Ozone in the Dairy Industry: Comparison
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The dairy farming industry has generally improved in terms of equipment and product performance, but innovation must be linked to traditional product specifications. During cheese ripening, the storage areas and the direct contact of the cheese with the wood must be carefully managed because the proliferation of contaminating microorganisms, parasites, and insects increases significantly and product quality quickly declines, notably from a sensory level. The use of ozone (as gas or as ozonated water) can be effective for sanitizing air, water, and surfaces in contact with food, and its use can also be extended to the treatment of waste and process water. 

  • ozone
  • dairy supply chain
  • cheese
  • product quality
  • microbial control

1. Microbiological: A Critical Control Point to Be Monitored

Some phases within the dairy industry are particularly critical from a microbiological point of view. Examples in the production of cheese are the thermization or pasteurization of milk which must be sufficient to inactivate all human pathogenic bacteria, as well as reduce the endogenous microbial load [76][1]; the syneresis which may be more or less marked, influencing the aw (water activity index) and therefore the possibility of development of the various microorganisms; the salting phase which determines a change in the microbiological habitat and, therefore, also the variation of the microbial ecosystem (from the population of starter lactic acid bacteria, SLAB, to that of non-starter bacteria, NSLAB); stewing, during which the optimal environment is recreated between temperature and relative humidity for the proliferation of the microorganisms responsible for the acidification of the “pasta”; finally, the maturation phase is a fundamental one in which the NSLAB microorganisms will be responsible for the changes that will occur in the cheese and which will influence the consistency, the sensory properties of the product, and its shelf-life. For the maturation of some aged cheeses, a further criticality must be considered: the presence of mites, arthropods that tend to settle in the wooden boards on which the cheeses are placed to age, on which the parasites feed, causing serious economic losses to the companies [77][2].
A fundamental element for the control of microbial loads is the disinfection of systems, instruments, and rooms: the microorganisms that can colonize environments are in fact many and companies carry out cleaning operations every day using water vapor at high temperature and pressure and chemical products that are chlorine-based; these are costly operations from both an economic and an environmental point of view. Furthermore, wastewater is rich in organic matter, an element that makes it difficult and expensive to dispose of, as well as representing an excellent growth medium for microflora.
The genus Pseudomonas includes spoilage bacteria, responsible for undesirable odors and flavors or unusual pigments of foods [78][3], often isolated at several stages of the dairy production environment (P. fluorescens, P. koreensis, P. marginalis, P. rhodesiae, P.fragi, P. putida, P. entomophila, P. mendocina, and P. aeruginosa) [79][4].
The genus Listeria comprises L. monocytogenes, a Gram-positive foodborne pathogen that can form biofilms and persist for a long period on surfaces and food processing environments, being able to cause frequent contaminations of the finished products [80,81,82][5][6][7].
The genus Salmonella comprises pathogenic bacteria found in various types of foods, like fresh and fermented dairy products, adapting to an acid environment [83,84,85,86][8][9][10][11].

2. Ozone Technology in Dairy Farming

Ozone is used upstream of the dairy supply chain, starting from animal husbandry: it is an alternative to traditional methods, which involve the use of hot water and chemical products, for the sanitization of the pipes that carry the milk from the rooms of milking to collection tanks. Using ozone reduces costs (in reference to the purchase of disinfectants and the use of hot water) and the environmental impact (as it does not release residues); moreover, the effectiveness of ozonized water in the disinfection of animals, surfaces and tools has been seen. In livestock farms, the ozone can be used for the sanitization of the air in the stables: at low concentrations, it contributes to the elimination of bad odors and any pathogens present in the air [26][12]. Ogata and Nagahata [87][13] studied its potential in the treatment of bovine mastitis, a problem that afflicts farms, causing important economic losses: according to the researchers, as many as 60% of treated mastitis heals without the use of antibiotics (6–30 mg of ozone).

3. Efficacy of Ozone on the Sanitization of the Working Surface Areas and Equipment

Hot water with chemicals is usually used for cleaning and disinfection processes, generating large energy and chemicals consumptions. Many studies evaluated the use of ozonated water and gas to clean and sanitize equipment and different surfaces in dairy locations [55][14], (Table 1).
Guzel-Seydim et al. [88][15] evaluated the use of ozone for the treatment of stainless-steel surfaces to remove milk residues, demonstrating its greater effectiveness compared to the traditional treatment with hot water at 40 °C. 15 min treatments using either warm water (40 °C) or ozonated cold water (10 °C) were conducted. The results show that Chemical Oxygen Demand (C.O.D.) values are reduced by 84% (ozone treatment) against 51% (hot water treatment). Furthermore, many researchers have evaluated the effectiveness of ozone against microorganisms that can colonize metal surfaces.
Greene et al. [89][16] studied the effect of ozonated deionized water against psychrophilic contaminating microflora (Pseudomonas fluorescens and Alcaligenes faecalis) on stainless steel plates, showing that a 10-min exposure with a concentration of 0.5 ppm decreased the microbial growth by more than 4 log10. For the experimentation, stainless steel plates were incubated in UHT pasteurized milk and inoculated with pure cultures of Pseudomonas fluorescens (ATCC 949) or Alcaligenes faecalis (ATCC 337). Since these are metal surfaces, the corrosive power of ozone must always be considered. The use of ozonated water is recommended, instead of hot water and chlorine, when the surfaces of the milk processing equipment are not damaged. Recently, the potential of disinfection effectiveness of single and synergistic ozone (10 ppm for 15 min) and UVC (1 cm or 15.56 mW cm−2 for 15 min) treatment for the sterilization of bacteria and fungi (Escherichia coli, Staphylococcus aureus, Candida albicans, and Aspergillus fumigatus) was studied on different material surfaces (stainless steel, polymethyl methacrylate, copper, surgical facemask, denim, and a cotton-polyester fabric) [90][17].
In the work of Dosti et al. [91][18], fresh 24-h bacterial cultures were treated with ozone (0.6 ppm for 1 min and 10 min), chlorine (100 ppm for 2 min), or heat (77 ± 1 °C for 5 min), showing its effectiveness against food spoilage microorganism in synthetic broth. The bacterial biofilm on the metal coupons was significantly reduced by ozone and chlorine, but with no significant difference between ozone and chlorine, except for P. putida (ozone was more effective than chlorine).
Greene et al. [47][19] observed that 0.4–0.5 ppm of ozone, pulsed into water at 21–23 °C for 20 min per day over a 7-day period, caused a certain degree of weight loss of all materials tested (i.e., aluminum, copper, stainless steel, and carbon steel), but only weight loss for carbon steel was significant. Therefore, special attention is required when the treatment is used for dairy chilling water systems with copper or carbon steel parts.
Megahed et al. [92,93][20][21] studied the effect of gaseous-ozone and aqueous-ozone (from 1 to 10 ppm) in commonly used devices of the dairy industry (plastic, nylon, rubber, and wood) contaminated by cattle manure-based pathogens. It was observed that aqueous-ozone treatment at a concentration of 4 ppm or greater reduced bacterial load below detectable limits within 2 min of exposure predominantly on the plastic surface, while other surfaces were largely decontaminated after 4 min of treatment. Gaseous O3 cannot be an alternative to aqueous O3 in reducing the manure-based pathogens to a safe level, particularly in complex environments. The results obtained are in line with different previous studies [94][22].
It is important to consider the contamination of surfaces by spoilage bacteria, like Pseudomonas spp. and pathogenic bacteria, like Listeria monocytogenes and Salmonella spp., that lead to recurrent food contamination, with problems related to the shelf life and safety of dairy products.
Biofilm development in food processing facilities is prevented by using common chemical sanitizers, but their use has some disadvantages, such as damaging environmental impacts and harmful consequences for human health [52][23]. The use of ozone could be a promising technique to prevent spoilage or pathogenic bacteria contamination [22][24].
Shelobolina et al. [95][25] studied the effect of dissolved ozone (5 ppm for 20 min) on Pseudomonas biofilm on various surfaces, and they observed that ozone can be effective. Biofilms on plastic materials showed inactivation effects, such as for glass, while biofilms grown on ceramics were more difficult to inactivate. Therefore, it is important to use non-porous materials in industrial and clinical settings. It has also been demonstrated that ozone can have an antimicrobial effect in association with other technologies: for example, ozone water and hydrogen peroxide solution were effective on P. fluorescens biofilm.
A sequential treatment with 1.0 and 1.7 mg L−1 of ozone, followed by 0.8 and 1.1% of hydrogen peroxide, showed synergistic disinfection effects [96][26].
In a study of a cheese production plant, the gaseous ozonation of 2 ppm was provided during a weekend for 15 min (first treatment) and for 120 min (second treatment), when staff were not present. Testing for L. monocytogenes was carried out for a total of 360 environmental samples, over a period of 12 months, in 15 areas before and 15 areas after ozonation: there was a significant reduction in L. monocytogenes isolations from 15.0% in pre-zoning samples to 1.67% in post-ozonation samples in all areas, to include the ozonation regime in the hygiene-health program. No negative effects of ozonation treatment were noted on the surfaces and equipment [97][27].
Instead, Shao et al. [98][28] studied the effect of ozone water on mature S. aureus and Salmonella spp. biofilm on stainless steel surfaces, using acidic electrolyzed water, ozone water, or ultrasound (40 kHz) alone, and combinations of ultrasound and disinfectants. They detected less than 0.8 Log CFU (colony-forming units) cm−2 of cells reduction in biofilm exposed to ozonized water (16 mg L−1) for 20 min. Only in a few studies was the efficacy of ozone explored against biofilms formed by bacteria belonging to the Salmonella genus [99][29].
Indoor air in the dairy industry constitutes a source or a vehicle of microbial contamination, causing food safety and product shelf-life problems. Masotti et al. [100][30] monitored the air microbial load in the dairy plant and evaluated the impact of air disinfection through ozonation or chemical aerosolization by hydrogen peroxide. Ozonated air had a constant flow rate of 40 L min−1, and it generated 1.5 g ozone per hour (11–12 p.m. and 1–3 a.m. from Friday to Sunday). Hydrogen peroxide aerosolization was realized producing particles in the range of 5–15 μm at a concentration of 5–15% for 16 and 20 min. Both techniques were effective against airborne microorganisms.
Table 1.
Ozone sanitization in working surface areas and equipment.
Area Treatments Target Result References
Ozone sanitization in working surface areas and equipment Ozonated cold water (10 °C) for 15 min Ozone treatment on stainless steel surfaces to remove milk residues Chemical Oxygen Demand values are reduced by 84% Guzel-Seydim et al., 2000 [88][15]
10 min exposure with 0.5 ppm of ozonated deionized water Effectiveness of ozone against microorganisms that can colonize metal surfaces Microbial growth decreased by more than 4 log10 Greene et al., 1993 [89][16]
Fresh 24-h bacterial cultures were exposed to ozone (0.6 ppm for 1 min and 10 min), chlorine (100 ppm for 2 min) or heat (77 ± 1 °C for 5 min) Ozone, chlorine and heat applications were compared for killing effectiveness against food spoilage bacteria in synthetic broth Ozone and chlorine significantly reduced the biofilm bacteria adhered to the metal coupons as compared to the control. No difference between ozone and chlorine inactivation of the bacteria. Ozone killed P. putida more effectively than chlorine Dosti et al., 2005 [91][18]
).
Torlak and Sert [102][32] studied ozone treatment to eliminate Cronobacter sakazaki (responsible for fatal infections in infants) from milk powder. They treated both whole and skimmed milk with different results: for the skimmed milk, the initial levels of Cronobacter were reduced by 2.71 and 3.28 log, following an exposure of 120 min at the concentration of 2.8 mg L−1 and 5.3 mg L−1, and the whole milk sample showed a smaller logarithmic reduction.
Rojek et al. [103][33] used pressurized ozone (5–35 mg L−1 for 5–25 min) to safeguard skim milk by diminishing its microbial masses, and this treatment looked foremost to decrease the number of psychrotrophs by more than 99%.
Sheelamary and Muthukumar [104][34] totally wiped out Listeria monocytogenes from both crude and marked milk tests through gaseous ozonation, with a mean viable count of 5.5 and 5.7 log10 CFU mL−1, respectively. The results showed complete removal of Listeria monocytogenes after 15 min of treatments with a controlled flow rate of 0.5 m L−1 of oxygen (0.2 g h−1 of ozone).
Cavalcante et al. [101][31] evaluated in crude milk that ozone gas at 1.5 mg L−1 for 15 min was found to decrease bacterial as Listeria monocytogenes, Enterobacteriaceae, and Staphylococcus, and contagious checks by up to 1 log10 cycle.
A study by Alsager et al. [105][35] analyzed the decomposition of various antibiotic compounds in milk samples by the ozonation process. They observed that increasing concentration of gas ozone (75 mg L−1) caused a 95% reduction in antibiotics in milk samples.
This was confirmed by Liu et al. [106][36]: antibiotic residues in pasteurized milk reach tolerance levels by O3 in a vortex reactor (2.16, 4.54, and 6.12 mg h−1), and no antimicrobial activity was detected in the milk treated. The results showed that post-ozonation of 400 mL of 5.52 μM various antibiotics for 20–40 min, their residue concentrations (50.8–84.1 μg L−1) satisfy the relevant maximum residue limits. Therefore, antibiotic residues in milk arrive at tolerance levels by O3 in a vortex reactor. Increasing the input O3 concentration, O3 flow rate, and temperature can accelerate the degradation and shorten the time to meet the relevant MRLs.
de Oliveira Souza et al. [107][37] studied the effect of gaseous ozone on the inactivation of Escherichia coli O157:H7 inoculated on an organic substrate and the efficacy of ozonated water in controlling the pathogen. It was inoculated in milk with different compositions: unhomogenized whole milk (UHWM), homogenized whole milk (HWM), skim milk (SM), lactose-free skim milk (LFSM), and lactose-free whole milk (LFWM), which were sterilized by boiling in the laboratory to guarantee the elimination of microorganisms and cooled before inoculation. Milk was utilized only as a substrate, contemplating the differences in fat and lactose contents. The same procedures were also conducted in distilled water for comparison. Escherichia coli O157:H7 was inoculated in different ozonated water treatments (35 and 45 mg L−1 for 0, 5, 15, and 25 min). In a second experiment, the water was ozonated at 45 mg L−1 for 15 min. E. coli O157:H7 was exposed for 5 min to the ozonated water immediately after the treatment, and after storage for 0.5, 1.0, 1.5, 3.0, and 24 h at 8 °C. The results showed that in lactose-free homogenized skim milk, was observed a reduction of 1.5 log cycles for ozonation periods of 25 min at the concentrations tested. Overall, ozonated water was effective in all treatments, but the efficiency of ozone is influenced by the composition of the organic substrates. The refrigerated ozonated water put in storage for up to 24 h was effective to control E. coli O157:H7.
The degradation of mycotoxins and aflatoxins is also possible through the use of ozone [108][38]. As reported by Ismail et al. [109][39] when milk samples were exposed to gas ozone (80 mg min−1 for 5 min), and aflatoxins were reduced by 50%.
Table 2.
Ozone milk treatment.
Area Treatments Target Result References
Ozone milk treatment Ozone concentration of 2.8 mg L−1 and 5.3 mg L−1 for 120 min Ozone treatment to eliminate Cronobacter sakazaki from skimmed milk powder Initial levels of Cronobacter

were reduced by 2.71 and 3.28 log		 Torlak and Sert, 2013 [102][32]
Pressurized ozone concentration of 5–35 mg L−1 for 5–25 min Safeguard skim milk by diminishing its microbial masses Pressurized ozone decrease

the number of psychrotrophs

by more than 99%		 Rojek et al., 1995 [103][33]
Controlled flow rate 0.5 m L−1 of oxygen (0.2 g h−1 of ozone) for 15 min Gaseous ozonation treatment to reduce Listeria monocytogenes from both crude and marked milk tests Gaseous ozonation totally wiped out Listeria monocytogenes from both crude and marked milk tests with a mean viable count of 5.5 and 5.7 log10 CFU mL−1, respectively. Sheelamary and Muthukumar, 2011 [104][34]
20 min exposure with 0.4–0.5

ppm of ozonated water at 21–23 °C for 7 days		 Effect of ozone on more materials
Ozone gas at 1.5 mg LWeight loss of all materials tested, but only weight loss for carbon steel was significantly −1 for

15 min		 Ozone gas bubbling effects

in the quality of raw milkGreene et al., 1999 [47][19]
Ozone decreases bacterial and contagious checks by up to 1 log10 cycle Cavalcante et al., 2021 [101][31] Ozone sanitization in working surface areas and equipment
Gas ozone concentration of 75

mg L−1		Ozone concentrations from 1 to

10 ppm for 4 min		 for 10 minEffect of gaseous-ozone and aqueous-ozone in commonly Decomposition of various antibiotic compounds in milk samples by ozonation process

used devices of the dairy industry		 All surfaces were largely decontaminated after 4 min treatment Gas ozone caused a 95% reduction in antibiotics in milk samples Alsager et al., 2018 [105][35]Megahed et al., 2018, 2019 [92,93][20][21]
Ozone concentrations between 300 and 1500 ppm for 10–480 s Effect of gaseous ozone and aqueous ozone in commonly

used devices of the dairy industry		 Escherichia coli and Staphylococcus aureus died for 99.99%
Ozone milk treatment Ozone concentrations of 2.16, 4.54, and 6.12 mg h−1 at selected volumes (1 L of each) and times

of 2, 4, and 6 h		Kowalski et al., 1998 [94][22]
Use of ozonation in a vortex reactor for removing antibiotics from milk Post-ozonation of 400 mL of 5.52 μM various antibiotics for 20–40 min, their residue concentrations were 50.8–84.1 μg L−1 satisfying the relevant maximum residue limits Liu et al., 2022 [106][36] Pseudomonas fluorescens, Staphylococcus aureus, and Listeria monocytogenes were treated with ozonized water (0.5 ppm) by immersion in static condition, ozonized water under flow conditions, and gaseous ozone at different concentrations (0.1–20 ppm) (20–60 min) Inactivation of foodborne bacteria biofilms by aqueous and gaseous ozone
In the first experiment, ozone concentrations were 35 and 45 mg L−1 for 0, 5, 15, and 25 min. In a second experiment, the water was ozonated at 45 mg L−1 for 15 minAqueous ozone under static conditions resulted in an estimated viability reduction of 1.61–2.14 Log CFU cm Efficacy of ozonated water in controlling Escherichia coli O157:H7−2 after 20 min, while reduction values were higher (3.26–5.23 Log CFU cm−2) for biofilms treated in dynamic conditions. With gaseous ozone, the highest concentrations estimated a complete inactivation Marino et al., 2018 [52][23]
Reduction of 1.5 log cycles for 25 min in lactose-free homogenized skim milk. The refrigerated ozonated water put in storage for up to 24 h was effective to control E. coli de Oliveira Souza et al., 2019 [107][37] Ozone concentration of 5 ppm for 20 min Effect of dissolved ozone on Pseudomonas biofilm on various surfaces The biofilms growth on the different materials were inactivated Shelobolina et al., 2018 [95][25]
Ozone sanitization in working surface areas and equipment
Gas ozone concentration of 80 mg min−1 for 5 min Degradation of aflatoxins on milk samples Aflatoxins were reduced by 50% Ismail et al., 2018 [109][39] Ozone concentration of 1.0–1.7 mg L
Ozonation (9.99 mg min−1),

UV light radiation (4.99 J cm		−1 followed by 0.8–1.1% of hydrogen peroxide Antimicrobial effect of ozone water in combination with a hydrogen peroxide solution −2) and pulsed electric field processes (13.15 µs)Synergic treatments showed an antimicrobial effect against Pseudomonas fluorescens biofilm Tachikawa et al., 2014 [ Synergistic effect of ozonation,

UV and pulsed electric field processes on the decrease of aflatoxin in probiotic milk		 Reduction of levels of aflatoxins (total and M1). Lactobacillus acidophilus in the final product of 106 CFU g−1 Khoori et al., 2020 [110][40]96][26]
Ozone concentration of 2 ppm for 15 min Monday to Friday and 120 min Saturday and Sunday Reduce the environmental colonization of Listeria monocytogenes
Ozone milk treatment by means of an ozonation regime in all production areas 3.5 g ozone h−1 at 20 ± 2 °C for 60 minIt showed a reduction in Listeria monocytogenes isolations from 15.0% in pre-zoning samples to 1.67% in post-ozonation samples in all areas Eglezos and Dykes, 2018 [97][27]
Effect of ozone treatment against milk concentrate and whey concentrate, evaluating the physicochemical and textural properties and the effect on the microbial, antibiotic, and aflatoxin content Ozone treatment reached 18.9% and 9.9% degradation of the aflatoxin M1 in milk concentrate and whey concentrate, respectively Sert and Mercan, 2021 [111][41] Ozonated water with 16 mg L−1 of ozone for 20 min Effect of acidic electrolyzed

water, ozone water, or

ultrasound on Staphylococcus aureus and Salmonella

5. Ozone Technology for Dairy Wastewater Care

In the dairy industry, water is used in a wide range of uses, generating significant volumes of wastewater rich in organic matter. It is currently purified with physicochemical and biological methods. Many studies have evaluated the possibility of using ozone for their treatment (Table 53), with the aim of recovering and reusing water [113,114,115,116][42][43][44][45].
Table 53.
Ozone for wastewater treatment.
Area Treatments Target Result References
Ozone in wastewater treatment Ozone concentration of 30 mg dm−3 at 25 °C for 5 min Reduce the content of organic pollutants in wastewater Thanks to its microflocculation effect, the effectiveness of the following nanofiltration phase increases and, consequently, the reduction of COD is performed; in addition, there is a 40% increase in the biodegradability of nanofiltration residues Làszlò et al., 2009 [117][46]
Ozonation combined with hydrogen peroxide (30% w/w, [H2O2] = 9.007 mol L−1 and density of 1.1 g mL−1) and catalyzed by manganese (1.71 g L−1) in alkaline conditions (MnSO4.H2O, 98% of purity) Degradation of organic matter in synthetic dairy wastewater The optimal condition for the ozonation catalyzed by manganese at alkaline medium (C.O.D. removal of 69.4%) can be obtained in pH 10.2 and Mn2+ concentration of 1.71 g L−1, with COD removals above 60%. dos Santos Pereira et al., 2018 [118][47]
Ozone in wastewater treatment 20 min of reaction time with Mn-Fe-Ce/γ-Al2O3 catalyst, which has a dosage of 12.5 mg·L−1·min−1 of ozone. pH = 9, and the catalyst dosage was 15 g·L−1 Mn-Fe-Ce/γ-Al2O3 for catalytic ozonation of dairy farm wastewater The COD removal ratio of dairy farming wastewater can reach 48.9%. The BOD/COD increased from 0.21 to 0.54. Therefore, when the catalyst dosage increases (0–25 g L−1), the C.O.D. decreases Li et al., 2020 [119][48]
spp. biofilm on stainless steel surfaces
Less than 0.8 Log CFU cm
−2
of
Five levels of ozonation flow rate (1, 2, 3, 4, 5 L min−1) were tested under the conditions that pH was 7.5 and reaction time was 60 min Effect of ozone on undiluted dairy farm liquid digestate that contains high levels of organic matter, chromaticity and total ammonia nitrogen After cascade pre-treatment, TAN, TN, COD, and chromaticity were reduced by 80.2%, 75.4%, 20.6%, and 75.8% respectively Zhu et al., 2022 [120][49]


cells reduction in biofilm exposed to ozonized water
All pre-treatments were performed with ultrasound (US 200 W), ozone (4.2 mg O3 L−1) and US combined with ozone (US/ozone) for 10 min, 20 min, and 30 min, respectively Dairy wastewater has been pre-treated with ultrasound, ozone and US combined with ozone to study the fate of enteric indicator bacteria and antibiotic resistance genes, and anaerobic digestion. US/ozone pre-treatment was effective in the inactivation of enteric indicator bacteria. Total coliforms and enterococci were reduced by 99% and 92% after 30 min Chen et al., 2021 [121][50]
Ozone in wastewater treatment Continuous type O3 treatment system (43.26, 87.40, and 132.46 mg L−1) Continuous type O3 treatment system to eliminate pathogens such as Salmonella Typhimurium and Escherichia coli O157: H7 in liquid dairy waste Ozone reduced E. coli O157: H7 and S. typhimurium and the reductions increased with the exposure time, particularly at 87.40 or 132.46 mg·L−1 Chang et al., 2022 [122][51] Shao et al., 2020 [98][28]
Ozonated water with an ozone concentration of 1 ppm and 1–2% of malic acid for 20–40 min The combined effect of malic acid

and ozone as sanitizer to inhibit the biofilm formation by Salmonella typhimurium on different food contact surfaces		 Combination of malic acid with ozone reduced the biofilm formation on plastic bags, as well as on PVC pipes, suggesting it as an effective disinfectant for food contact surfaces Singla et al., 2014 [99][29]
Ozone sanitization in working surface areas and equipment Ozonated air had constant flow rate of 40 L min−1 and it generated 1.5 g h−1 of ozone

(11–12 p.m. and 1–3 a.m. from Friday to Sunday). Hydrogen peroxide aerosolization was realized producing particles in the range of 5–15 μm at a concentration of 5–15% for 16–20 min.		 Effect of ozonation or chemical aerosolization through hydrogen peroxide to monitor the air microbial load in the dairy factory and in evaluating the air disinfection Ozonation and hydrogen peroxide aerosolization were effective techniques in the inactivation of airborne microorganisms Masotti et al., 2020 [100][30]

4. Ozone Treatment in Milk Production

In the treatment of raw milk, the use of ozone is debated: in fact, in contrast to the traditional thermal pasteurization process, ozone treatment minimizes the loss of the nutritional properties of milk, but it does not always show a total inactivating capacity on microorganisms [101][31] (Table 2
Làszlò et al. [117][46] confirmed the ability of ozone (30 mg dm−3) to reduce the content of organic pollutants in wastewater at 25 °C for 5 min; thanks to its microflocculation effect, in fact, the effectiveness of the following nanofiltration phase increases and, consequently, the reduction of COD is performed; in addition, there is a 40% increase in the biodegradability of nanofiltration residues.
Additionally, dos Santos Pereira et al. [118][47] investigated the degradation of organic matter in a synthetic dairy wastewater. In this study, dairy wastewater has a chemical oxygen demand of 2.3 g L−1, and they treated this wastewater through an oxidation process using ozonation combined with hydrogen peroxide (30% w/w, [H2O2] = 9.007 mol L−1 and density of 1.1 g mL−1) and catalyzed by manganese (Mn2+) in alkaline conditions (MnSO4.H2O, 98% of purity). It was observed that the optimal condition for the ozonation catalyzed by manganese at alkaline medium (chemical oxygen demand removal of 69.4%) can be obtained at pH 10.2 and Mn2+ concentration of 1.71 g L−1, with COD removals above 60%.

6. Use of Ozone Technology in Cheese-Making Processes and in Storage/Maturation Condition

Many studies demonstrate the effectiveness of ozone in preventing the growth of mold in maturing environments against cheese mites (Table 64).
Gibson et al. [124][52] demonstrated the bacteriostatic effect by applying two different concentrations of ozone in the ripening phase of Cheddar cheese, verifying its effectiveness both at concentrations of 3–10 ppm for 30 days and at those of 0.2–0.3 ppm for 63 days with a difference of 6 percentage points in favor of the highest concentrations. Gabriel ‘yants’ et al. [125][53] confirmed these observations: the application of ozone under refrigerated conditions on Russian and Swiss-type cheese prevented mold growth for four months without damaging the sensory properties and chemical composition, while growth was observed on the control sample already after one month of storage. They stored the cheeses under refrigeration (2–4 °C, 85–90% RH) with or without ozonation of the air in the room. Periodic treatments were undertaken with concentrations of 2.5–3.5 ppm of gaseous ozone for 4 h at 2- to 3-day intervals, preventing mold growth on packaging materials for up to 4 months.
Other studies verify the effectiveness of ozone against the bacterial population: Morandi et al. [71][54] applied 4 ppm of gaseous ozone for 8 min at different stages of maturation and showed the effectiveness in controlling Listeria monocytogenes (artificially surface-inoculated up to 103 CFU g−1) on Ricotta Salata di Pecora (below 10 CFU g−1) and, limited to the first days of maturation on Gorgonzola PDO (Protected Designation of Origin) and Taleggio PDO, the effect was a complete elimination; Cavalcante et al. [126][55] proposed treatments with ozonated water (2 mg L−1 for 1–2 min) for washing Minas Frescal cheese during storage, highlighting, once again, the ability to reduce the initial microbial load (by approximately 2 log10 cycles) without damaging the sensory and physical–chemical properties of the product. However, the treatment did not show efficacy in controlling the growth rate of the surviving microflora.
Serra et al. [127][56] have shown the effectiveness of ozone treatment against the fungal spores that colonize the maturing rooms, but they have also shown that the treatment may not eliminate the molds present on the surface of the cheeses. They studied the effect of ozone to reduce molds in a cheese-ripening room (3500 m3) with a temperature of 5 ± 1 °C and a relative humidity of >80%. The efficacy of this treatment was evaluated in air and on surfaces through sampling on a weekly basis over a period of 3 months. The results demonstrated that ozonation decreased the viable airborne mold load, but not on surfaces, resulting in a 10-fold reduction compared with the level observed for the control. The mold genera usually isolated in the air were Penicillium, Cladosporium, and Aspergillus (89.9% of the mold isolates). They used 8 g h−1 as the rate of ozone generation.
In their study, Guzzon et al. [128][57] explored the microbiota of the red–brown defect in smear-ripened cheese and how to prevent it using different cleaning systems. Red–brown pigmentation can occasionally form in this type of cheese, such as Fontina, during the ripening process due to an over-development of the typical microbiota present on the rind. The microbiota of the shelf had a role in this defect in cheese. Different systems were tested for cleaning the wooden shelves (WS): washing with hot water and ozone treatment. The results showed that Actinobacteria, dominant on the WS, indicated to be responsible for the red–brown pigmentation; they were also in traces in the defected samples. Galactomyces and Debaryomyces were the main species for the yeast population, with Debaryomyces as the most dominant species on the shelves used during the maturation of the red–brown defective cheese. Even if the hot water treatment decreased the microbial load of shelves, only the use of ozone guaranteed a total elimination of yeast and bacteria, with no red–brown defect on the cheese rind. The ozone treatment (OT) was done using ozone in two different forms: gaseous (dry) or water-dissolved (wet). Therefore, washing plus ozone treatment ensures that the WS will be safe for the next ripening process. After the study, some trained experts reported that the wheels ripened on shelves cleaned by wet ozone treatment did not have the red–brown defect and no difference in cheese color, taste, and texture.
In the study of Alexopoulos et al. [129][58], an ozone stream (2.5–3 ppm) for 0, 10, 30, and 60 s was used on the surface of freshly filled yogurt cups (240 g) before storage for the development of the curd (24 h) to prevent cross-contamination from spoilage airborne microorganisms. Additionally, the brine solution was bubbled with ozone at different times and applied for the ripening of white (feta type-400 g) cheese. Ozone gas was supplied for 0, 10, 20, and 30 min to the brine, where the cheese sample was then left for ripening for a period of 2 months. Products were monitored for microbial load and tested for their sensorial characteristics. In ozonated yogurt samples, data showed a reduction in mold counts of about 0.6 Log CFU g−1 (25.1%) by the end of the monitoring period against the control samples. In white cheese matured with ozonated brine (1.3 mg L−1 O3, NaCl 5%), the treatment for two months decreased some of the mold load without offering advantages over the use of traditional brine (NaCl 7%). However, some alterations in the sensorial quality were observed, perhaps due to the organic load of the brine that soon neutralizes ozone. For the cheese samples ripened in 60 min ozonated brine, there was a reduction in the attribute of flavor, according to the global acceptance. Therefore, factors of time and concentration of ozone are essential, and they must be configured.
The efficacy of ozone is non-selective towards harmful or useful bacteria. It becomes challenging when lactic acid bacteria (LAB—one of the most significant groups of probiotic organisms), commonly used in fermented dairy products, is destroyed by the application of ozone. In this work, LAB was present in all yogurt samples in an adequate number (S. thermophillus 9.1 ± 0.5 Log CFU g−1 and L. delbrueckii ssp. bulgaricus 8.8 ± 0.6 Log CFU g−1). Although a drastic decline was observed after 50 days of storage by the end of the monitoring time, nevertheless no statistical differences were observed between the control and the ozonated samples. Therefore, ozone did not affect good microflora, maintaining the functional character of the yogurt, with LAB values higher than 7 Log CFU g−1 for a period of 70 days.
Segat et al. [130][59] used ozone treatment to reduce the microbial spoilage load on “mozzarella” cheese, concluding that the cooling water treatment (15 °C) with ozone (2 mg L−1) improves the microbiological qualities, positively affecting the shelf-life of the products. The results showed that “mozzarella” cheese samples that were cooled in water and pretreated were characterized by low microbial counts as compared to control samples cooled with non-ozonated water, following 21 d of storage (by 3.58 and 6.09 log10 CFU g−1 lower total plate counts and Pseudomonas spp. counts, respectively, compared to control samples).
Sert et al. [112][60] showed the effect of ozone on butter samples in their work, considering that the treatment can be applied to produce butter as a non-thermal technology. Raw cream was ozone-treated for 5 (OT-5), 15 (OT-15), 30 (OT-30), and 60 (OT-60) min. The ozone application was conducted at 8 ± 2 °C and, after the treatment, the churning was carried out at 4 ± 2 °C. Then, the samples were packaged and stored at 4 °C for 24 h. The ozone treatment increased the firmness, consistency, and fat particle size values of samples. Instead, it decreased the color parameter b value, so the yellowness of butter. OT-60 caused 2.01 log reduction in Staphylococci, and over 15 min, the ozone completely inactivated Salmonella and yeast-mold, while coliform was not detected in OT-30 and OT-60. The results showed a higher microbiological quality in butter from raw cream with ozone treatment. Instead, OT decreased the oxidative stability of butter and up to 15 min increased the spreadability characteristic of the sample.
Panebianco et al. [131][61] studied the effect of ozone on Listeria monocytogenes contamination and the resident microbiota on the Gorgonzola cheese rind. Concentrations of 2 and 4 ppm for 10 min were used against L. monocytogenes and resident microbiota of cheese rind samples stored at 4 °C for 63 days. Results showed that in ozonized rinds the final loads of L. monocytogenes were ~1 log CFU g−1 higher than controls. No significant differences were found for the other microbial determinations and the resident microbiota between ozonated and control samples.
Recently, Tabla and Roa [132][62] studied the effect of gaseous ozone in soft cheese ripening, in particular its effect on the rind microorganisms, evaluating the sensorial quality. The experiment was conducted at different production scales. On a pilot plant scale, the samples were inoculated with Mucor plumbeus, Kluyveromyces marxianus, and Pseudomonas fluorescens, while on an industrial scale they evaluated the effect of ozone on naturally present microorganisms. For the experiment at pilot plant scale, cheeses were surface-inoculated after salting to a final concentration of 105 spores g−1 of rind for Mucor plumbeus and 107 CFU g−1 of rind for Kluyveromyces marxianus and Pseudomonas fluorescens. The ripening was conducted in 8 m3 ripening rooms for 30 days with gaseous ozone at concentrations of 0, 2, and 6 mg m−3. For the experiment at dairy plant scale, a total of 30 cheeses with 15 days of ripening were subjected to maturation with gas ozone at concentrations of 2 mg m−3 for 45 days. The results showed that gaseous ozone treatments had a significant fungistatic effect on cheese surface molds for pilot and industrial scale, but its effect on yeast growth was only observed at pilot plant scale, and for Pseudomonas ssp., the ozone concentrations used did not affect it. For the sensory quality of the cheese, its typicality was not influenced by gas ozone, and the ozonized cheeses had significantly higher scores than the control samples for the appearance of the rind and color, preventing cheese rind discoloration.
In ripened cheeses, an important problem is the presence of mites that live and grow on the rind of the product. Some studies have shown that the most represented species in the dairy industry are Acarus siro, Tyrophagus casei, T. longior, T. palmarum, and T. putrescentiae [36,37,38][63][64][65]. The ripening environment is optimal for the proliferation of mites, above all by the high relative humidity and temperatures that hover around 12–13 °C; it has, in fact, been seen that at temperatures of °C these parasites do not survive, but there is also an excessive prolongation of the ripening times which makes them inapplicable [133][66], and in most cases the presence of mites in environments of maturation is associated with important economic losses. The intake of mites can cause the appearance of symptoms ranging from dermatitis to allergic rhinitis and asthma, up to gastro-intestinal problems. It has been seen that water at a temperature higher than 71.1 °C [36][63] eliminates the mites in a few seconds; moreover, there are several effective chemical treatments, and the most used substances are organophosphate compounds. However, chemical products leave residues and, in some cases, such as that of methyl bromide, they have been banned, as they have harmful effects on human health. For this reason, it is essential that research identifies alternative control methods. Therefore, the interest in the methods of sanitizing the maturing environments with ozone is growing. The effectiveness of ozone in killing mites has been ascertained thanks to a few studies, even in dairy environments. For example, Pirani [59][67] verified the effectiveness of ozone treatment for the control of mite infestations on matured speck products.
In a new study, Pecorino cheese samples were treated for 150 days overnight with gaseous ozone (200 and 300 ppb, 12 °C, and 85% R.H. for 8 h per day). It was observed that, starting from 25 days of storage, 200 ppb of ozone reduced the growth of mites and significantly reduced bacteria, molds, and yeast count, starting from 75 days of storage. Furthermore, it was observed that no significant differences were shown between the control samples and ozone treatment at 200 ppb for the centesimal composition of Pecorino cheese. However, treatments with ozone 300 ppb contained microbiological and mite growth but did not have the same positive impact on some aspects of overall quality [134][68].
Table 64.
Use of ozone technology in cheese making processes and in storage/maturation conditions.
Area Treatments Target Result References
Use of ozone technology in cheese making processes and in storage/maturation condition Ozone concentrations of 3–10 ppm for 30 days and of 0.2–0.3 ppm for 63 days Effect of ozone in the ripening phase of Cheddar cheese Higher ozone concentrations showed 6% more bacteriostatic action than lower concentrations Gibson et al., 1960 [124][52]
Cheese refrigeration at 2–4 °C, 85–90% RH with or without ozonation of the air in the room. Periodical treatments were done with concentrations of 2.5–3.5 ppm of gas ozone for 4 h at 2- to 3-day intervals Application of ozone under refrigerated conditions on Russian and Swiss-type cheese to prevent mold growth Ozone prevented mold growth for four months, while growth was observed on the control sample already after one month of storage. It prevented also mold growth on packaging materials for up to 4 months Gabriel ’yants’ et al., 1980 [125][53]
4 ppm of gaseous ozone for 8 min at different stages of maturation Efficacy of ozone against Listeria monocytogenes (artificially surface-inoculated up to 103 CFU g−1) On Ricotta Salata below 10 CFU g−1 and, limited to the first days of maturation on Gorgonzola PDO and Taleggio PDO, the effect was a complete elimination Morandi et al., 2009 [71][54]
Ozonated water (2 mg L−1

for 1–2 min)		 Ozonized water treatments for washing Minas Frescal cheese during storage Reduction of the initial

microbial load (by 2 log10 cycles)		 Cavalcante et al., 2013 [126][55]
Use of ozone technology in cheese making processes and in storage/maturation condition Cheese preserved with a temperature of 5 ± 1 °C and a relative humidity of >80% for 3 months. It was used 8 g h−1 as the rate of ozone generation. Efficacy of ozone treatment against molds present on the surface of cheeses and of the fungal spores that colonize the maturing rooms Ozonation reduced the viable airborne mold load, but not on surfaces, resulting in a 10-fold reduction compared with the level observed for the control Serra et al., 2003 [127][56]
Gaseous ozone was produced of 40 g h−1 instead ozone water was done by coupling the same generator of ozone to a water bath (nominal volume of 100 L) Explore the microbiota of the red–brown defect in smear-ripened cheese and how to prevent it Ozone treatment guaranteed a complete elimination of yeast and bacteria, with no red–brown defect on the cheese rind Guzzon et al., 2015 [128][57]
Use of ozone technology in cheese making processes and in storage/maturation condition Ozone stream (2.5–3 ppm) for 0, 10, 30, and 60 s was used on the surface of freshly filled yogurt cups (240 g) before storage for the development of the curd (24 h). The brine solution bubbled with ozone was applied for the ripening of white (feta type, 400 g) cheese. Ozone gas was supplied for 0, 10, 20, and 30 min to the brine where cheese sample was then left for ripening (2 months) Prevent contamination from spoilage airborne microorganisms In ozonated yogurt samples, data showed a reduction in mold counts of about 0.6 Log CFU g−1 (25.1%) by the end of the monitoring period against the control samples. In white cheese matured with ozonated brine (1.3 mg L-1 O3, NaCl 5%) the treatment during two months decreased some of the mold load Alexopoulos et al., 2017 [129][58]
Cooling water (15 °C) with ozone (2 mg L−1) Reduce the microbial spoilage load on Mozzarella cheese Samples cooled in water pre-treated were characterized by low microbial counts, following 21 d of storage (by 3.58 and 6.09 log10 CFU g−1 lower total plate counts and Pseudomonas spp. counts, respectively, than in control samples) Segat et al., 2013 [130][59]
Use of ozone technology in cheese making processes and in storage/maturation condition The ozone application was conducted at 8 ± 2 °C and for the treatment, an ozone generator with a capacity of 3.5 g ozone h−1 was used for 5, 15, 30, and 60 min Effect of ozone on butter samples The results showed a higher microbiological quality in butter from raw cream with ozone treatment Sert et al., 2020 [112][60]
Concentrations of 2 and 4 ppm

for 10 min were used against L. monocytogenes and resident microbiota of cheese rind samples stored at 4 °C for 63 days		 Effect of ozone on Listeria monocytogenes contamination and the resident microbiota on Gorgonzola cheese rind In ozonized rinds, the final loads of L. monocytogenes were ~1 log CFU g−1 higher than controls Panebianco et al., 2022 [131][61]
Use of ozone technology in cheese making processes and in storage/maturation condition For the experiment at pilot plant scale the ripening was conducted in 8 m3 ripening rooms for 30 days with gaseous ozone at concentrations of 0, 2, and 6 mg m−3. For the experiment at dairy plant scale, a total of 30 cheeses with 15 days of ripening were subjected to maturation with gas ozone at concentrations of 2 mg m−3 for 45 days Effect of gas ozone in soft cheese ripening, in particular its effect on the rind microorganisms The results showed that gas ozone treatments had a significant fungistatic effect on cheese surface molds for pilot and industrial scale Tabla and Roa, 2022 [132][62]
Pecorino cheese samples were treated for 150 days overnight with gaseous ozone (200 and 300 ppb, 12 °C, and 85% R.H. for 8 h per day) Effect of gas ozone on the mite pest control and on microbiological growth during Pecorino cheese ripening/storage Starting from 25 days of storage, 200 ppb of ozone reduced mites and bacteria counts; molds and yeasts started from 75 days of storage. Ozone 300 ppb did not have the same positive impact on some aspects of overall quality. Grasso et al., 2022 [134][68]

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