Title of Paper: EFFECTS OF SODIUM HYPOCHLORITE ON THE PHYSICAL QUALITY AND MICROFLORA OF FRESH-CUT MANGO

Transcription

1 Title of Paper: EFFECTS OF SODIUM HYPOCHLORITE ON THE PHYSICAL QUALITY AND MICROFLORA OF FRESH-CUT MANGO CV. NAMDOKMAI Authors: Nanyarat Ratanasatien and Sirichai Kanlayanarat Address for Correspondence: Dr. Sirichai Kanlayanarat Division of Postharvest Technology School of Bioresource and Technology King Mongkut s University of Technology Thonburi Thungru, Bangkok 10140, Thailand

2 EFFECTS OF SODIUM HYPOCHLORITE ON THE PHYSICAL QUALITY AND MICROFLORA OF FRESH-CUT MANGO CV. NAMDOKMAI ABSTRACT Ripe mango fruit slices cv. Namdokmai were treated with 50, 100, 150 or 200 ppm sodium hypochlorite (NaOCl) for 2 min before storage for 3 days at 20 C. Slices dipped in water served as control. NaOCl did not significantly influence firmness loss. Firmness increase to levels higher than that of the prestorage level (day 0) was even noted in slices treated with ppm NaOCl after 3 days of storage. Surface color degradation was indicated by decreases in lightness (L*) and b* values with storage ppm NaOCl resulted to increased L* and b* higher than that of the control. At 200 ppm NaOCl, L* and b* decreased more rapidly than that of the control. However, it was most effective against microbial growth shown as lowest yeast and mold counts (YMC), total bacterial count (TBC) and E. coli count (ECC) among treatments ppm NaOCl also reduced TBC but not YMC. Only 150 ppm NaOCl reduced ECC similar to the effect of 200 ppm NaOCl. ECC reduction by NaOCl was noted only after 1 day of storage while at the later part of the storage period, NaOCl was ineffective and even resulted to higher counts. 50 ppm NaOCl did not control microbial growth. Key words: Mango; Mangifera indica L.; minimaly processing; fruit slices; chlorine; quality; microbiology; postharvest life

3 INTRODUCTION Mango (Mangifera indica L.) fruit slices are a common fresh-cut product in Thailand s rapidly growing horticultural fresh-cut produce industry. This is brought about by the increasing consumer demand and consumption of ready-to-eat, convenience food. Freshcut or minimal processing operations usually include washing, peeling, slicing, and packing in film packages or containers with film overwrap before distribution or retail display in ambient or refrigerated shelves. Because of the wounded nature of the fresh-cut produce which is also devoid of the protective skin, shelf life particularly of ripe fruit slices is very short, only one day at ambient and 2-3 days at refrigerated temperature. Shelf life is limited by rapid firmness loss, surface discoloration, overripening and microbial contamination or decay. Consequently, postharvest losses are serious and product prices are incoherently much higher than that of the intact fruit equivalent. To address this problem and meet the consumers expectations of product quality and safety, appropriate treatments maintaining freshness and wholesomeness of the fresh-cut produce over reasonable period of time during marketing and distribution need to be formulated. Chlorine wash is an important sanitation step in fresh-cut produce processing because of concerns about the detrimental effects of microbial contaminants on product quality and shelf life and the microbiological safety of the produce. Chlorine is important to ensure clean wash water and prevent the wash water itself from becoming a contaminating step in the processing operation. If the wash water is contaminated by spoilage and human pathogens present in the product or introduced on product contaminated in the field, the chlorine additive of the wash water could ensure that the pathogens are killed and cannot contaminate the product. On the other hand, chlorine wash could eliminate or reduce the pathogen load in the product. It has been calculated based from plate count studies that initial microbial population in fresh-cut produce, at least for mesophilic bacteria, ranged from CFU/g

4 (Nguyen-the and Carlin, 1994). Chlorine wash has been found to reduce the initial population of bacteria in lettuce (Beuchat and Brackett, 1990a), the numbers of epiphytic bacteria and Listeria monocytogenes present in fresh-cut chicory endive (Bennik et al., 1996), and the initial population of mesophilic aerobic microorganisms in shredded carrots by at least 90% (Beuchat and Brackett, 1990b). Chlorination has no residual effect and reinfection would become a problem if good sanitation, careful handling and proper temperature management are not employed in subsequent processing steps (Zagory, 1999). It has been obtained that chlorine treatment did not influence the bacterial population on lettuce after 4 days at 5 o C (Beuchat and Brackett, 1990a), the subsequent growth patterns of mesophilic aerobes on carrots during storage at 5 o C (Beuchat and Brackett, 1990b), and the growth patterns of mesophilic aerobic microorganisms, psychrotrophic microogranisms, yeasts and molds on chopped tomatoes (Beuchat and Brackett, 1991). L. monocytogenes also grew better on disinfected produce than on non-disinfected or water-rinsed produce due probably to the fact that disinfection diminishes the competing epiphytic microflora (Bennik et al., 1996), including probably Pseudomonas floursencens which has been shown to be an inhibitor of L. monocytogenes due to production of iron-chelating siderophores (Babic et al., 1996). Sodium hypochlorite (NaOCl) could be used to prepare the chlorine solution generally at a concentration of pmm (Hong and Gross, 1998). A highly available and cheap source of NaOCl is the commercial bleach which contains 5.25% NaOCl. However, there are concerns on its use. NaOCl at % was found to increase firmness loss and chill-induced watersoaking in fresh-cut tomato slices stored at 5 o C (Hong and Gross, 1998). In addition, hypochlorite being a halides could increase oxidation of ascorbic acid (Bielski, 1982). It was then found that washing in 100 ppm NaOCl induced significant

5 oxidation of reduced ascorbic acid in intact and sliced strawberries although the total ascorbic acid content was not affected (Wright and Kader, 1997). This study examined the effects of different rates of NaOCl application on the microbial population, quality changes, and shelf life of ripe fruit slices of Namdokmai mango, Thailand s leading and most popular variety. MATERIALS AND METHODS Experimental Treatments Ripe fruits of Namdokmai mango were procured from a local market, sorted for uniformity in size and shape and freedom from defects, washed in tap water and rinsed. The fruits were peeled using a very sharp stainless steel knife. The pulp or flesh from both sides of the fruit was sliced and the pulp from each side was further divided into 6 equal slices. The slices were then dipped for 2 minutes in water (control) or 50, 100, 150 or 200 ppm NaOCl prepared using the commercial bleach (5.25% NaOCl). After rinsing and air-drying, the slices were placed in foam trays and stored at 20 o C with 90-95% relative humidity. The study was done following completely randomized design experiments using four replications with 6 slices per replicate per observation period. Observation of responses was done at daily interval. Measurement of Responses Color change. Changes in surface color of the slices were determined using a Minolta DP-301 colorimeter taking the b* value as a measure of degree of yellowing and lightness (L*) value. Firmness. Firmness was measured using TA-TX2 texture analyzer equipped with a 500 kg load cell and 2mm-diameter plunger set to pierce 5 mm deep from the fruit surface. Cross head and chart speed were 100 mm/min and 300 mm/min, respectively.

6 Microbiological assay. Total plates counts for bacteria, yeasts and molds, and pathogenic organisms (Eschirichia coli, Salmonella sp.) were determined following standard procedures. Results were analyzed by conducting analysis of variance using the general linear models procedure by SAS (SAS Institute, Cary, N.C.) for completely randomized design experiments and Duncan s multiple range test (DMRT) or LSD analysis for mean comparison. RESULTS Surface Color Changes Figure 1 illustrates the changes in surface color of NaOCl-treated and untreated fruit slices with storage at 20C. In the untreated control, L* values decreased after 2-3 days of storage (Figure 1A). NaOCl at 200 ppm resulted to a more rapid decrease in L* than that of the control as early as after one day of storage and was sustained up to the third and last day of storage. At ppm NaOCl, slices seemed to maintain higher L* than the control. This was clearly manifested after 2 days of storage. At much lower NaOCl concentration of 50 ppm, erratic changes in L* were noted. On the other hand, the b* value, which indicates degree of yellowing, decreased in the untreated slices (Figure 1B). Again, 200 ppm NaOCl induced a more rapid decrease in b* similar to its effect on L*. In slices treated with ppm NaOCl, b* increased with storage and the values were consistently higher than that of the control. Firmness Firmness slightly decreased with storage (Figure 2). There was no clear effect of NaOCl concentration on firmness changes particularly during the first 2 days of storage. After 3 days of storage, all NaOCl-treated slices, except those treated with 100 ppm NaOCl,

7 had higher firmness than the untreated slices. Slices treated with ppm NaOCl had the highest firmness among treatments at this period. Firmness values obtained were even higher than that recorded at the start of storage, indicating tissue firming or hardening. Yeast and Mold Population Yeast and mold count (YMC) increased from about 4 log CFU/g at the start of storage (day 0) to about 4.8 log CFU/g after 2-3 days of storage in the untreated slices (Figure 3A). 200 ppm NaOCl reduced YMC. This was distinctly noted after 1-2 days of storage. The other NaOCl treatments had no significant influence; they even resulted to relatively higher YMC values than that of the untreated slices, recorded at about 5 log CFU/g or higher after 1-2 days of storage. Total Bacteria Total bacteria count (TBC) was initially about 4 log CFU/g (Figure 3B). It increased with storage and the rate of increase was most rapid in untreated slices and those treated with 50 ppm NaOCl. After 2-3 days of storage, TBC in these treatments was about 4.8 log CFU/g. NaOCl at ppm reduced TBC and the degree of reduction increased with increasing NaOCl concentration. However, differences among these treatments were small, with TBC values ranging only from log CFU/g. Pathogenic Organisms No microorganisms as potential food safety or human hazard were detected except for the E. coli bacterium (Figure 3C). E. coli count (ECC) in the untreated slices slightly increased from 4.4 log CFU/g at day 0 to about 4.6 log CFU/g after 3 days of storage. NaOCl at higher concentrations of ppm reduced ECC after 1 day of storage. Thereafter, NaOCl was ineffective and at ppm, even resulted to higher ECC values of about log CFU/g compared to that of untreated slices.

8 CONCLUSION NaOCl has no distinct beneficial effect on the physical quality of ripe mango slices stored at 20C. It seems to induce tissue firming or hardening at the later part of the storage period, particularly when ppm NaOCl was used. 200 ppm NaOCl also induced more rapid loss of desirable surface color. However, it was only the NaOCl concentration that consistently resulted to reduced fungal and bacterial population. Lower concentrations of ppm NaOCl may be effective against bacterial growth but not against yeasts and molds. 50 ppm NaOCl was not effective in controlling microbial growth. LITERATURE CITED 1. Association of Official Analytical Chemists (AOAC) Official Methods of Analysis. VA, USA pp. 2. Babic, I., Roy, S., Watada, A.E. and Wergin, W.P Changes in microbial populations on fresh-cut spinach. Intl. J. Food Microbiol. 31: Bennik, M.H.J., Peppelenbos, H.W., Nguyen-the, C., Carlin, F., Smid, E.J. and Gorris, L.G.M Microbiology of minimally processed, modified atmosphere packaged chicory endive. Postharvest Biol. Technol. 9: Beuchat, L.R. and Brackett, R.E. 1990a. Survival and growth of Listeria monocytogenes on lettuce as influenced by shredding, chlorine treatment, modified atmosphere packaging and temperature. J. Food Sci Beuchat, L.R. and Brackett, R.E. 1990b. Inhibitory effect of raw carrots on Listeria monocytogenes. Appl. Env. Microbiol. 56: Beuchat, L.R. and Brackett, R.E Behavior of Listeria monocytogenes inoculated into raw tomatoes and processed tomato products. Appl. Env. Microbiol. 57:

9 7. Bielski, B.H.J Chemistry of ascorbic acid radicals. In: Seib, P.A. and Tolbert, B.M., eds., Ascorbic Acid: Chemistry, Metabolism and Uses. Advances in Chemistry Series, Vol Amer. Chem. Soc., Washington, DC. Pp Hong, J.H. and Gross, K.C Surface sterilization of whole tomato fruit with sodium hypochlorite influences subsequent postharvest behavior of fresh-cut slices. Postharvest Biol. Technol. 13: Nguyen-the, C. and Carlin, F The microbiology of minimally processed fresh fruits and vegetables. Crit. Rev. Food Sci. Nutr. 34: Wright, K.P. and Kader, A.A Effect of slicing and controlled atmosphere storage on the ascorbate content and quality of strowberries and perimmons. Postharvest Biol. Technol. 10: Zagory, D Effects of post-processing handling and packaging on microbial populations. Postharvest Biol. Technol. 15:

10 L value A control 50 ppm 100 ppm 150 ppm 200 ppm Day of Storage b value B control 50 ppm 100 ppm 150 ppm 200 ppm Day of Storage Fig.1. Lighteness(L*)(A) and b*(b) of " Namdokmai " mango slice treated with ppm NaOCl for 2 min before storage at 20 O C.

11 Firmness (N) Day of Storage control 50 ppm 100 ppm 150 ppm 200 ppm Fig. 2 Firmness of Namdokmai mango slices treated with ppm NaOCl 2 min before stored at 20 o C

12 log cfu/g Day of Storage A control 50 ppm 100 ppm 150 ppm 200 ppm log cfu/g Day of Storage B control 50 ppm 100 ppm 150 ppm 200 ppm log cfu/g C control 50 ppm 100 ppm 150 ppm 200 ppm Day of Storage Fig. 3 Yeast and Mold (A), total bacteria(b) and E.coli(C) counts of Namdokmai mango slices treated with ppm NaOCl 2 min and stored at 20 o C.