In Vivo Studies

Fruit responses to heat treatments depend on the condition of the fruit prior to treatment, the commodity concerned, the temperature and duration of treatment, as well as the mode of heat application. The physiological responses of different fruits, vegetables, or flower species to hot water treatments can vary by season and growing location, and can be due to differences in climate, soil type, season, production practices, fruit maturity at harvest, and fruit size [24,42-45]. Hot water treatments prevent rot development in numerous temperate, subtropical, and tropical fruits and vegetables [13,14,16,17,46,47].

A dip time of 1 to 2 minutes in 55°C water for Galia melons (Cucumis melo L.) was an optimal antifungal treatment, while a higher temperature or longer exposure time resulted in heat injury to the fruits [48]. However, the optimal HWRB treatment to reduce decay while maintaining fruit quality after prolonged storage and marketing was 59 ± 1°C for 15 seconds [15].

Hot water treatment at 44 or 46°C for 15 minutes delayed Botrytis cinerea proliferation on artificially inoculated or naturally infected strawberries

(Fragaria x ananassa Duch. Tudla) [49,50]. Dipping strawberries inoculated with botrytis at 63°C for 12 seconds, followed by controlled atmosphere (CA) storage (15kPa CO2) was found to reduce rot development during a short or long storage regime [51]. Peaches and nectarine infected with Monilinia fructicola were immersed in hot water at 46 or 50° C for 2.5 minutes to control decay. These treatments reduced the incidence of decayed fruit from 82.8 to 59.3 and 38.8%, respectively [52]. Mature green plums (Prunus salicina Lindl. cv. Friar) were treated in water at 40, 45, 50, and 55°C for 40, 35, 30, and 25 minutes, respectively, and stored at 0°C for 35 days plus 9 days of ripening at 20 to 25°C. Decay symptoms were retarded in fruits treated at 45 and 50°C, while decay symptoms were severe in fruits of the control and those treated at 55 and 40°C [53].

HWRB at 55°C for 15 seconds significantly reduced decay development in Penicillium expansum--inoculated apple fruit after 4 weeks at 20° C, or in naturally infected (P. expansum) apple fruit after prolonged storage of 4 months at 1°C plus 10 days at 20°C [27]. Recently, Lunardi et al. [54] reported that prestorage hot water immersion at 47° C for 3 minutes significantly reduced rot development caused by white rot (Botryosphaeria dothidea) on Fuji apples after prolonged CA storage. In vivo studies of inoculation of peach and nectarine fruit with Monilinia fructicola followed by HWRB at 55 or 60°C for 20 seconds gave 70 and 80% decay inhibition, respectively, compared with the control [41]. The inhibition percentages of M. fructicola with HWRB were similar, if HWRB was applied shortly after inoculation or 24 hours later. In contrast, the sensitivity of P. expansum spores inoculated into wounds increased when the fruit were treated with HWRB 24 hours after the inoculation, compared with treatment just after inoculation [41]. Treating fruit with HWRB at 60°C for 20 seconds and then dipping them into a cell suspension (108 cells/ ml) of Candida spp. 24 hours after inoculation with P. expansum reduced decay development by 60% compared with the controls, but did not reduce rot development caused by M. fructicola [41].

Many types of fresh produce from the Solanaceae benefit from hot water treatments. Potato tubers were dipped at 55°C for 5 minutes in a commercial continuous hot water treatment plant [55]. The frequency of eyes colonized by Pseudomonas pustulans, Helminthosporium solani, and Rhizoctonia solani was reduced to virtually zero and the effect persisted on tubers subsequently stored at 4 and at 15°C for up to 16 weeks. Results with Colletotrichum coccodes were inconclusive. Treatment suppressed Penicillium spp. which, however, rapidly colonized the eyes during storage, leading to higher contamination levels in the treated than in the untreated tubers. With tubers inoculated with Phoma foveata, good control was achieved when the incubation period before treatment was 10 days but not when the fungus was more established 42 days after inoculation [55]. Potato tubers inoculated with Erwinia carotovora and Fusarium solani were safely stored for 12 weeks at either 8 or 18°C without spoilage if dipped in a 57.5°C hot water bath for 20 to 30 minutes [39].

The effectiveness of hot water dipping on the control of grey mould, caused by Botrytis cinerea, and black mould, caused by Alternaria alternata, and on sweet red pepper quality was investigated. Dipping naturally infected or artificially inoculated fruit at 50° C for 3 minutes completely inhibited or significantly reduced decay development caused by B. cinerea and A. alternata, respectively [36]. Heat damage was observed on fruit dipped for 5 minutes at 50°C, or at 55°C for 1 minute or longer [36]. Significant differences in the incidence of decay were found between temperatures and in the time-temperature interaction, but not between times of dipping [36]. Treatment of peppers with hot water at 53° C for 4 minutes was found to be effective in reducing decay after 14 and 28 days of storage at 8°C. Treatment at 45°C for 15 minutes was less effective in maintaining pepper quality during storage [56,57]. HWRB of red and yellow sweet bell pepper cultivars at 55°C for about 12 seconds significantly reduced decay incidence while maintaining quality compared both to untreated control and to most other commercial treatments [25].

A hot water dip at 50° C for 2 minutes was more effective than 1 kGy of gamma radiation in reducing Botrytis cinerea and Rhizopus stolonifer decay in inoculated light-red tomatoes (Lycopersicon esculentum Mill. cv F-121) [58]. Mature green tomatoes (L. esculentum Mill. cv Sunbeam) were treated in water for 1 hour at 27, 39, 42, 45, or 48°C, and then ripened for 14 days at 20 or 2°C. Treatment at 42°C reduced decay by 60%, whereas other water temperatures were less effective [59]. HWRB of freshly harvested tomatoes at 52°C for 15 seconds, or dipping the fruit at 52°C for 1 minute, significantly reduced decay development and chilling injury after 3 weeks' storage at 2 or 12°C and additional 5 days at 20°C [33]. These two prestorage heat treatments did not affect other quality parameters such as fruit firmness, total soluble solids, or acidity [33].

Fresh broccoli floret (Brassica oleracea L. Italica group) is a highly perishable fresh vegetable when held at ambient temperatures; it becomes unmarketable within 1 to 3 days. Immersion in hot water at 50 or 52°C for

2 minutes was most effective in controlling decay development and reducing yellowing [75].

Several recent studies have shown that hot water immersion for 2 to

3 minutes at 53°C significantly reduced decay development in a wide variety of citrus cultivars [43,61-64]. Hot water dipping at 53°C for 3 minutes gave beneficial effects on decay control of Tarocco blood oranges (Citrus sinensis L. Obsek) fruit harvested in February and March but was detrimental for fruit harvested in April [43]. No decay symptoms were detected after 90 days' storage at 4°C in March grapefruit treated with 45° C water for 3 hours [65]. A hot water dip for 2 minutes at 52 to 53°C inhibited the development of decay in lemons inoculated with P. digitatum [63]. When the fruit were heat treated 1 day after inoculation, no decay occurred within 6 days after the treatment, whereas fruit dipped in water at 25°C reached 100% decay after 4 days. When the hot water dip was applied 2 days after inoculation, 90% of the treated fruit remained healthy [63]. The optimal HWRB treatment to reduce decay development while maintaining fruit quality of kumquat was 55°C for 20 seconds [28]. When organic citrus fruits were treated with HWRB

at 56, 59, and 62°C for 20 seocnds after artificial inoculation with P. digitatum, decay development in infected wounds was reduced to 20, 5, and less than 1%, respectively, of that in untreated control fruits or fruits treated with tap water [30]. A 20-second HWRB treatment at 59 or 62° C reduced decay in Star Ruby grapefruit that was artificially inoculated with P. digitatum, by 52 and 70%, respectively, compared with control unwashed fruit. Tap water wash (~20°C) or HWRB at 53 or 56°C were ineffective [31]. Green mold incidence caused by P. digitatum was reduced from 97.9 and 98% on untreated lemons and oranges, respectively, to 14.5 and 9.4% by a brief 30-second HWRB treatment at 62.8°C [34].

Hot water immersion also inhibits decay development on tropical and subtropical fruits. Dipping Kensington Pride mango fruits in hot water at 52° C for 5 minutes together with the fungicide benomy gave good control of stem end rot caused by Dothiorella dominicana and Lasiodiplodia theobromae [40]. Treatments consisting of hot water only or hot water followed by the fungicide prochloraz gave only partial control of stem-end rot. All treatments gave good control of anthracnose caused by Colletotrichum gloeosporioides [40]. The effectiveness of different postharvest treatments to control different levels of quiescent infections of Alternaria alternata causing alternaria rot in mango fruits during storage was compared. A combined HWRB treatment at 48 to 62° C (depending on the cultivar) for 15 to 20 seconds with 225 mg/ml prochloraz was the most effective treatment for control of alternaria rot in fruit with a high relative quiescent infected surface [66]. However, the effectiveness of postharvest HWRB and prochloraz applications are dependent on the quiescent infected area of the fruit by A. alternata at harvest [66]. Hot water treatments reduced body rot caused by Colletotrichum spp. in ripe avocado fruit with 40 and 41°C for 30 minutes [67]. However, treatment at 42°C for 30 minutes increased body rot compared to the other HWTs in one season, but there was no benefit of HWT times longer than 30 minutes [67]. Stem rot caused mainly by Dothiorella spp. was also reduced by HWT at 40 and 41°C [67]. HWRB at 55°C for 20 seconds significantly reduced chemical use (prochloraz) to control decay development caused mainly by Penicillium spp. in litchi fruit [29].

Food safety has become a very important issue for fresh and minimally processed products [68]. Minimal processing of vegetables provides convenience to the food industry and retail consumers, but may result in limited shelf life and marketing because of undesirable physiological and pathological changes [69,70]. Very little research has been done to evaluate the efficacy of hot water treatment on minimally processed products. A 4-minute water wash of green onion at 52.5° C reduced the aerobic plate count by 1 to 2 logs compared with water wash at 20°C [70]. A similar reduction in microbial population of soybean sprouts and watercress after a 30-second water dip at 60°C was reported by Park et al. [71].

Immersing spot-inoculated apple fruits at 80 and 95°C for 15 seconds produced a reduction of more than 5log in Escherichia coli O157:H7 [72]. Several pasteurization procedures for alfalfa (Medicago sativa) seeds were investigated to disinfect completely inoculated Escherichia coli (Migula) Castellani and Chalmers ATCC 25922 [73]. Hot water treatments (85°C for 9 seconds) were equally or more effective than 20,000 ppm calcium hypochlorite treatments, yielding a reduction of 2 log CFU/g [73]. Li et al. [74] reported that the population of Listeria monocytogenes on cut iceberg lettuce treated at 50° C for 90 seconds steadily increased throughout storage at 5° C for up to 18 days.

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