Which of the following conditions is are used in the pasteurization of milk?

Pasteurized Milk Versus Extended Shelf Life Milk

Pasteurized milk must be stored under refrigeration and has a relatively short shelf life. In contrast, UP or UHT milk (heated to 125–138 °C for 2–4 s) can be stored for up to 3 months under refrigeration and milk subjected to ultra high temperature (UHT) treatment (heated to 135–140 °C for a few seconds) can be stored for 3–6 months at ambient temperatures. In some countries, the convenience of longer shelf life and less refrigeration capacities have led to a significant shift away from pasteurized milk toward UHT and UP milks. Studies with consumer panels have revealed that US consumers can distinguish between pasteurized and UHT milk. Most consumers prefer pasteurized milk because of the flavor (UHT milk tends to have a cooked flavor) and the “fresh” image associated with pasteurized milk (Lewis, 2010).

Fluid Milk Processing

Pasteurized milk processing varies from plant to plant and among countries. Usually, raw milk is stored in silo tanks before processing. In the United States, raw milk can be stored for up to 72 h at refrigeration temperature (legally below 7.2 °C but preferably at or below 4 °C). From the silo(s), raw milk is clarified, preheated, separated, standardized, homogenized, pasteurized, and cooled. Finally, the cooled pasteurized milk is pumped to a storage tank until packaged (FDA, 2013; Partridge, 2008; Varnam and Sutherland, 1994).

Several features help assure the safety of HTST pasteurization. A flow diversion valve (FDV), controlled by a temperature detector, automatically diverts milk back to the balance tank if the milk is not at the required temperature at the exit of (or at the entrance to) the holding tube. Furthermore, use of a positive displacement pump ensures that the pressure of the pasteurized milk is higher than that of the raw milk, thereby prevents the mixing of raw milk into the pasteurized milk stream within the regeneration section of the pasteurizer. Other pumps can also be used to generate the pressure differential, which should be monitored continuously. Additional safety features include an indicator thermometer, positively sloped holding tube, vacuum break, and placement of the balance tank below the inlet valve to the system. While the process described above, and illustrated in Figure 1, is typical for milk pasteurization, there are many alternatives. The milk may be cold separated before pasteurizing. Not all milk is standardized for fat content online prior to homogenization. Energy can be saved if the cream flow is homogenized immediately following the separator and then added into the skim milk flow. Occasionally, whole milk is not homogenized, as some consumers prefer a cream layer on the top of the milk. This cream layer is particularly obvious when milk is bottled in clear glass containers.

Figure 1. Production line for pasteurized milk. Milk passes balance tank, regeneration section, separator, online standardization with homogenized cream, heating section, holding tube, regeneration section, and cooling section.

milk,

cream,

skim milk,

standardized milk,

cooling medium,

heating medium,

diverted flow. 1, Balance tank; 2, product feed pump; 3, flow controller; 4, plate heat exchanger; 5, separator; 6, constant pressure valve; 7, flow transmitter; 8, density transmitter; 9, regulating valve; 10, shutoff valve; 11, check valve; 12, homogenizer; 13, booster pump; 14, holding tube; 15, flow diversion valve; 16, process control.Source: Tetra Pak, Inc.

15.2.3 Divergent Opinions—The Beginnings of the Advocating of Raw Milk

In the 1920s and 1930s, a debate was occurring between two subsets of scientific professionals: the biochemists and physiologists that were primarily concerned with human nutrition and the bacteriologists and public health workers that were more concerned with controlling the spread of infectious diseases. The physiologists and biochemists were concerned that pasteurization could cause thermal destruction of critical nutrients in milk and downplayed the contribution of milk to foodborne illnesses. At the same time, the bacteriologists and public health workers were proponents of mandatory thermal treatments citing that there was no convincing evidence that pasteurization damaged the nutritive value of milk (Wilson, 1938).

Dairy producers and processors joined in the debate by marketing the value of their respective raw or pasteurized milk in newspaper advertisements. These advertisements ranged in size from single line classified statements to full page displays to communicate the relative benefits and/or to discredit the value of the alternative product. Below are selected statements from such advertisements published in the 1930s in Oregon:

Pasteurized-milk advertisements:

Dr. McCallum, the man who discovered vitamins in milk, says in his book on nutrition that all city milk supplies should be pasteurized. Mayo Brothers, world famous physicians and surgeons in Rochester, MN, pasteurize all milk from their two herds of pure bred cows.

“Mothers, would it not be best to take the advice of such authorities as quoted above and see that your loved ones use only pasteurized milk? Milk to be pasteurized by Sunny Brook Dairy is produced exclusively on two of the finest dairies in this locality, having fine, healthy Jersey and Guernsey cattle. Sunny Brook Pasteurized Milk costs no more than ordinary raw milk.”—advertisement for Sunny Brook Dairy in The Corvallis Gazette-Times, September 6, 1934

Raw-milk advertisements:

“Pasteurization does not remove contamination, but it only an abortive attempt to destroy bacteria. There is no argument, and it is granted that Pasteurization will make milk reasonably safe for human consumption, that might otherwise be unfit.”—advertisement for Superior Quality Raw Milk Group in The Corvallis Gazette-Times, February 16, 1934

The pasteurization of milk takes none of the filth out of it but it does destroy some of the bacteria. The excessive amount of liquidized filth which it contains is cooked or heated and you’re having it served to you cooked instead of raw.

We have read volume after volume, numerous pamphlets published by high-powered individuals and advertising concerns and distributed by firms or individuals who sell pasteurized milk. And we have yet to read one article or hear one lecture, or even a quotation from purported eminent professors or doctors who sponsor the cause of pasteurization, which tells the actual facts about milk relative to sanitary production and proper handling which alone means good, pure milk.

“When properly pasteurized the usually called harmful bacteria are killed. But the dead carcasses remain in such number that they will and do cause milk when pasteurized to decay or rot before souring. Putrification will begin much sooner than in raw milk. You have often noticed a bad smell in pasteurized milk after it has become a little old. It is then starting to decay. Who wants to use milk full of dead carcasses of bacteria so prevalent and so inevitable in practically all pasteurized milk, when it is possible to get a good or even fairly good raw milk.”—advertisement for Clover Leaf Dairy in The Corvallis Gazette-Times, April 27, 1934

Protect your growing children’s teeth by giving them plenty of wholesome, tasteful raw milk, in which the normal chemical relationship is not disturbed. Calcium and Phosphorus are important in the prevention of tooth decay, be sure they are not affected by pasteurization.

“My dairy is open to the public at any and all times, and it is suggested that you pay this dairy a visit Sunday, April 29, and observe for yourself the conditions under which this milk is produced, then visit the other dairies and permit your own personal judgment to dictate to your conscience where best to secure your milk supply.”—advertisement for Mt. View Dairy in The Corvallis Gazette-Times, April 28, 1934

Research studies on the influence of pasteurization on nutrient degradation occurred throughout the 1930s and 1940s. Studies demonstrated measurable changes in a number of nutrients (lactalbumin coagulation, rennin coagulation time, casein clot texture, soluble calcium and phosphate concentrations, iodine, vitamin B1, vitamin C) between raw and pasteurized milk; however, these changes were determined to be minimal or negligible with animal feeding studies demonstrating no significant differences between the two types of milk (Wilson, 1938). The growing body of scientific evidence supported pasteurization as an effective means to reduce milk-borne illnesses with little to no risk of having a negative impact on the nutritional needs of the population.

Despite these findings, criticism of pasteurization continued, including the publication of “The Case Against Pasteurization of Milk: A Statistical Examination of the Claim that Pasteurization of Milk Saves Lives.” The main thesis of this book was that the consumption of raw milk contaminated with low levels of Mycobacterium tuberculosis served as a benefit to public health as a form of vaccination of children that would prevent more serious tuberculosis cases in adults (Kay, 1945).

Deteriorative Reactions and Indices of Failure

Pasteurized milk quality deterioration is the result of physicochemical or microbial changes in the product perceived by the consumer through off-odors and/or tastes (Papachristou et al., 2006a, 2006b). Among these defects, light-induced off-flavors (physicochemical defects) are probably the most common in milk and are attributed to two distinct causes. The first, a burnt sunlight flavor which develops during the first 2–3 days of storage and is caused by degradation of sulfur-containing amino acids (methionine) of the whey proteins. The second is a metallic or cardboardy off-flavor (lack of freshness) that develops two days later and does not dissipate. This off-flavor is attributed to light-induced lipid oxidation (Marsili, 1999). Light exposure, especially at wavelengths below 500 nm, also causes destruction of light-sensitive fat-soluble vitamins, mainly vitamins A and E but also water soluble vitamins such as B2, B6, B12 and vitamin C (Zygoura et al., 2004; Moyssiadi et al., 2004; Papachristou et al., 2006a, 2006b). Depending on the intensity of the light-oxidized flavor, consumers may vary in their ability to detect this defect; some may find the milk objectionable while others detect no specific off-taste.

The light-oxidized defect develops in milk as a result of its exposure to sunlight or to fluorescent lighting common in store dairy cases (especially bulbs with wavelengths below 620 nm). Light initiates a chemical reaction in milk that modifies specific components of proteins and fats, resulting in characteristic off-flavors. Exposure to sunlight for as little as 10–15 minutes (as short as 5 minutes on a very clear day with intense sunlight) is sufficient to cause the defect, while longer exposure times are generally required for fluorescent lighting. The closer the milk is to the fluorescent light source or the more intense the light, the quicker the development of the off-flavor (within 1–2 hours in some cases). In general, the defect is more common in milk packaged in transparent plastic or glass, although it can also occur in milk in more opaque containers with very intense light and sufficient exposure time.

The main chemical defect is lipid peroxidation. Lipid peroxidation can be described generally as a process under which oxidants such as free radicals attack lipids containing carbon-carbon double bond(s), especially polyunsaturated fatty acids (PUFAs) present in milk fat resulting in the same sensory changes as light-induced oxidation but through a different mechanism. The mechanism of light-induced oxidation begins with riboflavin which acts as a photosensitizer as shown in Fig. 1 (Skibsted, 2000). Riboflavin absorbs photons forming an excited singlet state (1Rib) which by intersystem crossing forms a triplet state (3Rib). Triplet riboflavin is subsequently deactivated to yield singlet-oxygen (1O2), important in protein oxidation (formation of dimethyldisulfide from methionine) (Type II reaction). Alternatively singlet oxygen acts as an oxidant to initiate free-radical processes by electron transfer and formation of substrate radicals and/or super-oxide anions (Type I reaction).

Microbiological changes involve growth of psychrotrophic bacteria (Gram-negative rods such as Pseudomonads and Alcaligenes) as a result of either inadequate pasteurization or post-pasteurization contamination leading to the formation of microbial flavor described as acidic, bitter, fruity, malty, putrid or unclean flavor. Considering the deterioration potential at refrigeration temperature, the predominant species identified were S. ureilytica (75%), A. urinaeequi (13.6%), and Lactooccus.lactis (8.6%) for simultaneous proteolytic and lipolytic activity, L. lactis (51.4%) and A. urinaeequi (25%) were identified among the proteolytic psychrotrophs, and A. lwoffii (88.9%) and E. kobei (31.9%) among the lipolytics (Ribeiro Júnior et al., 2018).

Pathogens such as coliforms (Aglawe and Wadatkar, 2012) were determined in Indian milk, Enterobacter spp. and Escherichia coli in Jamaican milk (Anderson et al., 2011), Staphylococcus aureus in Brazilian milk (De Oliveira et al., 2011), etc. Presence of Salmonella in pasteurized milk was due to improper pasteurization resulting from malfunctioning of a pasteurizer valve (Bergquist and Pogosian, 2000).

Post-pasteurization contamination of pasteurized milk with Bacillus cereus from packaging paper and board (Pirttijarvi et al., 1996) and filling machine (Eneroth et al., 2001) have also been reported. Likewise, Campylobacter jejuni has been implicated as the cause of foodborne disease associated with pasteurized milk in England and the USA. In one incident, under-processing of milk appeared to be the problem, while in the second C. jejuni survived batch pasteurization in a privately operated pasteurization plant in a boarding school. L. monocytogenes was the cause of an outbreak in the USA attributed to incorrect application of HTST pasteurization (Varnam and Sutherland, 1996). Salmonella has also been involved in at least two outbreaks, the first in Chicago, Illinois and the second in Cambridge, UK; both were attributed to contamination of pasteurized milk by raw milk. Finally, Yersinia enterocolitica has also been implicated in three large outbreaks of illness associated with pasteurized chocolate flavored milk in the USA. It appeared that the pathogen was introduced with the chocolate syrup added to milk after pasteurization.

Ortuzar et al. (2018) conducted a database study to identify, appraise, and summarize the prevalence and/or concentration of spore-forming bacteria throughout the fluid milk supply chain. The pasteurized milk supply chain was standardized to include the following steps: “milking machine”, “raw milk”, “bulk tank”, “transportation”, “silo”, “pasteurized milk” and “packaged milk”. In general, great heterogeneity was observed among studies on the contamination in milk samples with spore-forming bacteria. The findings showed that the concentration of spore-forming bacteria in milk samples increased within the range of 0.58–2.41 log cfu mL−1 from raw milk to pasteurized milk. Similarly, the prevalence of contaminated samples with sporeforming bacteria increased from 23% on farm, to up to 58% at the step of “pasteurized milk”. The study showed that the contamination of spore-forming bacteria originating from the farm remains stable with steady increases as the milk moves downstream.

Ziyaina et al. (2018) analyzed pasteurized milk (3.9% fat) stored at different temperatures for APC, psychrotrophic bacteria, acidity and pH in an attempt to determine product shelf life. Results indicated that the shelf life of pasteurized milk was 24, 36, and 72 h at 19, 15, and 13 °C respectively, as determined by APC and acidity indicators. However, milk stored at lower temperatures of 5, 7, and 10 °C had longer shelf life of 30, 24, and 12 d, respectively. A sharp increase in titratable acidity along with a decrease pH were observed when APCs reached 5.0 log cfu mL−1 at all storage temperatures.

UF-Domiati

Standardized pasteurized milk (12.5% TS, 3.5% fat) is concentrated by UF to 35% TS. The retentate is homogenized, heated to 75 °C for 1 min, and cooled to 34 °C; 1% mesophilic starter, rennet (3 g of rennet powder/100 kg), and 3–4% NaCl are added and treated as follows:

The retentate is poured into stainless steel trays (50 × 50 × 10 cm) and left to coagulate. The cheese is then cut manually into cubes (0.5 kg), wrapped in polyethylene sheet and packed into a 1-kg rigid plastic container (consumer package) or in 10/20-1 tins; the containers or tins are filled with 5% salted permeate and then closed.

The retentate is cast in plastic containers (750 g) placed in a moving conveyer in a constant temperature tunnel (40 °C), in which there is complete coagulation before the package leaves the tunnel. Salted brine (5% NaCl) is dosed, and packages are sealed with aluminum foil and covered with a plastic lid.

Pasteurization

The PMO code for the basic pasteurization principle is that ‘every particle of milk or milk product be heated to at least a minimum temperature and held at that temperature for at least the specified time in properly designed and operated equipment.’ Every plant should assess the adequacy of their pasteurization equipment to determine if it satisfies the basic principle of pasteurization. It is also necessary that dairy products containing higher fat and/or added sugars or which are viscous (e.g. frozen dessert mixes, cream, eggnog, etc.) also require higher pasteurization temperatures and/or longer times. The standard time and temperature pasteurization conditions for milk and high-solid dairy products are shown in Table 12.7.

Table 12.7. Minimum pasteurization temperatures and times recognized by the US Public Health Service and Food and Drug Administration

ProductTemperatureTimeReference method1. Milk145 °F (62.8 °C)30 minutesLTLT161 °F (71.7 °C)15 secondsSTHT191 °F (88 °C)1 secondUHT194 °F (89 °C)0.5 second201 °F (94 °C)0.1 second204 °F (96 °C)0.05 second212 °F (100 °C)0.01 second2. Milk products of 10% fat or more or added sugar (half/half, cream, chocolate milk)150 °F30 minutes166 °F15 seconds191 °F1 second194 °F0.5 second201 °F0.1 second204 °F0.05 second212 °F0.01 second3. Eggnog and frozen dessert mixes155 °F30 minutes175 °F25 seconds180 °F15 seconds

For all HTST pasteurizing systems, a properly designed, installed, and operating flow diversion device and properly operating pressure controls for regenerator systems are required to be installed. It is also recommended that all Grade A products as well as frozen dessert mixes must be pasteurized in the plant of final processing and packaging. The heat exchanger (presses) of HTST pasteurizer units need to be routinely opened and closely evaluated for stress cracks, pin holes, gasketing, cleaning, etc. Holes in regenerator and cooling plates can develop and cause contamination.

What methods are used to pasteurize milk?

What are the 3 steps in the pasteurization process?

What is the temperature used in pasteurization?

Which of the following is an effective heat treatment to pasteurize milk?