STABILITY OF MANGO CUBES PRESERVED BY HURDLE TECHNOLOGY

Fruit preservation by hurdle technology is based on combinations of low levels of various antimicrobial factors (hurdles) acting synergistically resulting in a shelf stable high moisture, fresh-like product. The objective of this work was to evaluate the stability of mango cubes obtained by combining the following hurdles: water activity reduction, pH reduction, and chemical preservation. Tommy Atkins mangoes, previously washed, sanitized and peeled, were cut into cubes of approximately 8 cm. The cubes were osmotically dehydrated under stirring (150 rpm) at 46C in a sucrose solution at 65.5Brix added with 2% citric acid and 0.2% potassium sorbate, during two hours. The product was drained, packed in low-density polyethylene bags and stored at room temperature (about 25C) during three months. The combination of hurdles on the final product (water activity, 0.96; pH, 3.99; potassium sorbate content, 396 mg.kg) was not effective to make it shelf stable, since the count of yeasts and molds increased. The cubes underwent pH reduction and color losses during storage. Furthermore, the acceptance of the product, as well as, the mango flavor intensity, decrease significantly with storage time.


INTRODUCTION
Conventional fruit preservation methods are usually based on a single preservation factor, applied at such a high level that causes noticeable sensory changes on the fruit.The hurdle technology, or combined methods technology (CMT), on the other hand, is based on combining low levels of two or more preservation factors (hurdles).No single factor is responsible for making the product stable, but rather the stability results from the synergism among the combined factors.The hurdle technology produces minimal sensory changes, which makes the products more acceptable than those obtained by conventional methods (AGUILERA & CHIRIFE, 1994).Since the water activity is only slightly reduced, the products obtained by hurdle technology are known as high moisture products.In the context of hurdle technology, the method generally used to reduce water activity of fruits is the osmotic dehydration, which consists in dipping the fruit in a hyperconcentrated solution.The resulting of osmotic pressure promotes water loss, reducing water activity of the fruit and favoring its stability.
The objective of this work was to evaluate the stability of high moisture mango (Mangifera indica L.) cubes produced by combining the following preservation factors: water activity reduction (by osmotic dehydration), pH reduction (by adding citric acid to the osmotic solution), and chemical preservation (potassium sorbate added to the osmotic solution).

MATERIAL AND METHODS
Tommy Atkins mangoes, purchased in the local market (Fortaleza, CE, Brazil), were washed, sanitized, peeled, cut into cubes of 8 cm 3 and osmotically dehydrated in a sucrose solution at 65 o Brix with added 2% citric acid and 0.2% potassium sorbate.Dehydration was accomplished in an open kettle at 46 o C under stirring (150 rpm) for 2 hours.The product was then drained in a sieve for 2 minutes.The dehydrated cubes were packaged in low-density polyethylene bags and stored at room temperature (about 25 o C) during three months.
At the beginning of the storage time, the product was characterized with regard to the potassium sorbate content (MAURO, 1992), water activity, pH, total titratable acidity, moisture and total soluble solid contents (AOAC, 1995).Additionally, sensory tests were carried out, named, a mango flavor intensity test, in a 7-point structured scale (1 -"very weak" to 7 -"very strong, fresh mango-like"), and acceptance tests, in 9-point structured hedonic scales (MEILGAARD et al., 1987).
Monthly during storage, the product was submitted to water activity, pH, and color (Hunter) analyses, as well as, the same sensory tests as those performed at the zero time of the storage.The data was submitted to regression analyses as a function of storage time, and the means were compared by Tukey's multiple range tests.Determination of mesophilic aerobes and counts of yeasts and molds (according to AMERICAN PUBLIC HEALTH ASSOCIATION, as described by DOWNES & ITO, 2001) were also carried out, to evaluate microbial stability of the product.
Figure 1 presents the frequency histograms from the sensory data, showing distribution of sensory responses to the tests applied to high moisture mango cubes at the beginning of the storage time.Most of the hedonic responses were within the acceptance ranges (higher than 5): 80% for appearance; 67% for flavor; and 87% for texture.The hedonic scores with the highest frequencies (modes) were the following: 8 for appearance (40%); 7 for flavor (23%); and 7 for texture (37%).According to 63% of the panelists, the mango flavor intensity of the processed product was near the middle of a scale in which the maximum score was labeled as "very strong, fresh mango-like".So, it is reasonable to suppos e that the product may have lost some flavor during processing.Actually, the solution flowing out of the fruit during osmotic dehydration consists of water and some natural solutes such as organic acids and some flavor compounds (Shi & Le Maguer, 2003).
Table 1 indicates that the acceptance of the product, as well as the mango flavor intensity, decreased significantly throughout the storage time.These changes may be partially attributed to microbial growth.In spite of the decreased acceptance, all the hedonic scores had remained in the lower half of the acceptance region (5-7 in the hedonic scale).
The results of the physico-chemical analyses of the product during storage are presented in Table 2.A significant pH reduction was probably due to microbial growth (Table 3).The product underwent orange color losses, since the color parameters a and b decreased with storage time.Actually, because of their high unsaturation degrees, carotenoids are highly susceptible to oxidation, which result in discoloration (RODRIGUEZ-AMAYA et al., 1984).
The counts of mesophilic aerobes were lower than 10 UFC.g -1 during 3 months, but the determination of yeasts and molds increased from less than 100 UFC.g -1 to values of about 10 4 -10 5 UFC.g -1 (Table 3).For this reason, the stability evaluation was interrupted at 3 months of storage, and the product was considered not stable.It may be attributed to a unsatisfactory combination of hurdles.Chanes et al. (1994) mentioned several stable fruit products obtained by similar combinations of hurdles, except by much higher sorbate concentrations.It is also important to know that the optimum functionality of the sorbate strongly depends on pH.At pH 4.0, about twice the sorbate concentration required at pH 3.0 is needed to achieve the same antimicrobial effect (PADILLA-ZAKOUR, 1998).So, future works might involve either high levels of the hurdles used in this work (higher sorbate concentrations and/or lower pH) or additional hurdles (e.g.edible coatings, storage under refrigeration), in order to obtain a product with higher microbiological (and possibly sensory) stability.

CONCLUSIONS
The high moisture mango cubes presented significant pH reduction and orange color loss during storage, while the water activity remained stable.
Although the product acceptance has remained within the acceptance ranges during three months of storage, the acceptance decreased significantly throughout time.The mango flavor intensity was considered remarkably lower than that of fresh mangoes since the beginning of the storage, and decreased further during storage.
The combination of hurdles (water activity = 0.96; pH = 3.97; potassium sorbate concentration = 396 mg.kg -1 ) was not effective to make the product microbiologically stable, since the counts of yeasts and molds increased with storage time.

FIGURE 1 -
FIGURE 1 -Distribution of sensory responses from high moisture mango cubes at the beginning of the storage time.

TABLE 1 -
F-test and Tukey´s test results for the sensory attributes of high moisture mango cubes as a function of storage time.
Bolded F values are significant (p ≤ 0.05).Values in the same raw followed by the same letter are not significantly different (p ≤ 0.05).

TABLE 2 -
F-test and Tukey´s test results for the physico-chemical attributes of high moisture mango cubes as a function of storage time.
Bolded F values are significant (p ≤ 0.05).Values in the same raw followed by the same letter are not significantly different (p ≤ 0.05).

TABLE 3 -
Results of the microbiological analyses of high moisture mango cubes.