HIGH-POWER ULTRASOUND

Ultrasound in combination with other physical and natural antimicrobial hurdles was used for inactivation of microorganisms. A further aim was to prevent any damage to cellular tissues, prevent the formation of toxic side products, and provide an environmentally friendly technic (Kentish and Ashokkumar, 2011). The application of pressure waves with frequencies ranging from 20 to 100 kHz is referred to as “high-power ultrasound” (HPU). The microbial inactivation is caused by intracellular acoustic cavitation (Bilek and Turantas, 2013).

The application of ultrasound at 120 W, 35 kHz, 15°C led to E. coli 0157Н7 inactivation levels close to 4 logI0 cycles in raw strawberry that were immersed in water (Alexandre et al., 2012). High power ultrasound at 45 kHz, 10 min, 25°C was effective in reducing initial loads of Salmonella typhimurium by 0.8 log10 cycles on cherry tomato (Sao Jose and Vanetti, 2012). Several examples found in the literature report inactivation levels between 2 and 5 log10 cycles for E. coli 0157H7 and Salmonella spp. in other fruits, such as apples or plum fruit by means of HPU treatments of 100 W, 40 kHz, 20°C, 10 min (Chen and Zhu, 2011; Huang et al., 2006)

Although this technology seems to be effective in combination with other chemical treatments, some problems are associated with the application of HPU, mainly regarding the accomplishment of sterilization criteria. Moreover, the application of ultrasonic treatment in water leads to a release of microorganisms into the washing medium, creating the risk of cross-contamination. Therefore, combined methods should complement the application of HPU to guarantee an appropriate level of consumer protection (Carrillo- Lopez et al., 2017).

PULSED LIGHT

Currently, irradiation, cold plasma, and pulsed light (PL) are some of the most effective treatments specifically aimed at reducing the microbial load on fruits and vegetables, particularly those that are specially exposed to the environment and microbiological contaminants in direct contact with the soil and water, and consequently contaminated at the surface.

The application of PL technology in food consists of light spectrum values ranging from ultraviolet (UV, 100-380 nm) to near-infrared (NIR, 1100 nm), with a considerable amount of light in the short-wave UV domain. Electromagnetic energy is accumulated in a capacitor during fractions of a second to produce light pulses within a short time (ns-ms). First of all, no residue remains on the food products after treatment, and lethal microbial doses are easily achieved, avoiding complex equipment or safety measures (such as those required in irradiation treatments).

Laboratoiy studies have demonstrated that PL disinfection technology is efficient and effective against Gram-positive and Gram-negative bacteria, yeast, molds, and viruses (Virnont et al., 2015). Nevertheless, the efficacy on real foods is still under investigation. In addition, the effects of PL treatments on food properties beyond safety and spoilage have to be further clarified.

This technology has been proposed as a non-thermal pasteurization process for food preservation and for decontaminating packaging materials. Successful inactivation effects can be found in the literature regarding the application of this technology to fruits and derivative products (Huang et al., 2015; Montgomery and Banerjee, 2014; Paskeviciute and Luksiene, 2011)

Valdivia-Najar et al. (2017) assessed the effectiveness of 4, 6, and 8 J cm"2 PL treatments to determine the microbial reduction after processing, and the evolution of microbial counts during 20 days of storage at 5°C in untreated and treated products. Treated fresh-cut tomatoes resulted in values of remaining bacterial contamination in the range of 0.7-1.8 log10 cycles, whereas the levels in raw sliced tomatoes were close to 8 log10 cycles. Molds and yeasts were also reduced by nearly 8 log10 cycles after PL processing. Microbial levels after the complete storage period were 2 log10 cycles lower than the microbial levels observed in untreated stored products.

Combined treatment employing photosensitization and PL has been studied to reduce the levels of artificially inoculated B. ceretis, L. monocytogenes, and S. Typhimurium in fresh fruits and vegetables, achieving more than 6 log10 cycles of B. cereus inactivation (Paskeviciute and Luksiene, 2011). The efficiency of PL is much greater and achieved in a much shorter tune than with continuous treatments, and it is even effective against spores in food processed by pulsed UV light achieving up to 4.05 log10 reductions of B. coagulans, B. cereus, and Alicyclobacillus acidocaldarius, among others, with the application of 23.72 J/mL UV dose (Gayan et al., 2013).

Specifically, postharvest application of UVC technology in fruit and vegetable has been associated with technological and nutritional benefits. Delay in the ripening of fruits and vegetables, and the enhancement of the potential of these raw foods to resist disease by means of the accumulation of phytoalexins are some of the most advantageous effects of UVC processing from a techno logical point of view (Darvishi et al., 2012). The use of UVC (X = 200-280 nm) radiation has a germicidal effect when applied directly to food surfaces, damaging the DNA of microbial contaminants.

Many fruits, including apples, strawberries, pears, tomatoes, and others, have been processed by UVC technology (Charles et al., 2008; Schenk et al., 2008; Taze et al., 2015).

Surface treatments with UV have generally been applied at X = 254 nm, 0.4-40 kJ/nr, in apple, blueberry, cantaloupe melon, grapefruit, mandarin, persimmon, and mango, among other fruits, achieving microbial reduction levels of 1-6 logI0 cycles in bacteria, yeast, and molds, including Penicil- lium expansion, E. coli 0157:H7, Pseudomonas spp., Listeria spp., Botrytis cinerea, and Penicillium digitation (Perkins-Veazie et al., 2008; Khademi et al., 2013;Turtoi et al., 2013). Several examples of microbial inactivation by this technology can be found in the literature, with reports that it is highly effective against Salmonella spp. and E. coli 0157:H7 pathogens. Apples and tomatoes were treated with doses between 1.5 and 24 mW/cm2. The maximum inactivation levels achieved were close to 2 log10 cycles for tomatoes inoculated with Salmonella spp., and 3.3 log10 cycles for apples inoculated with E. coli 0157:H7 (Yaun et al., 2004). Santo et al. (2018) also studied the effect of UVC radiation for the inhibition of£. coli and C. saka- zakii on minimally processed “Tommy Atkins” mangoes. UVC treatments of 2.5, 5, 7.5, and 10 kJ/m2were effective (2.2-2.6 log10 cycles reduction) against the pathogens studied, maintaining microbial levels below detection limits at 4°C, 8°C, and 12°C during the 10-day storage period.

Yoo et al. (2015) studied the effectiveness of titanium dioxide-UVC photocatalysis (TUVP) as a nonthermal technology for decontaminating raw orange surfaces. E. coli 0157:H7 spot-inoculated at levels of 7.0 log10 cycles on oranges (12 cm2) was reduced by 4.3 log10 cycles after application of 17.2 mW/cm2 treatments. This treatment was more effective than conventional chemical decontamination processes also studied by Yoo et al. (2015). Surface treatment of oranges with water, chlorine (200 ppm), and UVC alone (23.7 mW/cm2) achieved levels of E. coli 0157:H7 inactivation corresponding to 1.5, 3.9, and 3.6 log10 cycles, respectively. Furthermore, Yoo et al. (2015) also subsequently studied treatment of orange juice with

HHP (400 MPa, 1 min) and observed a synergistic effect between the two technologies applied consecutively (TUVP+HHP) in the final production of pasteurized orange juice, reducing bacterial counts below the detection limit (>7 log10 cycles).

NATURAL ANTIMICROBIALS

To avoid the microbiological, enzymatic, chemical, or physical changes derived from quality losses in fresh-cut fruits and unpasteurized fruit- derived products, the addition of natural compounds of plant origin to foodstuffs at different stages of the food chain for antioxidant and antimicrobial purposes is being investigated. Owing to consumer demand for healthy, fresh-like, safe foods that contain as low amounts of chemical preservatives as possible, the use of natural antimicrobials has increased in food research lines, a development that also leads to additional nutritional or functional value in the final products (Raybaudi-Massilia et al., 2009).

The achievements in recent years regarding the potential of natural antimicrobials to be bacteriostatic (inhibiting bacterial growth) or bactericidal (reducing the bacterial load in a contaminated product) are promising for cocoa polyphenols, essential oils (EOs) fiom citrus peel, ginger, rosemary, garlic, and oregano (Ayala et al., 2009), microalgae compounds (Pina-Perez et al., 2017), anthocyanins from acai (Belda-Galbis et al., 2015), stevioside from Stevia rebandiana Bertoni (Sansano et al., 2017), peptides (Rai et al., 2016), among others.

The effectiveness of organic acids against spoilage and pathogenic micro-organisms in fresh-cut fruits and fruit juices has been demonstrated by direct immersion of apples, oranges, and pears, among other raw fruits, generally reducing mesophilic bacteria, psychrophilic bacteria, mold, and yeast populations. Specifically, E. coli 0157:H7 and S. Typhimurium have been inactivated by ascorbic acid, citric acid, and calcium lactate (Bjorns- dottir et al., 2006).

EOs from plants have been demonstrated to have active compounds that can control or inhibit the growth of pathogenic and spoilage micro-organisms in fresh-cut fruits and derived products. In this context, the concentration applied, the characteristics of the matrix, and the type of microorganisms under consideration are influential factors for achieving an effective antimicrobial result. Concentrations between 0.015% and 0.7% of EOs from plants (carvacrol. cimiamon, cinnamaldehyde, citral, cinnamic acid, citrus, clove, eugenol, garlic, geraniol, lemon, lime, lemongrass, mandarin, oregano, and palmarosa) are sufficiently effective to reduce the pathogens and spoilage microorganisms of most concern in fresh-cut fruits (apple, pear, melon, orange, strawberry, tomato, and watermelon), such as E. coli 0157:H7, S. Enteritidis and Listeria spp. (Mosqueda Melgar et ah, 2008; Ayala-Zavala et aJ., 2009; Tzortzakis, 2009). According to the studies of Ayala-Zavala et al. (2009), garlic oil revealed a high potential for the preservation of fresh- cut tomato salad, achieving a marked inhibition activity against S. aureus and S. Enteritidis, and three fungi, Aspergillus niger, Penicillium cyclo- pi urn, and Fusarium oxysporum.

Other natural plant antimicrobials have been used to formulate novel fruit- based beverages, being used as complementary control measures to avoid microbial proliferation or to increase bacterial reduction in HHP processing or PEF treatments applied to these fruit-derivative products. High levels of L. monocytogenes inactivation (>5 log 10 cycles) have been achieved by a combination of HHP and Stevia rebaudiana Bertoni supplementation (2.5%, w/v) in an orange-mango-papaya fruit pulp extract (15:20:65, w/w/w) after application of an optimized treatment of 453 MPa for 5 min, maximizing the inactivation of PPO and peroxidase activities (Barba et al., 2014). Nutritional improvements and enhancement of freshness parameters have been reported as a result of supplementation of various fruit-based pasteurized products with natural antimicrobials, together with an increase in the refrigerated shelf life of the food matrices studied (Belda-Galbis et al., 2015; Rivas et al., 2015; Sanz-Puig et al., 2016).

 
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