The Impact of Thermoduric Bacteria on Milk Quality

Thermoduric bacteria play a crucial role in determining the hygiene quality of milk due to their ability to withstand pasteurization temperatures. These heat-resistant organisms, including spore-forming Bacillus cereus, survive the dairy farming environment and enter milk primarily during the milking process, often through cow teats tainted with soil, bedding, or feces.

Their survival through pasteurization can significantly impact milk quality. Despite common knowledge of spore-forming bacteria like Bacillus cereus, newer studies highlight non-spore-forming strains such as Streptococcus spp., Enterococcus spp., and Lactobacillus spp., with surprisingly high heat tolerance of up to 60–80 °C. This resilience leads to increased concerns during milk processing, where compromised milk residues or poorly managed silage inadvertently encourage thermoduric growth.

The challenge escalates as these bacteria contribute significantly to the aerobic plate count (APC) in pasteurized milk. Product manufacture can face hindrances due to these resilient organisms, potentially leading to product quality issues in the final dairy products.

Even more concerning is the adaptability of certain thermoduric strains to grow at refrigeration temperatures, posing additional obstacles to dairy farmers and processors. The proliferation of these bacteria isn't solely a concern for food safety and processing; it also ties into broader agricultural practices, such as rubberware management and silage quality control.

To mitigate the risks posed by thermoduric bacteria, it's crucial for dairy farms and processors to implement rigorous hygiene protocols, focusing on preventative measures like equipment sterilization, effective waste management, and ensuring optimal silage conditions. By prioritizing these strategies, the dairy industry can safeguard milk quality and maintain consumer confidence in dairy products.
The Impact of Thermoduric Bacteria on Milk Quality

Milk vat pasteurization

The term Pasteurization has been named so after its inventor, Louis Pasteur— the well-known French scientist. The pioneering investigations on such treatment were carried out in 1765 by Spallanzani.

Accordingly, there can be numerous combinations of time and temperatures for heat treatment to preserve the nutritional value of milk, different types of low-temperature pasteurization are commonly used onsite, such as vat pasteurization or low temperature, long time pasteurization (63°C for 30 min), HTST pasteurization (72°C for 15 s), and higher heat, shorter time pasteurization.

The long hold or vat pasteurization is a batch type method where the pasteurization is carried out at 63°C for 30 min.

The vat pasteurizer jacket is a double-walled covering. In the space between the walls, circulating water which heat the product in the vat. This unit is made up of the following sub-component parts, milk tank, water jacket, coil heater milk inlet and outlet valve, water inlet and outlet valve and insulation case.

Types of vat pasteurizers (Classification based on flow of heating medium)
1. Spray type – (A film of water is sprayed from a perforated pipe over the surface of the tank)
2. Flooded type
3. High velocity flooded type

Study demonstrated that vat pasteurization was an efficient and mild means of milk preservation resulting in only minor changes to the metabolites (J. Dairy Sci. 103).

In vat pasteurizers, an electric or air operated control can be connected with a timing clock so that the heat is shut off when the proper milk temperature has been reached and a bell rings when the proper length of holding time has elapsed.

Vat pasteurized milk tastes fresher, thus providing a taste similar to raw milk without the health concerns. It is reported that vat-pasteurized milk has become popular again as more small dairy processors are using it to appeal to customers seeking a more “farm-fresh” milk.
Milk vat pasteurization

Pasteurized cultured products

Cultured milk products (sour cream, cheese, cottage cheese and yoghurt) are good or health even if they’ve been pasteurized. To create cultured airy products, specific bacterial cultures are added to fluid milk.

The lactic acid produce also helps protect the product against unwanted growth of pathogens. Some of these cultured dairy products- particular yoghurt and kefir - are becoming increasingly popular as more research demonstrates the health benefits of probiotics.

Standard Grade A pasteurized cultured products
Temperature: Cooled to 45 °F or less and maintained thereat
Coliform limit: Not over coliforms 10 per ml.
Phosphatase: Less than 1 mg per ml, by Scharger Rapid Method or equivalent by other means.
Pasteurized cultured products

HTST pasteurization for milk

Milk is a primary example of a pasteurized product. Milk is a pasteurized under three different regimens:
*Low temperature, long time (LTLT) 63 ° C for 30 minutes
*High temperature, short time (HTST) 72 ° C for 15 seconds
*Ultrahigh temperature UHT 135 ° C for less 10 seconds

HTST pasteurization of raw milk employs a temperature of at 72 ° C for at least 15 seconds. The benefit of the HTST regimen is that there is equal or better killing of bacteria with considerably less nutrient loss.

Plate Heat Exchanger

Continuous processing is now the norm and the UK regulations require that pasteurized milk shall be produced by HTST. In HTST pasteurization raw milk held in a cool storage tank is pumped through a plate-type heat exchanger and brought to temperature.

The temperature and time is accomplished by pumping the heated milk through a holding tube of such length and diameter that it takes every milk particle at least 15 seconds to pass through the tune. At the end of the tube is an accurate temperature-sensing device and valve.
HTST pasteurization for milk

Grade A raw milk standard for pasteurization

Chemical, bacteriological and temperature standards have been established for grade A raw milk products intended for pasteurization, as well as for grade A pasteurized and bulk shipped heat-treated milk products.

In case raw milk for pasteurization the standard are as follows:

Temperature: cool to 50 F or less and maintained there at until process.

Bacterial limit: individual producer milk not to exceed 100,000 per ml prior to comingling with other producer milk.

Not exceeding 300,000 per ml as comingled mil prior to pasteurization.

Antibiotics: not detectable zone with the Baccilus subtilis method of equivalent.
Grade A raw milk standard for pasteurization

History of Pasteurization

History of Pasteurization
Pasteurization is named for the French scientist Louis Pasture (1822-1895). In 1856, 34 year old Louis Pasteur began his fourth year as the Head of Sciences at the University of Lille in France.

In the fall of the same year, Maurice D’Argineau, a local businessman, found Pasteur in his cramped corner lab.

D’Argineau’s consistent failure to make wine from his fields of sugar beets without it going sour was driving him to financial ruin. Pasteur was particular intrigued by the problem since it hinted at the involvement of one of his pet interests and he readily agreed to study the matter.

Although he first experimented with this process in 1862, pasteurization was not put to use until the early twenty century.

In the United States pasteurization was championed by Alice Catherine Evans (1881-1975), a microbiologists who worked for the US department of Agriculture.

Evans suffered from a disease known as brucellosis (undulant fever) and in 1918 she discovered that brucella, the bacterium that caused her disease, could be found in cow’s milk.

Scientists eventually determined that brucella was not the only milk borne bacterium. Milk can harbor other bacteria – such as E. coli, salmonella, and listeria – which can cause harmful and even life threatening infectious in the young, the old, pregnant women and the infirm.

Indeed, unpasteurized cow’s milk was a very common cause of tuberculosis, typhoid fever and salmonellosis.

Evans advocated on behalf of pasteurization for years after her discovery. Finally in the 1930s, milk pasteurization became mandatory under US law.
History of Pasteurization

Process of Pasteurization

Process of Pasteurization
Pasteurization, named after Louis Pasteur (1622-1895), its originator, was originally used to treat wine and beer, but soon came into use to treat milk as well, when it found that heating milk for a short time to below its boiling point killed microorganisms.

Pasteurization destroys 100 percent of pathogenic bacteria, yeasts and molds and 95 to 99 percent of other, nonpathogenic bacteria.

The process of pasteurization also inactivated many of the enzymes that cause the off-flavors of rancidity.

In the United States pasteurization was championed by Alice Catherine Evans (1881-1975), a microbiologists who worked for the US department of Agriculture.

Evans suffered from a disease known as brucellosis (undulant fever) and in 1918 she discovered that brucella, the bacterium that caused her disease, could be found in cow’s milk.

Scientists eventually determined that brucella was not the only milk borne bacterium. Milk can harbor other bacteria – such as E. coli, salmonella, and listeria – which can cause harmful and even life threatening infectious in the young, the old, pregnant women and the infirm.

Indeed, unpasteurized cow’s milk was a very common cause of tuberculosis, typhoid fever and salmonellosis.

Evans advocated on behalf of pasteurization for years after her discovery. Finally in the 1930s, milk pasteurization became mandatory under US law.

The advantages to be derived from pasteurization vary with the conditions under which the milk is produced and the efficiency with which the work is conducted.

If the milk comes from dairies where disease and uncleanliness prevail, pasteurization will prolong the keeping quality of the milk and also materially lessen the danger from disease germs.

If on the other hand, healthfulness and cleanliness receive the exacting attention which prevails on certified dairy farms, nothing can be gained by subjected milk to the pasteurizing process.
Process of Pasteurization

Processing of Milk – skim milk and low fat milk

Processing of Milk – skim milk and low fat milk
In plants producing fluid milk or milk product, all equipment, including tanks or vats, pasteurizers and coolers homogenizers, pipe lines and pumps, should be of sanitary design.

There should be no threaded pipe. Joints should be of the clamp type that can be easily disassembled for cleaning and sanitizing.

All surfaces contacting milk or milk products shroud be readily accessible for cleaning and sanitizing regardless of whether they are to be cleaned in place (CIP) or disabled.

The suitability of equipment for cleaning and sanitizing and the frequency with which this is done are equally important.

Skim milk (0.5% fat) and low fat milk (0.5 – 2.0% fat) are produced from whole milk passed through a centrifuge at high speed, after the milk has been heated to 90 – 110 degree C (32.2 – 43.3 degree C), to remove the butterfat as cream.

These products are usually fortified with vitamins A and D prior to pasteurizing and cooling.

In some cases, sodium caseinate (a derivative of casein, the main protein in milk) is also added.

The cream from the centrifuge may be separated as approximately 40% butterfat (heavy cream), 30% butterfat (all purpose cream), or 20% butterfat (light cream).

The creams may higher in butterfat may also diluted with skim milk to provide the various fat densities or to produce a product known as half and half (about 10.5% butterfat).

Since cream tends to spoil more quickly than milk, during pasteurization it is given a more drastic heat treatment than that given to milk.

When batch pasteurization is used cream is heated to 150 – 155 degree F and held at this temperature for 30 min prior to cooling.

When HTST method is used, cream is heated to 166 – 175 degree F and held at this temperature for 15 sec prior to cooling.

Table cream light cream r half and half) is usually homogenized after pasteurization.

All cream, after pasteurization, should be quickly cooled to 35 degree F and containerized.

It should be held at 35 – 40 degree F until consumed or subjected to additional processing.
Processing of Milk – skim milk and low fat milk

Flavor Treatment of Milk

Flavor Treatment of Milk
Milk is usually given what is called a flavor treatment to provide a product that is uniform in odor and taste.

During flavor treatment, milk is instantly heated to about 195 degree F (90.6 degree C) with live steam (injected directly into the product) after which it is subjected to a vacuum of about 10 in. (25.4 cm) in one chamber and to a vacuum of about 22 in. (55.9 cm) in another chamber.

The high vacuum treatment serves to regulate flavor, to cool the milk to about 150 degree F (65.6 degree C) and to evaporate water that may have been added through the injection of steam.

While the milk is still hot, it is usually homogenized by passing it through a small orifice that breaks up the fat globules to a small size, preventing the separation of cream from the milk.

The milk is then quickly cooled to about 35 degree F (1.7 degree C). This is done by the same general procedure used in heating, except that refrigerated water or brine, or directly expanded ammonia is used in the coils, vat jacket, outer tubes of the pasteurizer.

During HTST (high temperature short time) pasteurization and during flavor treatment and homogenization, milk is passed through the heating and cooling cycles at such a rapid rate that at no time is it held for long periods at high temperature.

After processing and cooling, milk is filled mechanically into containers, made of waxed or plastic-coated cardboard of different volumes up to 2 qt and of semi-rigid plastic containers of 2 qt. or 1 gal and the containers are sealed.

In this state, milk should be held as close to 32 degree F as possible until consumed.
Flavor Treatment of Milk

Processing of Milk

Processing of Milk
Milk as delivered to the processing plant, is first clarified while cold. Clarification consists of passing milk through a centrifuge similar to a cream separator but operated at low speed. This treatment is sufficient to separate out dirt and sediment that might be present, depositing them as a layer on the inner surface of the centrifuge bowl. The clarifier is not operated at sufficient speed to separate the cream from the milk. After clarification, the milk is usually pumped into a storage tank equipped with an agitator. While in the storage tank, the milk is sampled, and butterfat content is determined. It is then standardized by adding enough cream or skim milk (milk from which cream has been removed) to provide the fat content required by state regulations. The milk is then fortified with vitamin D at the rate of 400 USP units per quarts (0.95 liter).

The next step in fluid milk processing is to pasteurize it. During pasteurization, milk must be heated in all parts to 145 degree F and held at this temperature for 30 min, or it must be quickly heated to 161 degree F and held at this temperature for 15 sec, heated to 191 degree F and held for 1 sec or heated to 194 degree and held for 0.5 sec. then cooled. Pasteurizing at temperatures above 145 degree F is called the high temperature short time (HTST) method, and both heating and cooling are carried out over a short period of time. Milk may be pasteurized in insulated vats heated by coils carrying hot water, or in vats heated by hot water sprayed within a jacket surrounding the sides and bottom of the vat. With low temperature pasteurization, the milk is ordinarily agitated during heating and cooling. Plate heater and cooler or tubular heaters may be used to pasteurize and cool milk. When tubular heaters are used, the product travels in one direction through an inner tube while the hot water for heating or the refrigerated liquid for cooling passes in the opposite direction though an outer tube surrounding that which carries the product.
Processing of Milk