2021 Volume 27 Issue 5 Pages 695-710
This article reviews the research progress of non-thermal disinfection technologies in food and introduces the principles of non-thermal disinfection technologies such as ultra-high pressure, ozone, acid electrolyzed water, irradiation, magnetic sterilization, high-pressure carbon dioxide, and natural antibacterial. We discuss the characteristics and application progress of non-thermal disinfection technologies in food processing and analyze their development trend based on their application and promotion in food processing and storage.
In recent years, the incidence of foodborne diseases caused by pathogenic microorganisms has increased gradually. People have paid significant attention to food safety and quality, mainly during global food shortages after natural disasters and epidemic outbreaks. Because microorganisms are susceptible to temperature, heating has been largely employed as one of the conventional sterilization techniques to kill microorganisms in food. Heat sterilization technology is widely used in the food industry, which further guarantees food safety. For example, the canning process uses heat treatment technology to sterilize. Heat sterilization technology has been used in the food industry for many years to ensure food safety, extend shelf life, and maintain food quality (Kullananant et al., 2020, Pursito et al., 2020, Rivas et al., 2007). However, heat sterilization technology could also harm the quality of the food. The high temperature during the sterilization process may change the food's color, flavor, and nutrient loss (Hoac et al., 2006). Consumers' demand for the sensory and nutritional quality of food increases day by day, requiring both safety and not compromising food quality. Control of microorganisms in food without compromising nutritional and organoleptic properties. Non-thermal disinfection technology to minimize the loss of various nutrients in food, try to maintain the original flavor of food and improve the economy of disinfection technology, convenience, improve food packaging and storage conditions, extend the shelf life of food to meet the needs of the growing material life of consumers.
Non-thermal disinfection refers to a technology that uses chemical or physical methods to destroy microorganisms at room temperature or low temperature, which different sterilization methods have different sterilization mechanisms (Fig. 1). This technology is characterized by energy-saving ability, high efficiency, safety, and low cost and essentially maintains the natural color, aroma, and taste of the food. This article reviews the principles, vital technical aspects, and application limitations of non-thermal disinfection technologies and provides references for the industrial application of these technologies.
Diagram of the non-thermal disinfection mechanism described in this review
Foodborne illnesses are mainly caused by the consumption of contaminated food or water and can be contaminated at all stages from production, transportation to distribution. Food safety control involves chemical, physical and biological controls, of which biological and especially microbial controls are the most critical. Microbial contamination of food is the most significant problem facing consumers, regulatory agencies, and the food industry today (Penha et al., 2017). Microorganisms are highly susceptible to contamination of raw fruits and vegetables, dairy products, and meat products because these foods possess high water activity and rich nutrient content (Ghate et al., 2013). Most foodborne illnesses are caused by bacteria and have a high incidence, posing a severe threat to public health. Fresh vegetables and fruits are frequently contaminated with Bacillus cereus and Listeria monocytogenes (L. monocytogenes) (Paskeviciute et al., 2018). The major foodborne pathogens of dairy products are Salmonella spp., L. monocytogenes, Cronobacter spp., Staphylococcus aureus (S. aureus), and Escherichia coli (E. coli) (Cancino-Padilla et al., 2017, Fox et al., 2018). The primary foodborne pathogens associated with livestock and poultry products are Salmonella, enterohemorrhagic E. coli, Campylobacter spp., and Clostridium perfringens (Ferrari et al., 2019, Mor-Mur and Yuste, 2009, Tesson et al., 2020). The major pathogenic bacteria in aquatic products include Vibrio, Salmonella, L. monocytogenes, enterohemorrhagic E. coli, Campylobacter spp. and Shigella spp. (Dumen, et al., 2020, Novoslavskij, et al., 2016). In processing, microbial contamination of food with pathogenic bacteria is mainly confined to the food surface, where the risk of cross-contamination aspects involving personnel, equipment, and processing environment is significantly increased (López-Carballo et al., 2008). Disinfection or sterilization is an essential means of controlling microorganisms in food by using various means to kill microorganisms contaminated by the food itself, brought in from food packaging containers, introduced by operators and equipment during processing and blending, and present in the production environment, thus extending the storage period of the product and ensuring product quality and safety.
Heat sterilization technology can cause undesirable sensory properties of food, such as color degradation, water loss, and nutrient destruction. Therefore, in recent years, researchers have been interested in non-thermal physical disinfection technologies, including ultra-high pressure (UHP) sterilization, irradiation sterilization, and ultraviolet (UV) magnetic sterilization, ultrasonic sterilization, and blue light sterilization. These physical disinfection technologies can be used as alternative methods for conventional thermal sterilization technologies. Details of non-thermal physical disinfection technologies and their applications are shown in Table 1.
| Method | Key points | Advantages | Disadvantages | Applications |
|---|---|---|---|---|
| Ultra-high pressure sterilization technology | Destroys cell membranes, inhibiting enzyme activity and affecting genetic materials such as DNA | Extends the storage period and improves the quality (D'Incecco, et al., 2018) | Changes the appearance of food | Aquatic products preservation (Tong, et al., 2014) |
| Irradiation sterilization technology | After γ-rays interact with substances, part of the energy is transferred to atoms or molecules to generate a pair of cations and anions, which produce physical, chemical, and biological effects through ionizing radiation. The genetic material DNA will lose its replication ability due to the cleavage of chemical bonds (Della Schiava, et al., 2020) | No evident influence on the appearance, aroma, taste, and taste of food | Radiation is harmful to the human body (Annamalai, et al., 2020) | Extend the shelf life of fruits (Mahto and Das, 2013) |
| Ultraviolet sterilization technology | To destroy the molecular structure of DNA in microbial cells (2020) | High sterilization efficiency, safe, and reliable operation | No continuous sterilization ability | Inhibits the reproduction of microorganisms in meat products; inhibition of microorganisms in dairy products (Ollikainen and Riihimaki, 2012, Rossitto, et al., 2012) |
| Magnetic sterilization technology | Increases the probability of gene mutations, which can cause cell damage and apoptosis in animals and plants (Hashish, et al., 2008) | Maintain the nutritional content of food without affecting the flavor quality | Changes in the density and specific heat capacity of water (Yang, et al., 2020) | Improves the freshness of fruits and vegetables, reduces decay, and extends the shelf life (Zhang, et al., 2020b, Zhao, et al., 2018) |
| Ultrasonic sterilization technology | Ultrasonic cavitation effects cause local instantaneous high pressure and pressure changes, killing certain bacteria and viruses in the liquid (Chandrapala, et al., 2012) | Low energy consumption and short sterilization time | Limited antibacterial ability and single processing capacity | Extends the shelf life of beverages, soy sauce, milk, and beans (Leadley, et al., 2008) |
| Blue light sterilization | The bacteria produce ROS under blue light (405–470 nm) irradiation, causing oxidative stress leading to their death (Lukšiene, 2005) | Gram-positive can sense blue light and Gram-negative bacteria and fungi (Wang et al., 2016), and still has the potential to be antibacterial or bactericidal without photosensitizers and is less harmful to mammalian cells (Wang, et al., 2016) | Light pollution, hurting the operator's eyes, affecting the color of food (Dos Anjos et al., 2020a, Ghate et al., 2016) | Sterilization of fruits and vegetables, meat products, and milk (dos Anjos et al., 2020b, Ghate et al., 2017, Josewin et al., 2018, Sommers et al., 2017) |
Ultra-high pressure sterilization technology UHP processing refers to the soft packaging of food materials into a liquid medium (normally water). Sterilization at 100–1000 MPa pressure under room temperature or low-temperature conditions kills microorganisms of food and prevents microorganisms' growth and reproduction (Zhang et al., 2020a, Zhu et al., 2020). High pressure ruptures the bacterial cell wall and cell membrane, causing the leakage of the bacterial content, thereby killing the bacteria in the food (Agregán et al., 2021). UHP treatment is suggested to inactivate protein coagulation and related enzymes in microorganisms, resulting in bacterial death (Bridgman, 1914).
The critical elements of UHP are precise control of pressure, time, temperature, and pressure method. Generally, within a specific pressure range, the longer the pressurization time, and the higher the pressure, the more pronounced the effect of sterilization. Ye et al. (2013) found that the number of Vibrio parahaemolyticus (V. parahaemolyticus) in oysters significantly reduced after 15 d of cryopreservation after pressure treatment at > 250 MPa. After 90 s of treatment at 345 MPa, the number of V. parahaemolyticus in oysters reached an undetectable level (Calik et al., 2010). Even at lower temperatures, UHP can achieve better sterilization effects (Alpa et al., 2000). Reyes et al. (2015) found that UHP treatment could inhibit the growth of psychrotrophic bacteria and sulfur-producing bacteria, especially Shewanella, at 4 °C. The higher the temperature during pressurization, the more pronounced the sterilization effect. Aspergillus niger heat treatment (80 °C, 15 min) before/after high pressure at 200 MPa had a synergistic bactericidal effect (Tribst et al., 2010). Gou et al. (2010) found that UHP (400 MPa, 20 min) semi-dried squid fillets had a reduced number of psychrophilic bacteria by at least 4.7 logs. Besides, adopting different pressure methods can have a better sterilization effect. The synergistic effect can improve the sterilization effect and prevent the negative impact of UHP sterilization on food. Lee et al. (2003) found that using ultra-high pressure (300 MPa, 5 °C, 200 s) and nisin (10 mg/L, about 0.01%) to treat eggs contaminated with Listeria seeligeri (L. seeligeri). They found that using UHP (250 MPa, 300 MPa) alone, the reduction of L. seeligeri was not detectable, and nisin treatments at atmospheric pressure reduced the viable cell counts of L. seeligeri only slightly up to 10 mg/L of nisin concentration. However, when the same concentration of nisin and UHP is used at the same time, the number of L. seeligeri in eggs can be reduced from 107 CFU/mL to 102 CFU/mL.
After UHP treatment, the food can maintain its original nutrients and can be easily absorbed by the human body. The entire process is environmentally friendly, with no pollution and adverse effects on the environment. However, UHP technology can change the shape of the food and have a significantly adverse effect on the appearance of the food (Lowder et al., 2014). Further, because of the high investment cost of UHP (Sunil et al., 2018), its application is subject to certain limitations.
Irradiation sterilization technology Food irradiation sterilization is a sufficient food that uses the physical, chemical, and biological reactions generated by the interaction of ionizing radiation generated by radioactive sources such as cobalt-60 and cesium-137 with substances to kill bacteria and prevent mildew Sterilization technology (Ahn et al., 2013, Al-Bachir and Othman, 2013). Irradiation can break the DNA of microorganisms and denature enzymes and membrane proteins. Simultaneously, the water molecules in the microorganisms are ionized to produce free radicals (i.e., OH•, H•·, and H2O•), which leads to the inactivation of bacteria (Lung et al., 2015). The rays commonly used in food sterilization include X-rays, gamma rays (γ-rays), and electron rays. γ-rays have strong penetrating power and are suitable for internal sterilization of whole foods and various packaged foods. The FDA has approved irradiation sterilization technology to eliminate pathogens and spoilage microorganisms in food (FDA 2016).
Irradiation sterilization technology is widely applied and has a good sterilization effect. Irradiation can inhibit the growth and reproduction of bacteria in food. Sterilization by irradiation ensures the food's quality and is easy to manipulate without residue. Irradiation will not affect the appearance of food in most cases, but high-dose irradiation will affect the color, flavor, and physical properties of food. Joshi et al. (2018) used irradiation to purify fresh cucumber slices and found on the irradiation day (day 0), the population of Salmonella reduced by 4.96 log (CFU/g) than the unirradiated control. During the entire storage period, Salmonella's population in the irradiated samples continued to decrease, while the number of Salmonella in the control group remained unchanged. Meanwhile, irradiation treatment had no adverse effect on the quality parameters of cut cucumber slices. The Roma tomatoes irradiated by X-ray reduced the pathogens on the tomato surface and extended the storage period (22 °C) from 6 d to 20 d (Mahmoud, 2010). However, to have a better sterilization effect, increasing the radiation dose will also decrease food quality. According to the FAO/IAEA/WHO Expert Committee, ionizing radiation treatment of up to 10 kGy is allowed on any food, and it is not harmful to consumers (Annamalai et al., 2020). Therefore, consumers' acceptance of food irradiated by radioactive sources and safe handling of radioactive materials has become a bottleneck for food irradiation and sterilization. Also, the initial irradiation sterilization equipment's investment and maintenance cost is another bottleneck restricting the application of the food industry (Goodburn and Wallace, 2013).
Ultraviolet sterilization technology UV sterilization is the use of UV rays of appropriate wavelengths to destroy the RNA or DNA (the genetic material) in the cells of the microorganisms, causing cell death and achieving the effect of sterilization (Kataoka et al., 2020, Ollikainen and Riihimaki, 2012, Sun et al., 2019). UV rays are electromagnetic waves between visible light and X-rays. Its wavelength range is 10–400 nm. Based on the wavelength range, UV rays are divided into vacuum UV (10–200 nm), short-wave UV (200–290 nm), medium-wave UV (290–320 nm), and long-wave UV (320–400 nm) (Khan et al., 2008). The waveband of the artificial UV light source currently used for sterilization is 200–350 nm. Genetic damage caused by UV light is primarily responsible for microbial inactivation (Gayán et al., 2014). The primary mechanism of UV sterilization is DNA generates thymine dimer by absorbing UV, which interrupts DNA replication and transcription and induces the photochemical changes of microbial DNA, leading to microbial inactivation (Gayán et al., 2014, Krishnamurthy et al., 2008).
UV sterilization is widely used in vegetables and fruit surface disinfection. Mukhopadhyay et al. (2014) used UV (dose 0.6–6.0 kJ/m2) to eliminate pathogens on grape tomato and indicated that E. coli O157:H7 and Salmonella on the surface of each fruit were reduced by 2.3–3.5 and 2.15–3.1 log CFU, respectively. It can effectively control the tomato rot caused by Botrytis cinerea and prolong the fruit's freshness for 4 to 5 d (Sunil et al., 2018). Besides, UV treatment can reduce the growth and reproduction of bacteria in fruits (Pombo et al., 2009, Qian et al., 2009). UV sterilization has been proposed as a potential alternative to thermal and chemical disinfection methods. However, UV sterilization has some application limitations, such as UV permeability, shadowing effects from food surface properties (Gayán et al., 2014), and UV can alter photosensitive compounds in food (Koutchma, 2019).
Magnetic sterilization technology Magnetic sterilization technology uses polar and magnetic molecules and atoms to sterilize the organism when a magnetic field is applied (Haile et al., 2008). Under the action of the magnetic field, high-speed moving charged particles in the food collide with the food molecules and decompose the food molecules to produce highly active anions and cations. These ions can penetrate the cell membrane, interact with the protein and RNA in the microorganisms, and block the cells. Normal metabolism prevents cells from growing naturally and leads to cell death (Ma et al., 2003).
Gerencser (1962) found that high-strength magnetic fields can inhibit cell activity to a certain extent. Tenuzzo et al. (2009) showed that a constant magnetic field of 6 mT has a significant effect on cell viability, diffusion capacity, and morphology, and a constant magnetic field can interfere with the normal process of programmed cell death. Magnetic fields can also cause chromosomal conformation changes and micronucleus formation, and DNA damage can cause cell function weakening and even cell apoptosis (Phillips et al., 2009). Magnetic sterilization technology exhibits practical antibacterial effects on four vegetable juices and has no adverse effects on their quality (Lin et al., 2019). Studies have shown that the combination of pulsed magnetic fields and refrigeration can play a positive role in preserving fresh meat, which can control microorganisms without degrading meat quality (Goldschmidt Lins et al., 2017). Applying an alternating magnetic field to treat the cucumis melo L. cv Hetao before cutting helped reduce its wound reaction and maintained better quality and flavor (Jia et al., 2015). The static magnetic field treatment had an inhibitory effect on the pathogenic bacteria on the cucumber's surface, delayed the cucumber's decay, and the decay rate was s 57.2% lower than that of the control group (Zhao et al., 2018). However, different magnetic fields have different characteristics. Static magnetic fields cause polar water molecules to shake and generate additional heat, which has a negative effect on cooling (Yang et al., 2020), while alternating magnetic fields reduce the specific heat and viscosity of the solution (Holysz et al., 2007).
Ultrasonic sterilization technology Ultrasonic sterilization uses ultrasonic waves (frequency > 20 kHz) to act in the medium. When its strength exceeds a certain air threshold, the tiny air bubble core in the liquid shrinks and collapses during adiabatic contraction, and the bubble appears at 5 000 °C. The above high temperature and the temperature change rate of 109 K/s produce a powerful shock wave of up to 108 N/m2, which subjects the contents of microbial cells to strong shocks, thereby achieving the destruction of microorganisms (Watanabe and Fjjita, 2014). The ultrasonic cavitation effect causes a local instantaneous high temperature and alternating temperature changes, local instantaneous high pressure and pressure changes generated in the liquid makes the bacteria in the liquid lethal, inactivates the virus destroys the cytoderm of microorganisms, and kills pathogenic bacteria to extend the shelf life of food. Compared with traditional heat treatment and sterilization technology, ultrasonic sterilization can reduce energy consumption and food quality damage, extend shelf life, and achieve a high degree of automation.
With ultrasonic sterilization, the microbial reduction is closely related to operating parameters (temperature, power, and exposure time). Generally, increasing the ultrasound frequency and exposure time can achieve a higher reduction in microorganisms. Huang et al. (2018) used ultrasonic (frequency 42 kHz) to treat vegetables inoculated with E. coli and L. monocytogenes. They found that the leaves' bacterial concentration with bacteria decreased as the ultrasonic treatment time increased, and ultrasound treatment for less than 10 minutes would not affect fresh lettuce quality. The population of S. aureus significantly reduced after high-intensity ultrasound treatment (frequency 40 kHz, intensity 9.6 W/cm−2, 50 min) of chicken breasts (Pinon et al., 2020). Ultrasound (frequency 40 kHz, 10 min. ) can also inhibit the growth of green asparagus colonies in cold storage (Wang and Fan, 2019). The synergistic effect of ultrasound (frequency 20 KHz) and heat treatment (60 °C) to kill bacteria is better (Jambrak et al., 2018, Pagán et al., 1999). However, the sound waves generated by ultrasound can lead to cell rupture and tissue damage (Ngnitcho et al., 2017). Due to the limited antibacterial ultrasound ability, larger equipment should be considered when applied to the food industry (Deng et al., 2019).
Blue light sterilization technology Light sterilization is a non-pharmacological technology, including photodynamic therapy (PDT) and ultraviolet radiation (UVC), and has been widely studied as an alternative to traditional antibiotics (Dai et al., 2009). The advantage of light sterilization is that the killing effect is the same regardless of the bacteria's antibiotic resistance. Blue light (Blu-ray) sterilization technology selects Blu-ray for light sterilization, and its wavelength is 405–470 nm. Blu-ray can sense by Gram-positive bacteria, Gram-negative bacteria, and fungi and induce Blu-ray receptors to cause physiological reactions (Wang et al., 2016). It has potential antibacterial or bactericidal ability without photosensitizers. Also, Blu-ray is less harmful to mammalian cells (Kleinpenning et al., 2010). The mechanism of Blu-ray sterilization is cell death induced by oxidative stress caused by reactive oxygen species (ROS) generated by the endogenous photosensitizers of bacteria after absorbing Blu-ray (Dietel et al., 2007, Lukšiene, 2005). Photosensitizers in the ground state are converted to their single or trilinear states upon irradiation with Blu-ray, in the presence of oxygen, undergoing two types of energy transfer: type I that produces toxic oxygen species, such as hydrogen peroxide (H2O2), superoxide, or hydroxyl radicals; (2) type II that generates 1O2 (Hadi et al., 2020, Hu, et al., 2018).
Blu-ray sterilization in the medical field is more research application. Hamblin (2005) et al. found that the killing rate of Helicobacter pylori was 99.9% when irradiated with 405 nm Blu-ray, and Fukui (2008) et al. showed that Blu-ray (405 nm) had a significant inhibitory effect on the growth of Porphyromonas gingivalis (more than 75% inhibition compared to unirradiated), while Blu-ray at 430 nm and longer wavelengths did not significantly inhibit their growth. In clinical studies, pulsed Blu-ray also showed significant improvement in the treatment of acne vulgaris (Ammad et al., 2008, Noborio et al., 2007). Because of the effectiveness of Blu-ray sterilization and the absence of any thermal effect, scholars noticed it in the food field. Dos Anjos (2020a) et al. showed that after Blu-ray irradiation (413 nm, < 2 h, 720 J/cm2), all Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Salmonella Typhimurium, and Mycobacterium fortuitum presented a 5 log inactivation in milk. Blu-ray irradiation (460 nm, 4 °C, irradiance 92 mW/cm2) reduced Salmonella in orange juice by 3.3 log CFU/ml, and the same irradiance reduced Salmonella by 3.6 log CFU/ml when the sterilization temperature was increased to 12 °C. At the same time, Salmonella achieved a reduction of 4.8 log CFU/ml when the temperature was increased to 20 °C (Ghate et al., 2016). Blu-ray sterilization is still effective for solid foods. It was found that Blu-ray irradiation (460 nm) can sterilize fresh-cut fruits without any food additives and Blu-ray irradiation (irradiance 92, 147.7, and 254.7 mW/cm2) when used at ambient temperatures of 7 °C and 16 °C, can inhibit Salmonella spp. inoculated on the surface of fresh-cut pineapples (Ghate et al., 2017). Josewin (2018) et al. used Blu-ray (460 nm) in combination with riboflavin (25, 50, and 100 µM) for sterilization and found that Blu-ray at a dose of 2.4 kJ/cm2 reduced Listeria monocytogenes on the surface of smoked salmon by 0.7–1.2 log CFU/cm2 at 4 °C and 12 °C. Thus, it can be seen that the Blu-ray non-thermal sterilization technology in the food field is a relatively wide range of applications. However, although Blu-ray is harmless to the operator's skin, visual impairment is a concern, and operational standards should be developed to ensure safety.
In the past few decades, ozone sterilization, high-pressure carbon dioxide (HPCD) sterilization, and acid electrolytic water (AEW) sterilization, which be
belong to chemical disinfection technologies, have attracted significant attention from researchers and the food industry. These technologies are characterized by high bactericidal efficacy, broad-spectrum, low toxicity, and fewer by-products. Details of non-thermal chemical disinfection technologies and their applications are shown in Table 2.
| Method | Key points | Advantages | Disadvantages | Application |
|---|---|---|---|---|
| Ozone sterilization technology | A large number of hydroxyl radicals and new ecological oxygen generated during the reaction indirectly oxidize inorganic or organic matter in the water and enter bacterial cells to oxidize intracellular organic matter (Han, et al., 2009) | Efficient, convenient, and economical, no residue will remain in the processed product (Ueda, et al., 2010) | The relative humidity of the environment should be <45%; ozone has almost no killing effect on the suspended matter in the air | Eliminates odor, mycotoxins, and pesticide residues; extends shelf life (Agriopoulou, et al., 2016, Brodowska, et al., 2018, Liu, et al., 2016) |
| High-pressure carbon dioxide sterilization technology | Lowers the intracellular pH value and inhibits enzymes; disorders the intracellular electrolyte balance and cell metabolism (Garcia-Gonzalez, et al., 2010) | Maintains the original beneficial ingredients of food without pollution (Clifford, et al., 2006) | Limited continuous operation and abnormal operating parameters (temperature, pressure, and processing time) cause changes in food color and structure (Ferrentino and Spilimbergo, 2011) | Oyster (Meujo, et al., 2010), cooked ham (Ferrentino, et al., 2013), fruits and vegetables (Ferrentino, et al., 2015, Spilimbergo, et al., 2013) |
| Acid electrolyzed water sterilization technology | Characterized by low pH, high oxidation-reduction potential, and effective chlorine, which can rapidly and extensively kill bacteria (Mourad and Hobro, 2020) | Easy to operate, low cost, safe, and eco-friendly | Affects some nutrients in fruits and vegetables (Chen, et al., 2018); strong AEW is efficient but it is corrosive and unstable (Zhang, et al., 2018) | Fresh aquatic products, fruits and vegetables (Chen, et al., 2018, Zhao, et al., 2019) |
Ozone sterilization technology Ozone is an allotrope of oxygen, an unstable colorless gas, and a by-product in photochemical oxidation reaction (Guzel-Seydim et al., 2004). Ozone quickly penetrates cells by destroying cell walls, oxidizing DNA and RNA in cell walls, and decomposing proteins, lipids, polysaccharides, and other macromolecular polymers. It destroys the metabolism, growth, and reproduction of viruses and bacteria (Currier et al., 2001) by penetrating cell membrane tissues. It invades the cell membrane. It acts on the outer membrane lipoproteins and internal lipopolysaccharides, causing permeability aberrations of cells, leading to cell lysis and death. The genetic materials, parasitic species, parasitic, viral particles, bacteriophages, mycoplasma, and bacterial virus metabolites and endotoxins are dissolved and destroyed (Li et al., 2012).
Ozone can rapidly kill most microorganisms. The excess ozone after sterilization eventually reduces to oxygen without causing residues and pollution. Young (2010) found that ozone water destroyed the inner membrane of Bacillus subtilis spore through oxidation, causing its death. Kang (2013) et al. showed that after 30 min of treatment with 0.4 mg/L ozone water, E. coli and L. monocytogenes decreased by 1.26 log CFU/mL and 2.84 log CFU/mL, respectively, and the presence of metal ions could significantly increase the effect inhibition bacteria of ozone water. Low-concentration ozone water is fatal to Aspergillus flavus (Beuchat et al., 2010), and ozone molecules are more bactericidal than hydroxyl radicals (Facile et al., 2000). Ozone water treatment, usually performed by spraying and soaking, is convenient in the operation of continuous processing. As a safe and effective antibacterial agent, it has been widely used in the food industry. In addition to sterilization and extending shelf life, ozone can also remove odor and pesticide residues (Brodowska et al., 2018). However, excessive exposure to ozone oxidizes some of the food ingredients, resulting in flavor deterioration and color changes (Brodowska et al., 2018). Also, overexposure to ozone (> 1 ppm) can damage the operator's respiratory tract, lungs, and eyeglasses (Oner and Demirci, 2016). A good and safe workplace can also prevent explosions of highly unstable ozone.
High-pressure carbon dioxide sterilization technology HPCD refers to a sterilization technology that uses the molecular effect of HPCD to achieve sterilization under the condition of pressure < 50 MPa. The molecular effects of HPCD are suggested to decrease the pH of food. Carbon dioxide (CO2) molecules and bicarbonate ions have an inhibitory effect on the activity of microbial cells and physical damage to cell membranes, changing cell membrane permeability and spore activity, thereby achieving the effect of sterilization (Lin et al., 2010, Ribeiro et al., 2020, Werner and Hotchkiss, 2006). HPCD is recognized as a new type of pasteurization technology that is both safe and environmentally friendly and can be used to sterilize food (Ferrentino and Spilimbergo, 2011).
Studies showed that HPCD induced E. coli damage, leading to its death (Garcia-Gonzalez et al., 2010, Liao et al., 2010, Liao et al., 2011). HPCD treatment of Salmonella for 10–50 min (35–45 °C, 80–150 bar) caused the total loss of its colony activity (Kim et al., 2007). Using HPCD (13.7 MPa) treatment at 35°C for 2 h effectively killed Salmonella in chicken (> 95%) and egg yolk (> 100%) and killed Listeria in shrimp (> 99%), orange juice (> 99%), and egg yolks (> 99.4%) (Kim et al., 2008, Wei, et al., 1991). Using HPCD (12 MPa, 40 °C, 15 min) to treat fresh-cut carrots could effectively inactivate the natural microbial flora and maintain the content of bioactive compounds, antioxidant capacity, and enzyme stability (Spilimbergo et al., 2013). Although HPCD technology can maintain the original quality of the food, application in solid food faces challenges such as limited continuous operation and packaging after processing. Also, HPCD acidifies the medium (Valverde et al., 2010), which may affect food. Besides, limited continuous operation and abnormal operating parameters (temperature, pressure, and processing time) cause changes in food color and structure (Ferrentino and Spilimbergo, 2011). Therefore, more studies are needed on HPCD sterilization.
Acid electrolyzed water sterilization technology AEW is a new type of functional water. It is produced by electrolyte-containing hydrochloric acid (HCl) or a mixture of HC1 and sodium chloride (NaC1) through an electrolytic water generator (Qiao et al., 2017). At present, the research on the sterilization mechanism of AEW mainly focuses on the modification of metabolic fluxes and adenosine triphosphate production, an increase of membrane penetrability, the release of intracellular components that lead to necrosis of bacteria (Hati et al., 2012, Jeong-Hyeon, et al., 2017, Liu, et al., 2018). In addition, in the process of electrolysis, O3, OH•, and H2O2 are produced, which improves the sterilization efficiency of AEW (Hati et al., 2012).
AEW preservation and sterilization have an excellent sterilization effect. After treating salmon with AEW (pH 2.6, free chlorine 90 ppm) for 64 min at 35°C, the total number of colonies of L. monocytogenes decreased by 1.12 log values, and the total number of E. coli decreased by 1.07 log values (Ozer and Demirci, 2006). The total number of colonies reduced by 2.16 log values after treating carp fillets with AEW (pH 2.6, free chlorine 90 ppm) (Ozer and Demirci, 2006). Treatment of tilapia with AEW (pH 2.47, free chlorine 120 ppm) at 25°C for 10 min decreased the total number of E. coli and V. parahaemolyticus colonies 0.76 and 2.61 log values, respectively (Huang et al., 2006). AEW can also prolong the shelf life of aquatic products and maintain good sensory properties while sterilizing. Kim et al. (2006) found that slightly acidic electrolyzed water ice (SAEW-ice) at 4 °C effectively inhibited the growth and reproduction of aerobic psychrotrophic bacteria in saury and body fat oxidation of the fish and can maintain its good sensory quality. Meanwhile, SAEW-ice can extend the shelf life by 4–5 d compared with conventional ice water. AEW can inhibit the growth of bacteria in shrimp and significantly delay the color change and the increase of volatile basic nitrogen (p < 0.05) (Lin et al., 2013). This technology has broad-spectrum efficiency, safety, and environmental protection, the electrolyzed water generator structure is relatively simple, and the production cost is relatively low (Ma, et al., 2019). However, for fruit and vegetable ingredients (Chen et al., 2018), AEW can harm their nutritional content. In addition to this, higher concentrations of AEW can be corrosive and unstable, and the generation and release of chlorine gas during AEW production limit its application (Rahman et al., 2016, Zhang et al., 2018).
Natural sterilization technology is the use of natural biological antibacterial agents for antibacterial and fresh-keeping technology for food. Natural antibacterial agents come from nature and are obtained by extracting and purifying active antibacterial substances from animals, plants, and microorganisms (Gyawali and Ibrahim, 2014). Based on their source, natural antibacterial agents can be divided into three categories: plant source, animal source, and microbial source. Details of non-thermal natural antibacterial technologies and their applications are shown in Table 3.
| Natural antimicrobial | Major source | Target microorganism | Application | Reference |
|---|---|---|---|---|
| Plant source | ||||
| Hydrosols of thyme and rosemary | Spice of thyme and rosemary | E. coli O157:H7 Salmonella Typhimurium |
Fresh cut carrots and apples | Tornuk et al. (2011) |
| Capsicum annuum extract | Cayenne pepper and black pepper | Psychrotrophic bacteria | Fresh pork sausages | Martínez et al. (2006) |
| Polyphenolic compounds | Pomegranate and apple peels | S. aureus, Pseudomonas fluorescens |
Agourram et al. (2013) | |
| Phenolics and tannins | Coconut husk | L. monocytogenes, S. aureus, E. coli, Vibrio cholerae, Salmonella, and Typhimurium | Wonghirundecha and Sumpavapol (2012) | |
| Animal source | ||||
| Lactoferrin | Milk | Vibrio cholerae and S. aureus | Infant formula | Lonnerdal (2011) |
| Chitosan | Exoskeletons of crustaceans and arthropods | E. coli and S. aureus | Tiwari et al. (2009) | |
| Lysozyme | Eggs | Clostridium tyrobutyricum | Juneja et al. (2012) | |
| Microbial source | ||||
| Nisin | Mesophiles, Enterobacteriaceae, lactobacilli, and yeasts | Fresh-cut fennel | Martinez-Hernandez et al. (2017) | |
| Lactocin and bacteriocins | Lactobacillus curvatus CRL705 and Lactococcus lactis CRL1109 | E. coli species | Belfiore et al. (2007) | |
| List-shield (Phage) | L. monocytogenes | Beef | Ishaq et al. (2020) |
Plant source natural antibacterial agents The antibacterial effect of plant-derived natural antibacterial agents mainly uses biologically active compounds, including phenols, terpenes, fatty alcohols, aldehydes, acids, and isoflavones (Lai and Roy, 2004, Tiwari et al., 2009). The antibacterial efficacy of natural plant-derivedantibacterial agents usually depends on the compound's chemical structure and concentration. Dussault (2014) et al. studied the effect of essential oils on the growth rate of L. monocytogenes in ham and found that adding oregano and cinnamon at a concentration of 500 ppm reduced the growth rate of L. monocytogenes by 19% and 10%, respectively Li et al. (Li et al., 2009) found that tea polyphenols have suitable antibacterial and antioxidant activities, which can effectively inhibit the growth of microorganisms and delay fat oxidation, improve the gel properties of fish balls extend the shelf life. Fan (2008) et al. used a 0.2% tea polyphenol solution to soak that the carp's fresh-keeping period reached 35 d.
Animal source natural antibacterial agents Animal-derived natural antibacterial agents mainly include lysozymes, antibacterial peptides, and polymer carbohydrates. Lysozyme is an enzyme naturally present in poultry eggs and mammalian milk. It destroys the beta 1,4-glucosidic linkages in the bacterial cell wall peptidoglycan (Aziz and Karboune, 2018). It is usually used as a preservative for meat products, fish products, and dairy products (Cegielska-Radziejewska et al., 2009) to extend the shelf life of food (Tiwari et al., 2009).
The high stability of antibacterial film in a wide range of temperature and pH also confirms the possibility of making an antibacterial edible film (Bayarri et al., 2014). Antibacterial peptides are widely found in nature, especially lactoferrin (LF). LF is an iron-binding glycoprotein found in milk and has antimicrobial activity against viruses and bacteria (Cacciatore et al., 2020). Studies have shown that LF could kill Pseudomonas in fresh-cut fennel (Martinez-Hernandez et al., 2017) and mozzarella cheese (Caputo et al., 2015) and is approved as an antibacterial agent in meat products (Rybarczyk et al., 2017).
LF is permitted to be used in beef at a level of 65.2 mg/kg in the United States (FDA, 21 CFR.170.36(f)). In EU regulations, LF is considered a milk protein and can be used in food (EEC, 79/112). Also, in Asian countries, Japan and South Korea both list it as a natural food additive (Naidu, 2002). Chitosan is a biodegradable polysaccharide, naturally present in the exoskeletons of crustaceans and arthropods (Aziz and Karboune, 2018). Previous reports have shown that chitosan is effective against gram-positive and gram-negative bacteria, fungi (Abd El-Hack et al., 2020). After oysters were treated with chitosan at a concentration of 5.0 g/L and stored at 5 ± 1 °C, the shelf life was extended from 8–9 d to 14–15 d (Soni et al., 2018).
In addition, chitosan film can inhibit the growth of E. coli, Salmonella, and L. monocytogenes. However, the antibacterial mechanism of chitosan and its derivatives has not yet been fully elucidated.
Microbial source natural antibacterial agents Microbial natural antibacterial agents mainly use the antibacterial properties of microorganisms to inhibit other microorganisms. For example, nisin produced by the fermentation of lactic acid bacteria in a modified milk medium has been approved as a food additive in over 50 countries/regions as an antibacterial preservative in food (Ahmad et al., 2017, Tiwari et al., 2009). Previous reports have demonstrated that nisin is effective against L. monocytogenes and S. aureus in food (Campos et al., 2011). Besides, bacteriophages have been used for the biological control of foodborne pathogens to ensure food safety. Bacteriophages can invade bacterial cells, destroy bacteria's metabolism, and cause the death of bacteria; however, phages are harmless to mammalian cells (Bren, 2007). Bacteriophages have a wide range of applications. They can be used in meat, fresh fruits and vegetables, ready-to-eat food, infant formula, and pasteurized milk (Endersen et al., 2014).
Natural sterilization technology has unique advantages of being safe and non-toxic, preventing the production of drug-resistant bacteria caused by the abuse of antibiotics. Natural substance sterilization technology has increased consumer acceptance. However, the development cost of natural antibacterial agents and the negative impact on food quality have restricted the popularization and application of this technology. It is also necessary to further expand the exploration of natural sterilization technology, and the law should follow up the scope and dosage of natural sterilization technology in time.
Compared with conventional heat sterilization, non-heat sterilization shows significant advantages and is characterized by ensuring high-quality and safe food. Different non-thermal disinfection technologies have their sterilization application scenarios and can obtain better sterilization and preservation effects through synergy. Perhaps non-thermal disinfection technologies can replace or supplement traditional thermal sterilization technologies. Further exploration and research should be conducted on non-thermal disinfection technologies with simple operation, wide application range, and low investment. Besides, non-thermal disinfection technologies that are eco-friendly will have more excellent commercial value in the future.
Author contributions Conceptualization, ZhenKun Cui; software, Han Yan, and Hao Zhang; writing—original draft preparation, ZhenKun Cui and Han Yan; writing—review and editing, Hao Zhang and Tatiana Manoli; funding acquisition, ZhenKun Cui and Hao Zhang; supervision, Zhenkun Cui and Tatiana Manoli. All authors have read and agree to the published version of the manuscript.
Acknowledgements This research was supported by Henan Province Key R&D and Promotion Projects (212102110022 and 212102110017) and Henan Institute of Science and Technology Young Backbone Teacher Program (2018).
Conflict of interest The authors declare that they do not have any conflict of interest.
