Chitosan (Mw = 357 ± 39 kDa) with 96.2% N-deacetylation were obtained from VA & G Bioscience Inc. (Taoyuan, Taiwan). The non-micronized chitosan (NMC) was in powder form and prepared from crab shells. Before and after the micronization, the particle sizes of the micronization chitosan (MC) samples were estimated by the laser particle size analyzer (Analysette 22-Economy, Fritsch, Germany). To prepare one liter of 0.25%, 0.5% or 1% chitosan solutions, 2.5, 5.0 and 10.0 g of chitosan were dispersed in 900 ml of distilled water to which 50 ml of glacial acetic acid was added to dissolve the chitosan. The pH of the solution was adjusted to pH 5.0 with 0.1 M NaOH and the solution was made up to one liter. Acid solution without chitosan (pH 5.0) was used as control.

The papaya (Carica papara L., Tainung No. 2) used in the experiment was grown in Pingtung, Taiwan, and brought to the laboratory immediately after it was harvested. The fruit were selected for their uniformity, size, color, shape, and absence of damage and fungal infection. The 60 fruits were separated into groups of three, for treatment in triplicate. After washing, the fruit were peeled and sliced manually. The sliced papayas were then dipped into a chitosan coating solution for 1 min. After they had been air-dried for 30 min at 25˚C, the sliced fruit were placed into plastic trays, and over-wrapped with 30 × 20 cm PVDC film (Wu-Yu Chemistry Co., Japan). They were then stored at 4˚C to be later assessed.

A CIELAB colorimetry system was used to determine the color. Coloration was determined using a ColorPen^TM handy color difference photometer (Dr. Bruno Lange GmbH, Berlin, Germany), which recorded the spectrum of reflected light and converted it into a set of color coordinates (L, a^*, and b^* values). Color coordinates range from L = 0 (black) to L = 100 (white), –a^* (greenness) to +a^* (redness), and –b^* (blueness) to +b^* (yellowness). A Minolta standard white plate (X = 83.6, Y = 81.2, Z = 93.8) and a black plate were used to standardize the instruments.

Pulp (100 g) from 10 fruits was homogenized in a grinder and then centrifuged at 3500 rpm (Du-Pont, model Sorvall RC-5C) for 20 min to remove the pomace. The supernatant phase was collected to be analyzed for the amount of soluble solids (using a hand refractometer; ATAGO, model N1).

Three batches of 100 slices underwent each treatment. Ten slices were removed from each treatment daily. The slices were weighed regularly to determine weight loss, which was calculated cumulatively by comparing the weights of fruit immediately after slicing and treatment with chitosan after various storage times. The results were expressed as percentages.

A 10 g sample was obtained following homogenization in 90 ml 0.1% peptone water (Difico, 0118-17-0). Other decimal dilutions were prepared from a 10⁻¹ dilution. The total plate count was determined via the pour plate method, with Plate Count Agar (Difco, 0479-17) as the medium. The plates were incubated at 35˚C for 48 h. Three samples in each group were analyzed. All counts were presented as average values over three samples.

Three analyses of each sample were performed and each experiment was performed in triplicate (n = 3). The mean values and the standard deviation were calculated based on the data obtained. These data were then compared using the Duncan’s-multiple range method.

The L value of the sliced papayas decreased with storage time (Table 1). After four days of storage, the L values of the fruit treated with chitosan coating solution did not vary significantly. However, the chitosan-coated sliced papayas and uncoated sliced papayas were significantly different (p ≤ 0.05). The results indicate that chitosan coating had the ability to maintain the lightness of the sliced papayas. Surface color measurements showed that the a^* value of the sliced papayas increased with storage time (Table 1). The a^* value of uncoated sliced papayas

increased significantly from 50.66 ± 1.34 to 75.91 ± 0.67 during four days of storage. The increase in the redness was probably caused by an increase in the respiration rate and the promotion of enzymatic processes that were responsible for a drop in quality of the fruit, which involved browning and other reactions. The a^* value associated with the chitosan treatment was lower than that of the control. Also, the a^* values of the MC-coated sliced papayas were significantly lower than those of the NMCcoated. After four days of storage, the b^* value associated with the MC treatment (1%) was significantly lower than that of the control. The yellowness of the 1% MC-coated sliced papayas was not significantly changed after storage.

The total soluble solid contents did not vary significantly among the sliced papayas treated with 0.25%, 0.5% chitosan (non-micronized and micronized) and uncoated. The total soluble solid contents of all samples (coated and uncoated) were not significantly changed after two days of storage. However, the sliced papayas that had been treated with 1% MC had a lower total soluble solid content after four days of storage (Table 1). Minimal processed papaya is very perishable and has a short shelf life due to the increased tissue disruption, respiration, and acceleration of fruit ripening (Rolle & Chism, 1987; King & Bolin, 1989; Watada & Abe, 1990). The result indicates that the 1% MC treatment had the ability to delay the ripening of the sliced papayas during storage, which could prolong their shelf life.

Water loss or transpiration is another factor that affects the quality of fresh-cut papaya. A chitosan coating retarded the weight loss of the sliced papayas (Figure 1). Higher weight loss was observed in the control samples after two days of storage. After four days of storage, the weight losses of the control and 1% MC-coated sliced papayas were 5.82% and 3.86%, respectively. The weight loss was mainly due to transpiration and the leakage of juice from the pulp. Therefore, one of advantageous effect of chitosan coating on the loss of weight by papaya pulp was reducing transpiration and leakage of juice.

The total plate counts of uncoated, NMC-coated, and MC-coated sliced papayas were presented in Figure 2. The total plate counts of uncoated samples increased from 2.70 to 5.22 log CFU/g at the end of the storage. After four days of storage, the total plate counts of the control and 1% MC-coated sliced papayas were 5.22 and 3.45 log CFU/g, respectively. The chitosan coating on the sliced papaya effectively inhibited the growth of microorganisms.