temperature during direct dry salting should be between 87-92°F/31-33°C.

Higher temperatures result in a higher flush of whey which will:

  1. Carry away salt before it can be absorbed resulting in the cheese having sub-optimal salt % content.
  2. Carry away excess fat resulting in a greasy/seamy texture.

Temperatures below this range can result in a limited flush of whey and thus a limited brine forming around the curd pieces resulting in lower, sub-optimal uptake of salt.

Salt Type

US made Morton brand 25 pound bag of non-iodized Table Salt with anticaking sodium silicoaluminate
US made Morton brand 25 pound bag of non-iodized Table Salt with anticaking sodium silicoaluminate

Salt types are discussed in the article Salt Type’s, A To Z. For dry salting curds, standard dry crystalline non-iodized no anticaking additive sodium chloride salt is used.

It should be coarse granular sized (not rock or kosher grain size salt) rather than very fine to slow down the speed of dissolving into brine and absorption into the curds. As with high temperatures, fine gained salt can result in a rapid flush of whey and fat which can wash away other salt before being absorbed resulting in incorrect salt % content of the cheese type being made.

Salt Amount

As reviewed in the Salt’s Function, A to Z article, final salt content of the cheese type being made is critical. Thus follow the cheese making procedure accurately on amount/weight of salt being applied versus weight of final cheese(s).

Salt Application Method

Generally, rather than in one large application, dry salt should be sprinkled onto cut or milled curds in several equal increments with gentle stirring or mixing in between. This is for the same reason as correct temperatures and coarse grained salt are optimal, to slow down the rate of salt absorption into the curds.

The gentle stirring or mixing helps to distribute the salty whey fully around the curd pieces rather than just where the salt is applied. Note, excessive or rough stirring is not recommended as it will result in smaller pieces of curds and a larger unwanted size distribution of the curds.

Two to three such increments spaced ~10 minutes apart are common. After the last salting, the curds should be allowed to rest for ~10 minutes to stabilize before proceeding to the next step in the cheese making procedure, normally forming the cheese.

Wiki: Ingredients, When To Add

Malaka Brand Vegetarian Liquid Rennet Retail Box - CheeseForum.org

This Wiki Article discusses the several common cheese making ingredients which can be added to milk at the start of the cheese making process before the curd is formed. It provides a short description, Order to add, Measuring, When Add, and Adding Method. Which ingredients to add and their amounts are cheese type – recipe specific and not discussed here. If your recipe and the recommendations below differ, follow the recipe. This article is to understand Typical cheese curd ingredients are:

  1. Cheese Base such as milk, cream, or whey.
  2. Milk Enrichers such as creams or milk powder.
  3. Starter Culture to ripen the milk by creating lactic acid and aid in coagulation.
  4. Colourants to change the cheeses colour from common default white.
  5. Lipase as a flavour augmentor.
  6. Calcium Chloride to aid in rennet coagulation if using store bought pasteurized milk.
  7. Rennet as a coagulant.

Cheese Base

  • Description: Milk, Cream, Whey
    • Can be from single or multiple animals
    • Can be from combination of raw and pasteurized.
  • Order: First as base for cheese and needs to be heated or cooled to recipe temperature before other ingredients added.
  • Measuring: Standard volume or weight measuring devices.
  • When Add: Start of cheese making.
  • Adding Method: Pour into cheese vat with minimal foaming to minimize oxidation of milk.

2 – Milk Enrichers

Common Household Kitchen Food Colourants - CheeseForum.org
Common Household Kitchen Food Colourants - CheeseForum.org
  • Description: Creams, Milk Powder.
  • Order: Second as normally diluted into cheese base.
  • Measuring: Standard volume or weight measuring devices.
  • When Add: Directly after placing cheese base in vat as need to adjust temperature before adding other ingredients.
  • Adding Method: Pour liquids and dry powder directly in cheese base in vat with minimal splashing or foaming to minimize oxidation of milk. Stir until additional fluid is fully diluted or powder is fully dissolved.

3 – Colourants

  • Description: Annatto or other food grade dyes, normally liquid.
  • Order: After cheese base and any enrichers to ensure good dilution, before Starter Culture is added to minimize stirring after adding. Can be added before or after Lipase or Calcium Chloride (if used).
  • Measuring: Small volumes by drops from an “eye dropper” bottle, larger with liquid measuring device.
  • When Add: While adjusting vat ingredients temperature to recipe requirement as Colourants are normally inert and not temperature sensitive.
  • Adding Method: Normally highly concentrated thus to ensure optimal dilution, pre-dilute in non-fluoridated water then trickle into vat from low height to minimize splashing then stir in with vertical strokes of perforated ladle to thoroughly dilute (unless want streaky coloured cheese in which case stir less). Some colorants will degrade rennets’ effect, thus ensure your dilution container is cleaned before using it to dilute rennet.

4 – Lipase

Danisco Brand Mild Calf Lipase, 16 Ounces - CheeseForum.org
Danisco Brand Mild Calf Lipase, 16 Ounces - CheeseForum.org
  • Description: Normally manufactured freeze-dried product.
  • Order: After cheese base and any enrichers to ensure good dilution, before Starter Culture is added to minimize stirring after adding. Can be added before of after Colourants or Calcium Chloride (if used).
  • Measuring: By volume as can be partially rehydrated depending on storage practices.
  • When Add: Preferably after adjusting vat ingredients temperature to recipe requirement as Lipase is minimally temperature sensitive.
  • Adding Method: Sprinkle freeze-dried into bowl of non-fluoridated water to rehydrate for 20 minutes. Then trickle directly into the milk then stir in vertically with perforated ladle just barely breaking surface to minimize splashing and foaming to full depth of vat for 1 minute to ensure optimal dilution.

5 – Calcium Chloride

  • Description: Normally diluted aqueous liquid.
  • Order: After cheese base and any enrichers to ensure good dilution, before Starter Culture is added to minimize stirring after adding. Can be added before or after Colourants or Lipase (if used).
  • Measuring: Liquid measuring device.
  • When Add: While adjusting vat ingredients temperature to recipe requirement as Calcium Chloride is inert and not temperature sensitive.
  • Adding Method: Normally highly concentrated thus to ensure optimal dilution, pre-dilute in non-fluoridated water then trickle into vat from low height to minimize splashing then stir in with vertical strokes of perforated ladle to thoroughly dilute (unless want streaky coloured cheese in which case stir less). Some colorants will degrade rennets’ effect, thus ensure your dilution container is cleaned before using it to dilute rennet.

6 – Lactic Acid Starter Culture

Danisco's Choozit Product Line, MM100, 250 Dose Mesophilic Lactice Acid Producing Starter Culture - CheeseForum.org
Danisco's Choozit Product Line, MM100, 250 Dose Mesophilic Lactice Acid Producing Starter Culture - CheeseForum.org
  • Description: Natural in milk, whey from previous cheese making, concentrated buttermilk, manufactured liquid or frozen or freeze dried. Mesophilic or Thermophilic or combination.
  • Order: After other ingredients and before rennet (if used) after which do not want to stir as break curd.
  • Measuring: Set of small volume measuring devices for liquids and a mini scale for solids.
  • When Add: After other curd ingredients added and before rennet added (if used) and vat materials are at recipe’s recommended temperature.
  • Adding Method: Pour liquid or frozen into vat minimizing any splashing or sprinkle freeze-dried directly onto the milk and allow to rehydrate for 5 minutes. Then stir in vertically with perforated ladle just barely breaking surface to minimize splashing and foaming to full depth of vat for 1 minute to ensure optimal dilution. Note, after diluted, do not stir (unless adding rennet) during ripening period as excessive aeration can reduce the rate of acid production resulting in longer ripening times required to get to the correct pH before renneting.

7 -Rennet

  • Description: Normally diluted aqueous liquid, tablet, or freeze-dried.
  • Order: After other ingredients as do not want to stir as break curd while curd is forming until correct point to break or cut curd.
  • Measuring: Set of small volume measuring devices for liquids and a mini scale for solids, pill cutter if need part of tablet for tiny batches.
  • When Add: After all other curd ingredients added and ripening time or pH drop after adding starter culture as determined by recipe.
  • Adding Method: Dilute or dissolve rennet in small amount of non-fluoridated water, then pour liquid into vat minimizing any splashing. Note, tablets can take up to 45 minutes to fully dissolve. Then stir in vertically with perforated ladle just barely breaking surface to minimize splashing and foaming to full depth of vat for 1 minute to ensure optimal dilution. Note, after diluted, do not stir during coagulation period until point to break or cut the single large curd. Note, some colorants will degrade rennets’ effect, thus use separated clean dilution container or colourant dilution container is cleaned before re-using to dilute rennet in.

Wiki: Coagulation

Rennet coagulated good quality curd, checking with finger Clean break method.

All cheeses use coagulation of the milk as an essential cheese making step. This articles discusses how coagulation works and the different types generally used in cheese making.

General

After acidification, the next step in making cheese is coagulation. Coagulation of milk is the first step towards concentration of milk’s casein and fat and expulsion of whey made up of water and milk’s soluble components. During coagulation the casein micelles form long chains that branch in all directions and bond with themselves forming a three dimensional matrix that encompasses and all the milk including fat and water. Cheese makers call this matrix the curd.

The process of coagulation occurs through two different mechanisms, primarily acid coagulation and primarily enzyme coagulation. Each method results in two very different families of cheese. To be literally correct, rennet is the historical name of the product from animals, but in cheese making rennet is the generic term for all types of enzymes, whether of animal, plant, microbial or fermentation origin, that are used to coagulate milk.

The most common method is enzyme or rennet coagulation as it produces a lower moisture content and longer shelf life curd without excessive hardening. Virtually all hard cheese are made using rennet coagulation.

Lactic Acid Coagulation

General

Primarily lactic acid coagulated curd from store bought cow's milk after sitting overnight.
Primarily lactic acid coagulated curd from store bought cow's milk after sitting overnight.

Several soft cheeses such as cottage cheese, quark, and traditional cream cheese use lactic acid coagulation which occurs as response to a reduction in pH from production of lactic acid by the starter culture. Pasteurization including Ultra Pasteurization and UHT milks are fine for making Lactic Acid cheeses, they will give a thicker lactic acid set curd than when using raw milk.

The procedure for setting is to add a precise amount of mesophilic starter culture to milk at a temperature of ~21°C / 70°F, much lower than for rennet coagulated cheese, although some recipes – procedures use warmer temperatures.

The starter culture causes lactose to be converted to lactic acid, lowering the pH with full coagulation into a solid curd occurring around a pH of 4.6 – 4.7. This can take 4 to over 24 hours, depending on the temperature and the amount and activity level of the starter culture.

The curd that is formed from lactic acid coagulation is much weaker than from rennet coagulation and the curd more strongly resists the expulsion of whey. Thus the resulting cheese is softer and higher moisture than rennet coagulated cheeses.

As the resultant cheeses are softer and moister, they have a shorter shelf life and are consumed young and thus in some countries such as USA, milk is required to be pasteurized for health reasons.

Milk Type

Primarily lactic acid coagulated curd using store bought chocolate flavoured cow's milk.
Primarily lactic acid coagulated curd using store bought chocolate flavoured cow's milk.

For lactic acid coagulated cheeses, when cow’s milk is used, it is normally pasteurized or skimmed otherwise cream will separate during the long incubation time resulting in an non-homogeneous curd. Whole non-homogenized cow’s milk is used but the whole curd must be re-worked to form a homogeneous mixture. Some cheese making recipes-procedures use both acid and rennet coagulation, here rennet is normally added in significantly smaller amounts than when rennet coagulating milk as it is added not to cause coagulation but rather to enable better whey separation and better curd formation, and thus less cream separation when using non-homogenized milk.

Conversely non-homogenized sheep and goat’s milk do not easily cream and thus there is less need for rennet addition or homogenization in lactic acid coagulated cheeses, however some recipes still call for the addition of small amounts of rennet when using sheep or goat’s milk for lactic acid coagulated cheeses.

If using rennet in lactic acid coagulated cheeses, the lower setting temperature than for rennet coagulated cheeses still allows the non-enzymatic phase of rennet coagulation but not the second enzymatic phase.

Rennet Coagulation

General

Rennet coagulated good quality curd, checking with finger Clean break method.
Rennet coagulated good quality curd, checking with finger Clean break method.

Rennet coagulation originally used enzymes from the lining of the fourth stomach of calves and from the stomachs of kid or lamb as they have these enzymes naturally in their stomach to better digest milk and as that was what was commonly available. There are two prime enzymes in these stomachs that coagulate milk, initially chymosin and later after weaning, pepsin. It is the chymosin enzyme that is the stronger milk coagulant. Since the 1990’s other forms of “rennet” (fermentation, microbial) have been made.

Rennet coagulation is a two stage process involving an initial enzymatic phase during the first ~10 minutes where a chemical change is occurs as preparation for second non-enzymatic phase where the casein micelles start forming linked chains and eventually a full solid curd is formed, if enough calcium is present.

Rennet is discussed in detail in Wiki: Rennet, different types of rennet are discussed in Wiki: Rennet Types.

Milk Preparation

Milk is warmed to optimum rennet coagulation temperature of 30-36°C / 86-96°F. Higher temperatures up to ~40°C / 104°F result in faster coagulation times. Above 40°C / 104°F and the rennet becomes inactivated. Lower temperatures result in slower coagulation, below ~18°C / 65°F coagulation will not occur.

Starter culture, CaCl2, lipase, colourants such as Annatto and flavor additives must be added to the milk before renneting so that they are incorporated evenly in the curd. See Wiki: Ingredients, When To Add.

Other additives such as dill weed, caraway or cumin seeds, or smi dried fruit like apricots can be mixed in later, normally after the curd is cut and whey is drained.

Note, many rennet coagulated cheese making procedures have a pre-ripening time of up to 1 hour after adding the starter culture and before adding rennet.

Adding Rennet To Milk

Rennet coagulated store bought cow's milk curd, after diamond cutting, imprint of bowl from spinning bowl flocculation test.
Rennet coagulated store bought cow's milk curd, after diamond cutting, imprint of bowl from spinning bowl flocculation test.

Rennet is very concentrated, so adding it directly to the milk would cause it to set the milk in just that area and not in the overall milk. Even if one stirred it after adding directly, it would still coagulate in areas resulting in a poor curd formation. Therefore the common method of adding rennet is to first dilute / dissolve in cool non-chlorinated water before adding to the milk. Dilution amounts are discussed in the Animal Based Rennet webpage.

Best practices for rennet preparation and addition are:

  1. When ready to add rennet, dilute or dissolve rennet in cool un-chlorinated water. Chlorine is a strong oxidizing agent and rapidly destroys the rennet enzymes.
  2. Trickle the diluted rennet into the milk while stirring the milk with a skimmer for a maximum of 60 seconds in an up and down method without breaking to surface (no splashing). Do not dilute rennet in advance of adding to milk as its strength deteriorates when diluted.
  3. Stop the swirling of milk after stirring with skimmer to enable better coagulation.

Wiki: Acidfication

Generally, cheesemaking starts with acidification. This is the lowering of the pH (increasing acid content) of the milk, making it more acidic. Classically, this process is performed by bacteria. Bacteria feed on the lactose in milk and produce lactic acid as a waste product. With time, increasing amounts of lactic acid lower the pH of the milk. Acid is essential to the production of good cheese. However, if there is too much acid in the milk the cheese will be crumbly. If not enough acid is present the curd will be pasty.

Thus, for cheese makers, there are pH markers that you need to hit for each step of the process, for this you need a pH meter and to track the pH readings.

Wiki: Curds, When To Cut

Rennet coagulated cow's milk, testing for Clean Break.

This Wiki Article discusses the different methods to determine when to break or cut the milk curd after it has been coagulated, a critical control point in making cheese. When milk first coagulates, it has a natural tendency to contract and expel whey. This property is called syneresis and it depends on several factors, the largest being surface area of the coagulum or curd. Initially the milk is coagulated as a single large curd with a small ratio of surface area to volume which results in slower and less whey expulsion from the coagulum as the whey in the middle has a long distance to travel before being free from the curd.

The purpose of breaking (for lactic acid coagulated cheese) or cutting (for rennet coagulated cheese) the curd is to increase its surface area to volume ratio and thereby increase its expulsion of whey to reduce the water content of the curd and in the final cheese.

Lactic Acid Coagulated Cheeses

Primarily lactic acid coagulated cow's milk curd after ripening overnight.
Primarily lactic acid coagulated cow's milk curd after ripening overnight.

Lactic acid coagulated (i.e. no or minimal rennet) cheeses such as cream cheese and cottage cheese are normally formed as one large curd and broken either by stirring or ladling.

This should be done when the coagulum reaches a pH of 4.6-4.7, more acidic – lower pH and the curd will be brittle and “shatter”, higher pH and the curd is more rubbery and clumps together.

If not using a pH meter, a pH of 4.6-4.7 is normally reached when the curd either pulls away from the sides of the vat with about 1 cm/0.5 in of whey around the perimeter and on top of the curd or the curd tears in the middle with about 1 cm/0.5 in with whey released in the gap and again on top of the curd, click on pictures to enlarge and view.

Rennet Coagulated Cheeses

Cutting of rennet coagulated curds should happen when the curd is firm enough to cut cleanly:

  1. If cut too early then the curd be mushy and cut poorly into non-ideal uneven small pieces or fines resulting in 1) fat and casein loss to the whey and thus poor yield, and 2) lower moisture content in the cheese.
  2. If cut too late the curd will be very firm, tough, rubbery and will bunch up around the curd knife when cutting and i) tear into pieces of non-preferred uneven size, and ii) tend to resist whey expulsion and result in higher moisture content than wanted in your cheese. Additionally, the un-cut curd will already be expelling whey from its surface and shrinking resulting in uneven density cut curds, as dryer at surface and moister cut curds from interior.

The following are three common methods for determining when to cut rennet coagulated curds, from simple to accurate:

Method #1: Timing

This method simply involves using a set time after rennet is added, i.e. 20, 30, 45, 60 minutes to cut the curd.

While simpler for authors of cheese making recipe books, this method is not recommended as milk types and starter cultures vary widely around the world and across the season thus a non-optimal time of cutting is highly probable.

Method #2: Clean Break

Rennet coagulated cow's milk, testing for Clean Break.
Rennet coagulated cow's milk, testing for Clean Break.

This method involves inserting a clean finger or flat object into the curd at a ~45 degree angle to a depth of 5 cm/2 inches at an early time point, i.e. 20 minutes after renneting, and then lifting the finger or spatula or knife vertically to break the curd. You then observe the curd to see if in general:

  1. You have a sharp clean split of the curd and green whey accumulating in the pocket where the cut was made, in which case the curd is ready to be cut.
  2. You have a ragged mushy split with milky whey accumulating in the pocket where the cut was made, in which case the curd is not yet ready to be cut and more time, say another 10 minutes given for the curd to set before trying the test again.

This method isn’t perfect as the ideal firmness of the curd is a function of the milk, which varies over the season, and the cheese type being made. Note, some cheese making procedures call for varying level of firmness before cutting, for example Roquefort using sheep’s milk calls for firmer curd before cutting than blue cheeses using cow’s milk.

Method #3: Flocculation Point

This method is the most complex and the most accurate. For this you need to observe the onset of flocculation which usually occurs ~10-15 minutes after adding rennet. This time after adding rennet to the onset of flocculation is then multiplied by a factor, dependent on the cheese type being made, which determines when to cut the curd after the addition of rennet. This method is based on the optimum cutting time being proportional to the flocculation time and that there are two phases in curd formation. First is the enzymatic phase followed by the aggregation phase where the lattice or curd is formed. The flocculation time is at the start of the aggregation phase. These times are dependent on the levels of casein and calcium which vary with the source of the milk, thus this method takes into account the milking/lactation cycle through the year.

The factor applied to the flocculation time is a function of the cheese type being made, normally between 2 and 6, for example 2-2.5 for Swiss & Alpine types, 2.5-3 for cow’s milk Cheddar, 3.5 for Monterey jack, 4 for Feta & Blues, and 5-6 for soft cheeses such as Camembert & Domiati. So if flocculation time is 15 minutes, then for Feta or Blues, total time since adding rennet to when cut is 60 minutes.

Rennet coagulated cow's milk, spinning bowl flocculation time testing.
Rennet coagulated cow's milk, spinning bowl flocculation time testing.

The reason for the different multipliers for different cheese type recipes – procedures is because the curd at time of cutting will have different strength, young curd set will more readily release water when cut versus older curd set will release less. Thus this time, along with the cut curd size are large determinants in the final moisture content of the cheese.

Note, many aged goat cheeses are made from a hybrid lactic and rennet curd set, time to cut is usually performed when pH has dropped to 4.5 and cutting is performed by ladling the curds into forms.

Optical Flocculation Time Test:

This test uses visual observation of the flocculation of the milk and is normally used by very experienced cheese makers.

  1. When adding rennet to milk start a timer.
  2. Insert a metal spatula into the milk, withdraw it and observe the milk as it flows back off the spatula, it will drain freely.
  3. After the milk has pre-ripened for ~8-10 minutes, repeat every ~30 seconds. Initially the milk will thicken and then at the onset of flocculation at the surface of the milk, tiny white grains will be seen in the film of the milk off the spatula. That is your flocculation time.

Physical Flocculation Time Test:

This test uses a physical observation of the flocculation of the milk and is better for less experienced cheese makers.

  1. When adding rennet to milk start a timer.
  2. Float a small bowl on top of your milk, spin the bowl, it will spin freely because the milk has not flocculated.
  3. After the milk has pre-ripened for ~8-10 minutes, spin the bowl every ~30 seconds. When the bowl won’t spin, that is the flocculation time.

Flocculation time multipliers:

Flocculation Time Multiplier
(min:sec) 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
8:00 12:00 16:00 20:00 24:00 28:00 32:00 36:00 40:00 44:00 48:00
8:30 12:45 17:00 21:15 25:30 29:45 34:00 38:15 42:30 46:45 51:00
9:00 13:30 18:00 22:30 27:00 31:30 36:00 40:30 45:00 49:30 54:00
9:30 14:15 19:00 23:45 28:30 33:15 38:00 42:45 47:30 52:15 57:00
10:00 15:00 20:00 25:00 30:00 35:00 40:00 45:00 50:00 55:00 1:00:00
10:30 15:45 21:00 26:15 31:30 36:45 42:00 47:15 52:30 57:45 1:03:00
11:00 16:30 22:00 27:30 33:00 38:30 44:00 49:30 55:00 60:30 1:06:00
11:30 17:15 23:00 28:45 34:30 40:15 46:00 51:45 57:30 63:15 1:09:00
12:00 18:00 24:00 30:00 36:00 42:00 48:00 54:00 1:00:00 1:06:00 1:12:00
12:30 18:45 25:00 31:15 37:30 43:45 50:00 56:15 1:02:30 1:08:45 1:15:00
13:00 19:30 26:00 32:30 39:00 45:30 52:00 58:30 1:05:00 1:11:30 1:18:00
13:30 20:15 27:00 33:45 40:30 47:15 54:00 1:00:45 1:07:30 1:14:15 1:21:00
14:00 21:00 28:00 35:00 42:00 49:00 56:00 1:03:00 1:10:00 1:17:00 1:24:00
14:30 21:45 29:00 36:15 43:30 50:45 58:00 1:0:15 1:12:30 1:19:45 1:27:00
15:00 22:30 30:00 37:30 45:00 52:30 1:00:00 1:07:30 1:15:00 1:22:30 1:30:00
15:30 23:15 31:00 38:45 46:30 54:15 1:02:00 1:09:45 1:17:30 1:25:15 1:33:00
16:00 24:00 32:00 40:00 48:00 56:00 1:04:00 1:12:00 1:20:00 1:28:00 1:36:00
16:30 24:45 33:00 41:15 49:30 57:45 1:06:00 1:14:15 1:22:30 1:30:45 1:39:00
17:00 25:30 34:00 42:30 51:00 59:30 1:08:00 1:16:30 1:25:00 1:33:30 1:42:00
17:30 26:15 35:00 43:45 52:30 1:01:15 1:10:00 1:18:45 1:27:30 1:36:15 1:45:00
18:00 27:00 36:00 45:00 54:00 1:03:00 1:12:00 1:21:00 1:30:00 1:39:00 1:48:00
18:30 27:45 37:00 46:15 55:30 1:04:45 1:14:00 1:23:15 1:32:30 1:41:45 1:51:00
19:00 28:30 38:00 47:30 57:00 1:06:30 1:16:00 1:25:30 1:35:00 1:44:30 1:54:00
19:50 29:15 39:00 48:45 58:30 1:08:15 1:18:00 1:27:45 1:37:30 1:47:15 1:57:00
20:00 30:00 40:00 50:00 1:00:00 1:10:00 1:20:00 1:30:00 1:40:00 1:50:00 2:00:00
(min:sec) 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Flocculation Time Multiplier