4. Metodología
4.1 Análisis y Resultados
4.1.2 Análisis del formato de contenido publicado en la cuenta de Twitter del presidente
A panel of 10 untrained testers evaluated the taste of cranberries pretreated with 13 different techniques and then subjected to 5 h osmotic dehydration in a sugar syrup of 67.5° Brix at 50°C. An evaluation scale of 0–100 points was applied (0 = totally rejected; 100 = maximum preference). Figure 5.3 shows the average values. The lowest values were given to all chemically pretreated cranberries, medium to thermal and combined (chemical and thermal), and the highest to cranberries cut into halves.
According to comments of the testers, each of the chemical or thermal pretreatments
FIGURE 5.3 Results of taste panel acceptability evaluation. The numbers of the pretreatments refer to Table 5.1.
1 2 3 4 5 6 7 8 9 10 11
PRETREATMENT TYPE (Number)
0 10 20 30 40 50 60 70 80 90 100
TASTE ACCEPTABILITY, % CHEMICAL
MECHANICAL THERMAL COMBINED
12 13
caused some specific changes in the taste or texture of the berries. Thus, the mechanical cutting of cranberries into halves was the most effective pretreatment tested. The other mechanical pretreatment, perforating the skin, was not successful, probably because a very low proportion of the skin was opened to the osmotic syrup. If it were possible to obtain a high enough number of perforations (or surface area opened by perfo-ration), for example, 20–30% or more, this technique could also be practical.
5.3.2 SIMPLE DIFFUSSIVITY MODEL FOR CUT CRANBERRIES
After selection of the most effective pretreatment technique, attention was given to the mathematical description of the mass transfer between cut cranberries and the osmotic syrup. It is obvious that water and sucrose are mainly transferred through the cut surface of the berries, with only a small proportion being transferred through the uncut surface covered by the skin. Therefore, the entire mass transfer is the sum of two quantities: one related to the cut surface of the berry, and the second to the unaffected surface. Introducing a surface ratio parameter (α), which is the ratio of the cut surface area divided by total surface of the cut cranberry,
α = Fopen by cut/Ftotal (5.5) and introducing different moisture (or infused sugar) effective diffusivity for the cut (Deff cut) and the uncut (Deff uncut) surface, the following simplified model of mass exchange between cut cranberry and osmotic syrup is proposed:
Deff= αDeff cut + (1 – α)Deff uncut (5.6)
To verify this model, a special set of tests was performed. Cranberries of uniform size were cut into halves and additionally from both sides, to achieve approximately 50% of open surface. Samples of uncut cranberries (reference: α = 0), cranberries cut into halves (30% open surface:α = 0.3), and cranberries cut from both sides (for 50% of open surface:α = 0.5) were placed into the osmotic dehydration unit (Figure 5.1). The osmotic dehydration and infusion treatment was performed in a sucrose syrup of 67.5° Brix at 50°C.
Figure 5.4 shows examples of the dehydration kinetics. Figure 5.5 shows examples of sugar infusion kinetics of cut and uncut cranberries. At first, no significant differ-ences in osmotic process of either variety of cranberries were observed. As predicted, the water content of cranberries cut to 50% of open surface (α = 0.5) was reduced at a higher rate than that of those cut into halves (α = 0.3). The dehydration time of uncut or whole cranberries was many times longer than for cut cranberries. For the sugar infusion, a similar situation was observed (Figure 5.5).
Based on Fick’s second law of unsteady state mass diffusion (Crank, 1975), and assuming a spherical geometry and a uniform initial temperature and concentration distribution inside the berry, an effective coefficient of diffusion (Deff) was calculated from the following equations (Crank, 1975; Nsonzi and Ramaswamy, 1997):
WR= 6/π2 exp(–Deffπ2τ/a2) (5.7)
Deff= Ba2/π2 (5.8)
FIGURE 5.4 Kinetics of moisture removal (osmotic dehydration) from samples of cut and uncut (reference) cranberries.
FIGURE 5.5 Kinetics of sugar infusion into uncut (reference) and cut cranberries.
0 10 20 30 40
0 1 2 3 4 5 6 7
PROCESSING TIME, sec Thousands
WATER CONTENT, X, dry basis
Untreated
Halves
"Stevens"
Halves "Early Black"
Cut (50% open surface)
0 5 10 15 20
0 0.1 0.2 0.3 0.4
Thousands Untreated (whole) Halves "Stevens"
Halves "Early black"
Cut (to 50% open)
SUGAR CONTENT (S), kg dry basiskg
PROCESSING TIME, sec
whereτ is the process time; a is a characteristic dimension of the cranberry, and B is the slope of the kinetic curves ln(WR) or ln(SI) against time (t). Figure 5.6 shows the moisture reduction, and Figure 5.7 shows the sugar infusion.
Based on the statistical analysis (R2= 0.91 − 0.96), effective diffusion coefficient values were found to obey the following equations:
FIGURE 5.6 Calculation of the effective moisture diffusivity for moisture reduction of cranberries.
FIGURE 5.7 Calculation of the effective diffusivity for sugar infusion into cranberries.
0 7200 14400 21600
-6 -5 -4 -3 -2 -1 0 1
PROCESSING TIME, sec
ln(WR) Untreated
Halves "Stevens"
Halves "Early Black"
Cut (50% open surface)
-6 -4 -2 0 2
ln(SI)
0 7200 14400 21600
Untreated (whole) Halves "Stevens"
Halves "Early black"
Cut (to 50% open)
PROCESSING TIME, sec
For water loss (osmotic dehydration):
Deff= 2.38 10–9α + 1.7 10–11(1− α) (5.9) For sugar gain (infusion):
Deff= 10–9α + 1.65 10–11(1− α) (5.10) Equations (5.9) and (5.10) show that the effective diffusion coefficient for the cut area of a cranberry is around 100 times higher than that of the uncut surface of the berry. The values of effective diffusivity for dehydration and infusion varied with process temperature and concentration of the osmotic syrup. In this work, both parameters were kept constant to study the clear effect of pretreatment techniques.
For cranberries cut into halves (30% of open surface or α = 0.3), average effective mass diffusivities of about 7 × 10–10m2/sec and 3.1 × 10–10 m2/sec were obtained for dehydration and sugar infusion, respectively. In osmotic dehydration literature, similar values have been reported (Rastogi and Raghavarao, 1997; Kaymak and Kincal, 1994; Marcotte and LeMaguer, 1992; Lewicki et al., 1984). Lower values of Deff, i.e., 2–5× 10–10 m2/sec for dehydration and 0.2–2× 10–10 m2/sec for infusion, were determined by Nsonzi and Rawaswamy (1997) for blueberries that were untreated before the osmotic process.
To validate our mathematical model, shown in Equation (5.6), a special osmotic dehydration test for cranberries cut from one side to obtain 15% of open surface (α = 0.15) was performed. Experimental values of mass diffusivity of 4 × 10–10m2/sec for dehydration and 1.9 × 10–10 m2/sec for sugar infusion were obtained. These values compared well with values calculated from Equations (5.9) and (5.10) (3.7 × 10–10 m2/sec for dehydration and 1.64 × 10–10 m2/sec for sugar infusion). Therefore, this simple model can be applied to other fruits and vegetables cut before the next steps of processing.
5.4 CONCLUSIONS
From the point of view of efficiency and taste acceptability, the best practical pretreatment of cranberries before osmotic dehydration and infusion was to cut the berries into halves. Taking into account the ratio of the open surface from cutting to the total surface of cut cranberries, a simple proportional model for the effective moisture loss and sugar gain diffusivities was proposed. Within the range of experimental data, effective diffusion coefficients for moisture loss and sugar gain were approximately 100 times higher for the cut surface than for the uncut cranberries.
The model was validated experimentally by comparing theoretical values to experi-mental data generated for berries with 15% of open surface. This model could be applied to other fruits and vegetables that are cut before other processing operations.
NOMENCLATURE
a Characteristic dimension of cranberry, m
B Slope of the kinetic curves (absolute value from Equation 5.8), s–1
b Cranberry moisture content, w.b.
Deff Effective coefficient of diffusion, m2/sec F Surface of the single berry, m2
MR Mass reduction, g/100 g of initial mass S Sugar content, kg/kg dry basis
SG Solid gain, g/100 g of initial mass
SI Dimensionless sugar intake: (S – SEQ)/(S1− SEQ) WL Water loss, g/100 g of initial mass
WR Dimensionless water reduction: (X – XEQ)/(X1 – XEQ) X Moisture content, kg/kg dry basis
α Surface ratio: open by cut to total τ Process time, sec
ζ Coefficient of pretreatment efficiency (Equation 5.4)
SUBSCRIPTS
1, 2 Initial and final, respectively DEH Dehydration
EQ Equilibrium INF Infusion of sugar
N, P Without pretreatment and pretreated, respectively
ACKNOWLEDGMENTS
Financial support from the Office of Energy Research and Development of Canada (Program PERD) is gratefully acknowledged. Special thanks go to Francois Brunet for the HPLC sugar tests.
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