Response surface methodology for optimizing osmotic dehydration of papaya slices

Introduction

Osmotic dehydration refers to the immersion of food material in a concentrated solution of sugars, salts, or their mixtures, so that two counter-current mass transfers occur simultaneously: water migrates out of the food matrix while solute diffuses in [1] . The technique reduces the moisture content of fruits by 30-50% under mild temperatures, preserving heat-sensitive vitamins and pigments far better than conventional hot-air drying [2] . For a highly perishable tropical fruit like papaya, which deteriorates within 5-7 days of harvest at ambient temperature, osmotic dehydration can extend shelf life substantially while retaining colour, flavour, and nutritional value [3] . Process efficiency depends on the interplay among sucrose concentration, temperature, immersion time, fruit-to-solution ratio, and agitation [4] . Varying these factors one at a time is both laborious and incapable of capturing interactions. Response surface methodology overcomes this by fitting empirical polynomial models to a structured set of experimental observations, and has been applied to osmotic dehydration of mango, pineapple, and guava [5,6] . For papaya, however, published optimisation data remain scarce, particularly for cultivars grown in western Mexico. This research used a CCD within the RSM framework to identify the osmotic conditions that maximise water loss while minimising excessive solid gain in papaya slices.

Material and Methods

Reagents and chemicals

Commercial-grade cane sugar (99.5% sucrose) was purchased from a local sugar mill in Colima. Analytical-grade citric acid, potassium meta-bisulphite, and sodium hydroxide (Merck) were used for solution preparation and titrations. Distilled water was used throughout.

Material

Ripe but firm papaya fruits (cv. Red Lady) were procured from a horticultural farm in Tecomán, Colima State, during February-July 2023. Fruits were washed, peeled, deseeded, and sliced into uniform cubes (2 cm × 2 cm × 1 cm). Sucrose solutions at five concentrations (30, 37.5, 45, 52.5, and 60 °Brix) were prepared and maintained at the target temperatures using a thermostatically controlled water bath (±0.5 °C). The fruit-to-solution ratio was fixed at 1:5 (w/v) and agitation at 80 rpm.

Methods

A three-factor, five-level CCD with six centre points was generated using Design-Expert v13, yielding 20 experimental runs. After immersion for prescribed duration, slices were removed, blotted, and weighed. Water Loss (WL), Solid Gain (SG), and Mass Loss (ML) were calculated gravimetrically following standard equations [7] .

Responses were fitted to second-order polynomial models, and adequacy was assessed via R², adjusted R², lack-of-fit test, and coefficient of variation. Numerical optimisation targeted maximum WL, minimum SG, and maximum ML simultaneously, with desirability function weighting of 5, 3, and 4, respectively [8] . Each run was performed in triplicate.

The entire research was conducted in the pilot-plant facility of the Sierra Madre Institute. Water loss ranged from 14.8% to 42.1% across the 20 runs.

Results

Higher sucrose concentrations and temperatures drove greater WL, but the interaction between concentration and time was also significant (p=0.008). Solid gain varied from 3.6% to 14.3% and was most sensitive to concentration.

Discussion

The main findings are that sucrose concentration exerted the strongest effect on all three responses, followed by temperature and immersion time. These results agree with the hierarchy reported by Yadav and Singh [9] for mango slices and by Sharma et al. [10] for guava cubes. Where our data differ is in the magnitude of the concentration-time interaction, which was stronger than in prior papaya work by Jain and Tharanathan [11] , possibly because Red Lady cultivar tissues have higher initial porosity, allowing deeper solute penetration with prolonged contact. The quadratic nature of the WL and ML surfaces explains why beyond approximately 50 °Brix, further increases in concentration yielded diminishing returns: the osmotic pressure gradient saturates as the fruit surface equilibrates with the external solution [4] . The practical implication is that processors gain little by exceeding 50 °Brix and instead raise sugar costs. Temperature above 55 °C similarly began to soften tissues excessively, accelerating solid uptake disproportionally relative to water removal [12] .

Limitations

The research was limited to a single papaya cultivar and a fixed fruit-to-solution ratio of 1:5. Varying the ratio and testing additional cultivars could shift the optimal region. The models are empirical and valid only within the tested factor ranges. Solution recycling, which is common in commercial practice, was not evaluated and may alter solute properties over time [13] .

Conclusion

Osmotic dehydration remains an underused technology in the Mexican papaya industry despite its potential to reduce post-harvest waste. This research identified optimal conditions of 48.3 °Brix, 52.1 °C, and 4.2 hours for maximising water removal while limiting solid uptake in Red Lady papaya slices. The recommendation for smallscale processors in Colima is to adopt these parameters as a baseline, adjusting for local fruit maturity. Looking ahead, coupling osmotic dehydration with low-temperature convective or microwave-assisted finish drying could yield a shelf-stable product suitable for year-round distribution [14,15] .

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