RECyT

Year 23 / Nº 35 / 2021 /

 

Secado y estimación de coeficientes efectivos de difusión de recubrimientos con  contenido reducido de azúcar

 

Drying and effective diffusion coeficient estimation of confectionery coatings with low-sugar content

 

Bárbara E. Meza1,*, Daiana A. Beuter2, Luis A. Brumovsky2 y Juan Manuel Peralta1

 

1Instituto de Desarrollo Tecnológico para la Industria Química (INTEC), Universidad Nacional del Litoral (UNL) – Consejo Nacional de Investicaciones Científicas y Tecnicas (CONICET). Predio CONICET Santa Fe, Colectora Ruta Nacional 168, Km 0, Paraje El Pozo S/N. (3000). Santa Fe, Argentina.

2Facultad de Ciencias Exactas, Química y Naturales, Universidad Nacional de Misiones. Félix de Azara 1552. (N3300LQH). Posadas, Misiones, Argentina.

*E-mail: bmeza@intec.unl.edu.ar

 

Received: 16/02/2021; Approved: 27/04/2021

 

Resumen

Se estudió la cinética de secado y se estimaron los coeficientes efectivos de difusión de recubrimientos con contenido reducido de azúcarelaborados con un edulcorante comercial a base de stevia. Las formulaciones se elaboraron por triplicado utilizando cacao amargo, azúcar impalpable, una solución comercial con 7% p/p de glucósidos de steviol como sustitutivo del azúcar y aceite de soja. Se evaluaron diferentes porcentajes de stevia (75% y 50%) como edulcorante en reemplazo del azúcar, tomando como control la muestra con 100% de azúcar. El proceso de secado se llevó a cabo en una estufa con convección natural a 50 (± 1)°C y humedad relativa ambiente de 17 (± 2)%. La pérdida de humedad con el tiempo fue analizada aplicando el modelo matemático de capa fina logarítmico. Para la estimación de los coeficientes efectivos de difusión, se usó un modelo matemático simplificado derivado de la segunda ley de difusión de Fick. El cálculo de las humedades de equilibrio se realizó determinando las isotermas de desorción de agua a 50°C. A partir de los resultados obtenidos, se pudieron obtener satisfactoriamente parámetros de secado y coeficientes efectivos de difusión útiles para el estudio de la transferencia de materia que pueda ocurrir durante un proceso de recubrimiento.

Palabras clave: Baños de repostería; Desorción de agua, Stevia; Edulcorantes naturales.

 

Abstract

The drying kinetics and the estimation of the effective diffusion coefficients of confectionery coatings with low-sugar content were studied, made with a commercial stevia-based sweetener. Formulations were prepared in triplicate using defatted cocoa powder, icing sugar, commercial steviol glycosides solution (7% w/w) as sugar substitute, and soybean oil. Different contents of stevia (75% and 50%) were evaluated as sugar replacement, considering the formulation with 100% sugar as control sample. The drying process was carried out in an oven with natural convection at 50 (± 1)°C and relative ambient humidity of 17 (± 2)%. The loss of moisture over time was analysed by applying the logarithmic thin layer mathematical model. For the estimation of the effective diffusion coefficients, a simplified mathematical model derived from Fick's second diffusion law was used. The calculation of equilibrium moisture contents was performed by determining the water desorption isotherms at 50°C. The obtained results indicated that drying parameters and effective diffusion coefficients could be obtained, being useful information for further studies about mass transfer that may occur during a coating process.

Keywords: Confectionery coatings, Moisture desorption; Stevia; Natural sweeteners.

 

Introduction

The production of products with low sugar content is strategic for a sector of the industry dedicated to the production of products with high added value. This type of food, considered "healthy", is aimed at a large and specific sector of the population. These are people who want or need to consume products with reduced calorie content and an adequate nutritional profile, due to their physical and/or metabolic condition (e.g. people who are overweight, obese, diabetic and/or dyslipidemic) [1, 2].

Edible coatings provide foods with specific sensory characteristics, such as colour, gloss, texture and chocolate flavour. In addition, they allow control of technological variables such as shelf-life and storage conditions [3]. Sweet coatings, such as confectionery dips, are traditionally used to coat baked goods (biscuits, alfajores, cakes), and are made with simple sugars (sucrose or impalpable sugar), vegetable fats (oils and margarines) and other ingredients (cocoa and milk solids) [4]. Partially replacing this simple sugar with sweeteners is a novel strategy to produce coated foods with specific nutritional properties. For example, commercial stevia is a natural sweetener derived from the leaves of Stevia rebaudiana Bertoni and is considered a heterogeneous product composed of several steviol glycosides (steviosides and rebaudiosides). These glycosides are responsible for the sweetening capacity of stevia, which has up to 300 times more sweetening power compared to sucrose [5, 6].

The quality of coated foods will depend on the characteristics of the coating, the food to be coated and the interaction between the two. For this reason, the modification of the sugar content and the addition of sweeteners can generate technological problems in these products, associated with possible modifications in the behaviour during drying. The latter is mainly due to the fact that a decrease in sugar content leads to changes in water vapour diffusion and sorption properties of the coatings [7].

Considering the above, the aim of this work was to obtain the parameters of the drying process and to estimate the effective diffusion coefficients of coatings with reduced sugar content made with a commercial stevia-based sweetener. The information published in this work is not available in the current literature and is considered necessary for further studies related to matter transfer during a food coating process.

 

Materials and methods

Production of coatings

Coatings were made using the proportions described in table 1. The following commercial ingredients were used: impalpable sugar (San Diego, Santa Fe, Argentina), commercial solution with 7% w/w of steviol glycosides (Dulri SA, Buenos Aires, Argentina), bitter cocoa powder (Sucesores de José Salgado SAIC, Buenos Aires, Argentina) and soybean oil (Aceitera General Deheza, Córdoba, Argentina). In addition, glycerine was used as plasticiser (Cicarelli, Santa Fe, Argentina), soy lecithin as emulsifier (Laboratorios Yeruti SRL, Santa Fe, Argentina), potassium sorbate as preservative (Cicarelli, Santa Fe, Argentina) and drinking water.

The ingredients were weighed to obtain 300 g of each formulation in triplicate. They were mixed using a PE-BM65V planetary mixer (Peabody SA, Argentina) with a stainless steel whisk to minimise the incorporation of air bubbles. Speed 1 (~50 rpm) was used for 5 min and then speed 2 (85 rpm) for 5 min. The formulations obtained were placed in hermetically sealed plastic containers and stored at a temperature of 7°C for 16 h until further drying.

 

Table 1. Ingredients used to produce coatings made with cocoa and reduced sugar content.

 

Ingredient [% w/w]*

Code

Sugar

Cocoo

Stevia1

Glycerine

Fat2

Water

100% sugar

40

20

0

1

6

33

75% sugar

30

20

1

1

6

42

50% sugar

20

20

2

1

6

51

1Stevia = commercial solution with 7% w/w of steviol glycosides. 2Fat = mixture of soybean oil (5% w/w) and soybean lecithin (1% w/w). *In all formulations 0.1% w/w potassium sorbate was used as preservative.

 

Drying of the coatings

An aliquot of each formulation was weighed and placed in glass containers of known dimensions (9 cm diameter and 2 cm high) in triplicate. The containers were covered with aluminium foil to facilitate the detachment of the coatings after drying. The samples were dried in an electric oven with natural convection Dalvo TDC60 (Tecno Dalvo SRL, Santa Fe, Argentina) at 50 ( 1)°C and 17 ( 2)% relative humidity. Temperature was controlled using a Dhacel CD101 controller (Dhacel S.R.L., Buenos Aires, Argentina) and relative humidity was monitored with a Schwyz DAT 10 recorder (Schwyz, China). The containers with the samples were weighed using an analytical balance every 60 min for 7 h. The temperature inside the oven was monitored with a Schwyz DAT 10 logger (Schwyz, China). The temperature inside the oven was considered constant because variations of less than 2°C were recorded in the opening and closing cycles of the oven when handling the samples.

The loss of moisture () with time () relative to the initial moisture () of the samples was analysed by applying a semi-empirical thin film mathematical model (or logarithmic model) derived from the first term of the general solution of Fick's second law of diffusion [8]:

                                                                                                 (Ec. 1)

where is the moisture of the sample (dry basis) time  [kg kg-1],,  is the initial moisture of the sample (dry basis) , is the equilibrium moisture of the sample (dry basis) at study conditions[kg kg-1],  is the time min [min],, is the drying kinetic constant min-1 and the parameters and are empirical and dimensionless. The values of were determined using information obtained from water desorption isotherms at 50°C.

 

Determination of coating thicknesses

On the one hand, the initial thicknesses of the coatings were calculated in triplicate using the values of the densities, the initial weights and the radii of the glass containers used to dry the samples (4.5 cm). The densities of the formulations at 25 (± 1)°C were determined gravimetrically in quintuplicate by weighing on an analytical balance a given amount of sample in Eppendorf vessels with known volumes (1.89 cm3). The values obtained were 1209 (± 10) kg m-3, 1160 (± 12) kg m-3 and 1137 (± 12) kg m-3 for the 100% sugar, 75% sugar and 50% sugar formulations, respectively. The initial thickness values obtained were 1.31 (± 0,03) mm for 100% sugar, 1.50 (± 0,05) mm for 75% sugar and 1.67 (± 0,01) mm for 50% sugar, respectively.

On the other hand, the final thicknesses obtained after the drying process were measured (10 replicates) using a Schwyz ESP1-001PLA digital micrometer (Schwyz, Switzerland). In this case, the following values were obtained: 1.14(± 0,12) mm, 1.15 (± 0,11) mm and 0.97 (± 0,15)  mm for the 100% sugar, 75% sugar and 50% sugar formulations, respectively.

 

Isotermas de desorción de agua

The dried coatings obtained under the conditions specified above were cut into pieces of equal size (approximately 5 x 5 mm) and placed in small plastic containers duly labelled in triplicate. They were placed in an environment containing pure water (100% ambient relative humidity) at 25°C for 3 days for complete saturation. Then, the containers were distributed in duplicate in hermetically sealed glass jars containing different saturated salt solutions with known water activity (), according to the methodology previously used [9]. Eight salt solutions were used within the range of 0.189 to 0.848 obtained from the literature [10, 11] (Table 2). Subsequently, the flasks were stored in a natural convection oven at a controlled temperature of 50 ( 1 ) °C for 20 days. According to preliminary experiments, this time was sufficient for the samples to reach equilibrium moisture (constant weight). This temperature was selected because it corresponds to the temperature used for the drying process.

 

Table 2. Water activity () of saturated salt solutions at 50°C.

Salt

[-]

Potassium acetate (CH3CO2K)

0,189

Magnesium chloride (MgCl2)

0,305

Potassium carbonate (K2CO3)

0,427

Magnesium nitrate (Mg(NO3)2)

0,454

Cobalt chloride (CoCl2)

0,500

Sodium chloride (NaCl)

0,744

Potassium chloride (KCl)

0,812

Potassium nitrate (KNO3)

0,848

 

The moisture content was then determined on a dry basis by gravimetric method using a Yamato ADP310C vacuum oven (Yamato Scientific America Inc., California, USA) at 70 ( 1 ) °C and 0.01 atm pressure for 6 h. The experimental values of the equilibrium moisture contents were analysed by applying the theoretical model of GAB [12]:

                                                                                         (Ec. 2)

where is the monolayer moisture (dry basis) [kg kg-1]. The dimensionless caloric parameters and are related to the heat of sorption of the monolayer and multilayer region, respectively.

 

Estimation of the effective diffusion coefficients

A simplified mathematical model derived from Fick's second law of diffusion [13] was used to estimate the effective water diffusion coefficients () of the coatings at the drying temperature (50°C). For the development, an isotropic and homogeneous system with mass transfer on one of the coating surfaces was taken into account, assumed as a two-dimensional system and considered as an infinite flat plate of constant thickness and initial humidity . The surface of the plate in contact with the containing vessel was considered to be isolated to the transfer and the surface in contact with the air was considered to be in equilibrium with the surrounding medium. The latter consideration was based on the assumption that the resistance to water transfer in the layer surrounding the plate is negligible compared to its internal counterpart. In other words, the process has very large mass Biot number values (> 103) This can be verified by taking into account that the plate thicknesses ()considered are in the order of 10-3 m, that in food products it has been observed that the surface mass transfer coefficient  () is in the order of 10-2 m s-1 [14] and that  is in the order of 10-11 m2 s-1. Then, it is satisfied that . Therefore, the moisture content of the plate can be calculated with the following expression:

                                                           (Ec. 3)

where is the mass Fourier number ( ), is the effective water diffusion coefficient [m2 s-1] and is the total plate thickness [m].

 

Furthermore, considering that the summation terms of Eq. (3) are expected to decrease by an order of magnitude ( > 0.3), then the first term will be representative and hence:

                                                                                        (Ec. 4)

 

rearranging Eq. (4) and applying logarithms:

                                                                    (Ec. 5)

 

where  y . Thus, after fitting the experimental data to Eq. (5), plotting as a function of  , the coefficient can be estimated from the following expression:

                                                                                                                    (Ec. 6)

 

For this study, the values of  were obtained from the water desorption isotherms.

 

Statistical analysis

The determination of the parameters of the logarithmic model (Eq. 1) and the GAB model (Eq. 2) was carried out by means of non-linear regression. The existence of statistically significant differences between the values was assessed by applying ANOVA and, subsequently, Tuckey's test for comparison of means (95% confidence level).

The fit of the non-linear models was analysed by calculating the average percentage errors ( ) and the root mean square error (); while the fit of the proposed model for estimating the coefficients was assessed by calculating the coefficients of determination (R2) [15]. Statistical analysis was carried out using Statgraphics Plus 5.1 (Statgraphics Inc., Rockville, MD, USA).

 

Results and discussion

Water desorption isotherms

Figure 1 shows the values of the equilibrium moisture content obtained experimentally at 50°C and the theoretical curves fitted with the GAB model (Eq. 2). The shape of the obtained curves is characteristic of type III isotherms (according to the BET classification), which has been observed in foods with high content of sugars and water-soluble compounds [16].

 

Figure 1. Water desorption isotherms at 50°C of the coatings studied. The symbols are the average values (± standard deviations) of three experimental measurements and the lines correspond to the theoretical GAB model (Eq. 2).

 

The values of the GAB model constants are expressed in Table 3. As can be seen, the selected theoretical model presented a good fit (between 0.008 and 0.037 kg kg-1), consistent with the experimental errors obtained of less than 0.016 kg kg-1. The coefficient values were found to be in the range of 0.997 to 0.998, and were considered constant and independent of the type of sample analysed. The coefficients increased with increasing sugar replacement by the stevia-based sweetener, from values equal to 0.198 for the 100% sugar sample to 0.312 for the 50% sugar samples. These values are in accordance with the expected values for this constant, which, according to the literature, should be less than or equal to 2 for type III isotherms [17]. The values of, , corresponding to the moisture content of the monolayer, were found to be in the range of 0.065 to 0.067 kg kg kg-1, being considered constant and similar to data obtained in the literature for cocoa-based coatings [9].

 

Table 3. Parameters of the GAB model (Eq. 2) for the study of the water desorption isotherms at 50°C of the analysed coatings.

Code

 [kg kg-1]

 [-]

 [-]

 [kg kg-1]

100% sugar

0,065

0,198

0,997

0,037

75% sugar

0,067

0,258

0,998

0,023

50% sugar

0,066

0,312

0,997

0,008

 

The physical significance of the values of the constants found for each formulation would indicate that the water sorption mechanism varies with composition. According to Quirijns, van Boxtel, van Loon and van Straten (2005) [18], low values of accompanied with values of approximately equal to 1 would indicate that there is no localised water adsorption. In this case, the behaviour of the adsorbed water molecules in the monolayer does not differ from that of the molecules in the multilayer, which at the same time behave like liquid water.

 

Drying curves

Figure 2 shows the drying curves of the coatings analysed. The three coatings presented a similar behaviour, with a high initial drying speed followed by a slower stage. It can be observed that from the beginning of the drying process, there is no evidence of an initial period of adjustment or constant drying, indicating that the drying temperature was low enough for the samples to reach the air temperature quickly inside the oven. This means that in the formulations studied, drying occurred exclusively within the period of decreasing velocity, where mainly the diffusion of water vapour from the interior of the food towards the food surface takes place. The moisture content at infinite time asymptotically approaches the equilibrium value at ambient temperature and relative humidity of the air in contact with the food [19].

 

Figure 2. Drying curves at 50°C of the coatings studied. The symbols are the average values (standard deviations) of three experimental measurements and the lines correspond to the semi-empirical logarithmic model (Eq. 1).

 

The parameter values of the logarithmic model (Eq. 1) used to study the drying kinetics of the analysed samples are shown in table 4. According to the results obtained, the drying kinetics of the coatings could be modelled satisfactorily, since the maximum obtained was 5.44%.

 

As the sugar content decreases and is replaced by the stevia-based sweetener, the empirical constants vary, with the values of increasing and those of decreasing. At the same time, the values of , which represent the kinetic constants of the drying process, decreased with increasing sugar replacement, from 0.008 to 0.004 min-1. This result is due to the lower soluble solids content present in the formulations and the hygroscopic nature of sucrose that hinders the diffusion drying process in formulations with higher sugar content [7].

 

Table 4. Parameters of the logarithmic model (Eq. 1) for the study of the drying kinetics at 50°C of the analysed coatings.

Code

[-]

 [min-1]

 [-]

 [%]

100% sugar

0,669

0,008

0,326

1,27

75% sugar

0,858

0,006

0,152

2,44

50% sugar

1,027

0,004

0,000

5,44

 

The thin film concept applied to the mathematical modelling of the drying process is based on the fact that the size of the material is reduced to such dimensions that a uniform distribution of air and temperature over the material is assumed. The shape factor is integrated into the kinetic models to reduce the effect of product shape on the drying process. The kinetics of drying of various foods was successfully modelled using mathematical models based on the thin film concept [20, 21].

 

Coeficientes efectivos de difusión

The methodology used for the estimation of the values is detailed in Figure 3. In the linear regressions, R2 values greater than 0.96 were obtained, which is considered to be very good. The estimated values of the coating values studied were 1.80 ( 0.06) × 10-11 m2 s-1 for the 100% sugar samples; 3.60 ( 0.10) × 10-11 m2 s-1 for the 75% sugar samples and 4.50 ( 0.06) × 10-11 m2 s-1 for the 50% sugar samples. These values are similar, although an order of magnitude higher, to those published by Gosh, Duda, Ziegler and Anantheswaran (2004) [22] for coatings made with different contents of cocoa, coconut oil, soybean lecithin and sugar at 20°C (lower temperature than that studied in the present work).

 

Figure 3. Methodology used to estimate the effective water diffusion coefficients of the coatings studied. The symbols correspond to the averages of three experimental determinations and the lines are the linear regressions.

 

Statistically significant differences were found between the values mentioned, increasing with decreasing sugar content and increasing stevia content. According to information obtained from the literature, this behaviour is to be expected due to the higher amount of solids present in formulations with higher sugar content. It has been observed that in sweet coatings made with cocoa, diffusion through sucrose particles is much slower than through other components, such as fats and oils, because sugar is crystalline in nature. In addition, the presence of lecithin in formulations together with sucrose causes water diffusion to be much slower, because lecithin tends to absorb much more moisture than sugar at water activity values below 0.85 [7, 22].

 

Conclusions

In the present work, the kinetics of the drying process at 50°C were studied and the effective diffusion coefficients of reduced sugar coatings made with cocoa and a stevia-based sweetener could be estimated. From the results obtained, it was possible to model the moisture loss during drying of the coatings analysed satisfactorily, obtaining effective diffusion coefficients that are useful for the study of the transfer of matter that may occur between the coating, the substrate and/or the environment during a food coating process. Furthermore, the physical significance of the values of the GAB model constants found for each formulation would indicate that the water sorption mechanism varies with the composition of the coatings studied.

 

Nomenclature

           Empirical constant (Eq. 1) [-]

         Water activity [-]

         Mass Biot number () [-]

           Empirical constant (Eq. 1) [-]

          Theoretical constant related to the heat of sorption of the monolayer (Eq. 2) [-]

        Effective water diffusion coefficient [m2 s-1]

      Average percentage error [%]

         Mass Fourier number () [-]

           Drying kinetic constant (Ec. 1) [min-1]

          Surface mass transfer coefficient [m s-1]

          Theoretical constant related to the heat of sorption of the multilayer (Eq. 2) [-]

           Total plate thickness [m]

       Equilibrium moisture (dry basis) [kg kg-1]

        Monolayer moisture (dry basis) (Eq. 2) [kg kg-1]

         Moisture over time  (dry basis) [kg kg-1]

        Initial moisture (dry basis)  [kg kg-1]

   Root mean squared error [kg kg-1]

         Coefficient of determination [-]

            Time [min] (Eq. 1); [s] (Eq. 5)

 

Acknowledgements

This work was partially funded by CONICET (PIP Project 2015-11220150100185CO, Argentina), the UNL (CAI+D Projects 2016-50420150100002LI and CAI+D 2020-506201901000000005LI, Santa Fe, Argentina) and the National Agency for the Promotion of Research, Technological Development and Innovation (PICT Project 2019-209, Argentina).

 

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