Prediction of lycopene degradation during a drying process of tomato pulp

Journal of Food Engineering 74 (2006) 37–46 www.elsevier.com/locate/jfoodeng

Prediction of lycopene degradation during a drying process of tomato pulp Athanasia M. Goula, Konstantinos G. Adamopoulos *, Paris C. Chatzitakis, Vasilios A. Nikas Department of Chemical Engineering, School of Engineering, Laboratory of Food Process Engineering, Aristotle University of Thessaloniki, 541 24 University Campus, Thessaloniki, Greece Received 7 October 2004; accepted 5 February 2005 Available online 7 April 2005

Abstract Lycopene is the principle pigment found in tomatoes and is important not only because of the color it imparts but also because of the recognized health benefits associated with its presence. Heating and drying of tomato products under different processing conditions to manufacture tomato juice, pulp, powder etc. may cause degradation of lycopene. For an exact calculation of the rest concentration of a nutrient in a drying process one would have to know the material temperature and water concentration at each moment and the dependence of degradation reaction rate constant on temperature and moisture content. The objective of this study was to determine a mathematical model of the reaction kinetic of lycopene degradation to describe the rate of lycopene loss in a drying process of tomato pulp. Tomato pulps with different moisture contents were heated at specified temperatures for different time periods. A mathematical model giving rate constant of lycopene degradation as a function of material temperature and moisture content was derived from the changes of lycopene concentration in equal time intervals. This model was used to simulate the lycopene loss during two drying processes of tomato pulp. The first process was the concentration of tomato pulp with total solids concentration of 14% to approximately 40% final moisture content, whereas the second one was the spray drying of tomato pulp. It was concluded that there was a close agreement between the experimental and predicted values of lycopene loss during the tomato pulp concentration confirming the validity of the proposed model for this process. However, for the spray drying process a correction coefficient was introduced in the model, due to the more intense exposure of product surface to air. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Concentration; Degradation; Lycopene; Reaction kinetics; Spray drying; Tomato pulp

1. Introduction Traditionally, the major emphasis in the industrial processing of foods has been thermal processing for preservation and microbiological safety, with limited regard for nutritional quality. Over the past three decades, there has been an increased concern for food quality with a significant amount of work accomplished in the area of kinetics of nutrient destruction or general quality degra*

Corresponding author. Tel.: +30 2310 996205/996167; fax: +30 2310 996259. E-mail address: [email protected] (K.G. Adamopoulos). 0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.02.023

dation during thermal processing. It is self-evident that the number of possible degradation reactions in foodstuffs is very large and that in principle several reaction mechanisms may be involved. Usually, the rates of degradation follow the first-order reaction kinetics:


dC ¼kC dt

ð1Þ

where C is the concentration of a nutrient and k is the reaction rate constant, determined as a function of the material temperature. Limited research, however, has been achieved with regard to the kinetics of food quality degradation during

38

A.M. Goula et al. / Journal of Food Engineering 74 (2006) 37–46

dehydration. Generally, the problem of chemical conversions during drying is extremely complicated. The rate constants of the reactions depend on temperature, concentration of the reactants and concentration of water (water activity) (Bluestein & Labuza, 1988; Dumoulin & Bimbenet, 1998; Karel, 1975; Sokhansanj & Jayas, 1995). The question arises in which cases the temperature dependence may be described by the simple Arrhenius equation:
 DE k ¼ k 0  exp
ð2Þ RT where k0 is the so called frequency factor and DE is the activation energy. The frequency factor depends on the temperature and can be considered only as a constant when DE is sufficiently high. During the drying process, concentrations change also and it is not clear to what extent thereby the chemical changes are influenced. The influence of the water activity is important but insufficiently understood. Water acts as solvent for the chemicals of nutritional importance present in the product. As water is removed, the concentration of the chemicals increases. The loss of nutrient described by Eq. (1) is concentration dependent and thus would increase as dehydration progresses. On the other hand, some of the water-soluble compounds may act as catalysts to the decomposition process. These catalytic effects are greatly reduced as the moisture is removed. Although various oxidation reactions show a minimum rate of reaction at a certain water activity (Jayaraman & Das Gupta, 1995; Pan, Zhao, Dong, Mujumdar, & Kudra, 1998), in general, chemical reactions are slower as the water activity decreases (Karel, 1979). As during drying the product temperature increases and the water activity decreases, k may first increase when the temperature effect is dominating, whereas may decrease later on when the influence of the lower water activity becomes the dominating factor (Leniger & Bruin, 1977). According to Leniger and Bruin (1977), in the case of a first-order rate equation the average rest concentration of a component in a drying particle is given by:
Z te  C ¼ exp
k dt ð3Þ C0 0 where C0 is the initial concentration of a nutrient, te is the drying time and k is a function of temperature and moisture content. For an exact calculation of the rest concentration one would have to know the temperature and the water concentration at each moment and the dependence of k on temperature and moisture content (Escher & Blanc, 1977). The real problem with the current literature is that little work has been done on the kinetics of nutrient losses at constant temperatures and moisture contents.

The principal coloring matter responsible for the characteristic deep-red color of ripe tomato fruit and tomato products is lycopene, a C40 carotenoid polyene. Lycopene is important not only because of the color it imparts but also because of the recognized health benefits associated with its presence (Bramley, 2000; Franceschi et al., 1994; Micozzi, Beecher, Taylor, & Khachik, 1990; Southon, 2000). Heating and drying of tomato products under different processing conditions to manufacture tomato juice, pulp, powder etc. may cause degradation of lycopene (Klaui & Bauernfeind, 1981). Many researchers have studied lycopene loss in tomato products as a result of thermal treatments. Cole and Kapur (1957) examined the kinetics of lycopene degradation by studying the effects of oxygen, temperature and light intensity on the formation of its volatile oxidation products. Miki and Akatsu (1970) observed about 1–2% lycopene loss when heating tomato juice at 100 °C for 7 min. Noble (1975) found that heat concentration of tomato pulp resulted in up to 57% loss of lycopene. Sharma and Le Maguer (1996) reported that kinetics of lycopene degradation during heating of tomato pulp at 100 °C under different conditions followed a pseudo-first-order reaction with a reaction rate constant of 0.0023 min
1 when concentrating tomato pulp during heating. The rate of lycopene degradation was lower (k = 0.0017 min
1) when heating was done without concentration. According to Zanoni, Peri, Nani, and Lavelli (1999), during drying of tomato halves the lycopene content decreases to a maximum of 10% after drying at 110 °C and does not change during drying at 80 °C. However, to predict lycopene conversion rate during a drying process one would have to know the model giving its degradation rate constant as a function of material temperature and moisture content. With this knowledge and the time–temperature–moisture content distribution in the product during drying, process optimization procedures could be calculated where necessary. The aim of this work was to determine a mathematical model of the reaction kinetic of lycopene degradation to describe the rate of lycopene loss in a drying process of tomato pulp.

2. Materials and methods 2.1. Model development 2.1.1. Sample preparation Raw fresh tomato fruits (Cherokee variety) were purchased at local markets and stored at 5 °C before use. Damaged and over-mature fruits were discarded. Tomatoes were chopped into pieces with a knife and were ground into rough pulp using a StarMix blender (Model

A.M. Goula et al. / Journal of Food Engineering 74 (2006) 37–46

H.1, Starmix S.p.A., Schio, Italy). Subsequently, rough pulp was passed through a finisher (0.50 mm screen) to remove skins and seeds to get a thin tomato pulp. The pulp was stored in a refrigerator below 5 °C overnight before subsequent concentration, heating and lycopene analysis. 2.1.2. Concentration of tomato pulp To provide tomato pulp samples with different moisture contents, thin tomato pulp was concentrated to pulps of 95, 85, 65, 55, 50, 45, and 35% (wet basis, w.b.) moisture content (X). A Tokyo Rikakikai rotary evaporator (Model 1, Tokyo Rikakikai Co., Ltd, Tokyo, Japan) was employed for the concentration process. During evaporation tomato pulp temperature was 90 °C. Approximately 80, 125, 145, 155, 160, and 170 min were required to remove water of tomato pulp to moisture content of 85, 65, 55, 50, 45, and 35%. At the end of concentration process, the concentrated pulp was analyzed for moisture and lycopene content. 2.1.3. Heating of tomato pulp Approximately 100 g sample of tomato pulp of each total solids concentration was taken in a flask and was heated at specified temperatures (50, 60, 70, 80, and 90 °C) using a Gallenkamp waterbath (Model L12, Gallenkamp, London, UK). Pulp samples were drawn at heating intervals of 20, 40, 60, 80, and 100 min and analyzed for lycopene content. The heating was carried out in such a way so as to maintain the total solids content of the heating pulp constant by condensing the evaporating vapors inside the flask. 2.1.4. Total solids determination Total solids in tomato pulp samples were determined by a gravimetric method. About 3.00 ± 0.05 g of samples in triplicate were dried on aluminum dishes at 70 °C (AOAC, 1980). The dishes were kept in a desiccator for 30 min before recording their final weights. All measurements were done in triplicate and the averages of these triplicate measurements were used. Additional parallels were analysed if the single values from the triplicates deviated more than 0.60% from the triplicate mean. 2.1.5. Lycopene analysis The lycopene content (lg/g total solids) was spectrophotometrically determined on extracts in petroleum ether in triplicate at 505 nm (Gould & Gould, 1988) using a Helios UV–Visible spectrophotometer (Helios gamma, Thermo Spectronic, Madison, USA). All determinations were done in triplicate and the averages of these triplicate measurements were used. An amount of pulp to have 0.5–1.0 g total solids was accurately weighed. The sample was mixed with 75 ml of acetone and 60 ml of petroleum ether (65–100 °C) and blended

39

for exactly 5 min. The mixture was transferred to a 500 ml separatory funnel. A 9 cm funnel loosely plugged with glass wool and a wash bottle containing acetone facilitated this transfer and prevented tomato solids from entering the funnel. The extract was washed three times with distilled water. The funnel was shaken gently in an inverted position for 0.5 min. This step removed the acetone whose function was to remove the water of the sample, thus helping to prevent the formation of stable emulsions. The lower phase was discarded. The hyperphase was mixed with 20 ml of 90% methanol for 0.5 min and the hypophase was discarded. The hyperphase was mixed with 20 ml of 20% KOH in methanol for 0.5 min and the lower phase was discarded (saponification). The addition of 90% methanol was repeated and finally, the extract was washed three times with distilled water. The mixture was diluted with petroleum ether to 100 ml volume. Samples were analyzed immediately or refrigerated in darkness at 0 °C for a maximum of 72 h. The lycopene was quantified by using a standard curve of purified lycopene dissolved in petroleum ether in concentration ranging from 0.20 to 56.25 lg/ml. The lycopene standard used was 95% pure (Sigma Chemical Co., St. Louis, MO, USA). 2.1.6. Statistical analysis The data were analyzed using the statistical software Minitab (Release 13.32, Minitab Inc., State College, PA, USA). 2.2. Simulation of lycopene loss in a drying process The model expressing the lycopene degradation rate constant as a function of product temperature and moisture content was used to simulate the lycopene loss during two drying processes of tomato pulp. The first process was the concentration of tomato pulp with total solids concentration of 14% to approximately 40% final moisture content in the Tokyo Rikakikai evaporator at 75 and 90 °C. During concentration, weight loss, moisture loss and drying rate were calculated every 10 min by the value measured for tomato pulp weight, whereas tomato pulp temperature was continuously measured. Pulp samples were drawn every 10 min and analyzed for lycopene content. At the end of concentration process, total solids of the concentrated pulp were measured to validate the calculated weight losses. Tomato pulp drying and lycopene degradation kinetics were replicated twice at the two product temperatures. The second process was the spray drying of tomato pulp in a Buchi mini spray dryer (Model 191, Buchi Laboratoriums-Technik, Flawil, Switzerland). Eight different spray drying experiments were conducted in triplicate. In all experiments the atomizer pressure, the feed temperature and the feed rate were kept at 5 ± 0.1 bar,

A.M. Goula et al. / Journal of Food Engineering 74 (2006) 37–46 7.5

X (% w.b.)

T = 60oC

35 45 50 55 65 85 95

7.4 ln C

1.75 ± 0.05 g/min, and 32.0 ± 0.5 °C respectively, whereas the feed was medium concentrated tomato pulp with a total solids concentration of 14%. Tomato pulp was spray dried at air inlet temperatures (Ti) of 110 and 130 °C (±1 °C), drying air flow rates (Qa) of 17.50 and 19.25 m3/h (±0.18 m3/h) and atomizing agent flow rate (Qc) levels of 500 and 700 l/h (±20 l/h). In all experiments tomato pulp feed and tomato powder were analyzed for total solids and lycopene content. In a previous work (Goula & Adamopoulos, 2003, 2004), a numerical simulation of a spray drying process modelized with the computational fluid dynamics code Fluent 5.3 was described. The model was validated by experimental tests on a pilot spray dryer and was proved able to accurately predict the most important features of the dryer, such as fields of gas temperature and gas velocity inside the chamber, temperature and moisture history of the particles. Thus, the product temperature and moisture content at each moment during the spray drying experiments conducted in this work were determined by using the code Fluent 5.3.

7.3 7.2 7.1 7.0 0

20

40 60 t (min)

80

100

Fig. 2. The kinetics of lycopene degradation in tomato pulp with different moisture contents (X, % w.b.) heated at 60 °C (C in lg/g total solids).

7.5

X (% w.b.)

T = 70oC

35 45 50 55

7.4

ln C

40

7.3 7.2

65 85 95

7.1 7.0

3. Results and discussion

0

20

3.1. Kinetics of lycopene degradation The kinetics of degradation of most biological materials follows the first-order reaction: C ¼ C 0  expð
k  tÞ

ð5Þ

The concentration of lycopene in tomato pulp with different moisture contents heated at specified temperature for different time periods has been measured. The plots of ln C against time for each temperature and moisture content are shown in Figs. 1–5. From the distribution of the points we can see that the plots are approximately linear (R2 = 0.943–0.998) confirming the reaction of X (% w.b.)

o

T = 50 C

35 45 50 55 65 85 95

ln C

7.4 7.3 7.2 7.1 7.0 40

60

80

100

t (min)

Fig. 1. The kinetics of lycopene degradation in tomato pulp with different moisture contents (X, % w.b.) heated at 50 °C (C in lg/g total solids).

X (% w.b.)

T = 80oC

35 45 50 55

7.4

ln C

ln C ¼ ln C 0
k  t

20

100

ð4Þ 7.5

0

80

Fig. 3. The kinetics of lycopene degradation in tomato pulp with different moisture contents (X, % w.b.) heated at 70 °C (C in lg/g total solids).

or

7.5

40 60 t (min)

7.3 7.2

65 85 95

7.1 7.0

0

20

40 60 t (min)

80

100

Fig. 4. The kinetics of lycopene degradation in tomato pulp with different moisture contents (X, % w.b.) heated at 80 °C (C in lg/g total solids).

lycopene degradation to be first order. This observation is similar to that obtained by other researchers (Sharma & Le Maguer, 1996), who studied the kinetics of lycopene degradation in tomato pulp during heating at 100 °C. They concluded that the kinetics data followed a pseudo-first-order reaction. When other kinetic models were tried, the best fit model was zero order kinetics. However, theoretically it was not suitable because the rate of lycopene loss was dependent on the initial lycopene concentration.

A.M. Goula et al. / Journal of Food Engineering 74 (2006) 37–46 7.5

X (% w.b.)

T = 90oC

-7.00

45 50

7.3

55

7.2

65

ln k

ln C

-6.50

35

7.4

T (oC) 50 60 70 80 90

-7.50 -8.00

85

7.1

95

7.0 0

20

40 60 t (min)

80

100

Fig. 5. The kinetics of lycopene degradation in tomato pulp with different moisture contents (X, % w.b.) heated at 90 °C (C in lg/g total solids).

The slope of each line in Figs. 1–5 is the reaction rate constant k at a specified temperature and for a given moisture content. As it can be drawn from these figures, the reaction rate constant was dependent on moisture content, in addition to temperature. k at 50, 60, 70, 80, and 90 °C varies from 0.000261 to 0.000553 min
1, 0.000313 to 0.000676 min
1, 0.000390 to 0.000842 min
1, 0.000468 to 0.001028 min
1, and 0.000555 to 0.001216 min
1, respectively. This is not surprising as next to temperature, moisture content is now considered as probably the most important parameter having a strong effect on deteriorative reactions (Jayaraman & Das Gupta, 1995). A number of relations have been suggested to describe the dependence of reaction rate constants on moisture content or on water activity. Sometimes a linear relation between moisture content or water activity and reaction rate provides a satisfactory approximation of the behavior of a nutrient retention during drying. Beetner, Tsao, Frey, and Lorenz (1976) found that retention of thiamine and riboflavin in extrusion is affected by several factors (including moisture content), each of which could be approximated by a linear relationship that was valid within the narrow range of experimental values studied. However, several authors who studied browning of vegetables reported an exponential relation between the rate of browning and moisture content (Karel, 1979), whereas oxidation of potato chips was related to the function aw1=2 , where aw is the water activity (Quast & Karel, 1972). Labuza (1972) suggested a linear relation between the logarithm of the reaction rate constant and water activity. The plots of natural logarithm of k values versus moisture content for each temperature follow an approximate linear relationship as shown in Fig. 6 (R2 = 0.973– 0.997). As it can be concluded, chemical reaction is slower as the moisture content decreases from 95% to 55%. Some of the water-soluble compounds may act as catalysts to the degradation process. These catalytic effects are greatly reduced as the moisture is removed. The increased viscosity of the solution hinders the mobil-

41

-8.50 -9.00 20

30 40 50 60 70 80 90 100 moisture content, X (% w.b.)

Fig. 6. Relationship between natural logarithm of lycopene degradation rate constant (k, min
1) and tomato pulp moisture content (X, % w.b.) for different pulp temperatures (T, °C).

ity of the catalyst. In addition, as the water content decreases lycopene, which is not soluble in water, may form saturated solution and be precipitated. However, there is a minimum reaction rate constant of degradation when the moisture content of tomato pulp is between 50 and 55%. This trend is similar to that reported for bcarotene degradation during drying of carrot (Pan et al., 1998). In that case, there was a minimum reaction rate constant when the moisture content of carrot was between 30 and 40%. According to Jayaraman and Das Gupta (1995), carotenoid pigments in carrots are most stable at aw of 0.43. Generally, for the lipid-soluble nutrients, such as lycopene, there is a minimum in the reaction rate at aw of 0.3–0.4. This minimum is caused by the balance between catalyst hydration, mobility of catalysts, hydration of intermediates in the sequence, and free radical quenching (Bluestein & Labuza, 1988). However, for the lycopene degradation during drying of tomato the minimum reaction rate constant was at aw of 0.9. This higher aw may be explained by water sorption phenomena. Sorption of water with high sugar foods, such as tomato products, is likely to contain more liquid water at lower water activities. If more liquid water is present, the volume available for reaction is greater and the aqueous phase has a lower viscosity. According to Bluestein and Labuza (1988), the viscosity of the aqueous environment is one of the most important factors in controlling the destruction of nutrients. The lower the viscosity, the higher is the rate. The effect of product temperature and moisture content on lycopene degradation rate constant can be expressed by the following equations:
 2317 k ¼ 0:121238  expð0:0188  X Þ  exp
ðmin
1 Þ T for X P 55

ðR2 ¼ 0:996Þ

ð6Þ


 2207 k ¼ 0:275271  expð
0:00241  X Þ  exp
ðmin
1 Þ T for X 6 55 ðR2 ¼ 0:998Þ

ð7Þ

A.M. Goula et al. / Journal of Food Engineering 74 (2006) 37–46

where X is the product moisture content in % (wet basis) and T is the product temperature in K. In order to estimate whether lycopene was degraded into colorless form or was isomerized into another form of lycopene, lycopene extract was scanned from 250 to 550 nm. Identification of cis isomers was based on the appearance of extra maxima between 320 and 360 nm (Sharma & Le Maguer, 1996). The scanning pattern band for the unprocessed and thermally treated samples showed no difference in the three major long-wave absorption bands which were 445, 470 and 505 nm. However, the areas of peaks were smaller for the treated samples than for the unprocessed ones, indicating the lycopene loss. The peak of cis lycopene was observed at 362 nm, which was at the same wavelength as reported in the literature (Sharma & Le Maguer, 1996). The area of the cis lycopene was also smaller in the processed samples and a visible absorption band had not shifted in the scanning pattern, suggesting that thermal treatment of tomato pulp had not induced any isomerization. As described by Lovric, Sablek, and Boskovic (1970), Noble (1975), and OÕNeil, Schwartz, and Catignani (1991), an increase in absorbance at 360 nm, producing dissimilar absorbance curves, would have indicated trans ! cis isomerization of lycopene. Thus, the decrease in lycopene content reported here was due to an actual degradation of lycopene, rather than to a progressive conversion from the all-trans lycopene to a less strongly colored, less intensely absorbing cis form. This was in marked contrast with the increase in the levels of cis isomers during a range of heat treatments of tomato products (Schierle et al., 1997; Shi, Le Maguer, Kakuda, Liptay, & Niekamp, 1999). However, many studies have recently indicated that common heat treatments of tomato products do not result in a shift in the distribution of cis-lycopene isomers (Khachik et al., 1992; Nguyen & Schwartz, 1999).

Calculating the reaction rate constants k1, k2, . . ., kn from the mathematical model (6) or (7), the concentration of the lycopene corresponding to the time tn can be given as: ! n X C n ¼ C 0  exp
k i Dti ð8Þ 1

Fig. 7 shows the drying curves for the two concentration processes (75 and 90 °C). As it was observed, the drying rate increased on increasing the temperature. Approximately 180 min was required at 75 °C and 160 min at 90 °C to remove water of tomato pulp to moisture content of 40%. It means that increase of product temperature from 75 to 90 °C can shorten the concentration time by 10 min. Fig. 8 presents the experimental and simulated values of lycopene retention. During concentration at 75 °C a lycopene loss of approximately 10% occurred, whereas at 90 °C the lycopene degradation was slightly higher

X (%, w.b.)

42

100 90 80 70 60 50 40 30 20 10 0 0

20 40 60 80 100 120 140 160 180 200 t (min)

Fig. 7. Drying curves during concentration of tomato pulp at: (a) 75 °C () and (b) 90 °C (j).

3.2. Prediction of lycopene degradation during concentration of tomato pulp 1.00

Time Interval Temperature Moisture content Concentration

0

t1 Dt1 T1 X1

C0

C1

t2 Dt2    T2    X2   

 Dti Ti Xi

ti   

   tn Dtn Tn Xn

C2    Ci    Cn

where Ti and Xi are the mean values of temperature and moisture content in the relevant interval.

0.98 0.96

Ci/C0 (-)

For a first-order reaction, the change of a quality index can be obtained from Eq. (3) if temperature and water concentration at each moment are known. Let us divide the drying time into some intervals:

0.94 0.92 0.90 0.88 0.86 30

40

50

60

70

80

90

X (%, w.b.)

Fig. 8. Experimental (symbols) and simulated (lines) values of lycopene retention during concentration of tomato pulp at: (a) 75 °C (, - - -) and (b) 90 °C (j, ––).

A.M. Goula et al. / Journal of Food Engineering 74 (2006) 37–46

and resulted in a loss of 12%. It can be concluded that there was a close agreement between the experimental and predicted values confirming the validity of the proposed model for a concentration process.

43

70 65 60

T (oC)

55

3.3. Prediction of lycopene degradation during spray drying of tomato pulp

Experiment

50

1 2 3 4 5 6 7 8

45

The values of the variable operating conditions and the measured outlet air temperature (To) for each spray drying experiment are listed in Table 1. Data for To represent average values of the three replications. The repeatability for To expressed as the average standard deviation of the three replications was 0.29 °C. Fig. 9 shows drying kinetics of tomato pulp for each spray drying experiment, whereas Fig. 10 presents the relevant temperature profiles of the product. As it was observed, in all experiments drying was characterized by a short equilibration period, during which solid surface conditions came into equilibrium with the drying air. The constant rate period was not exhibited and drying moved quickly to the falling rate period characterized by a two-stage phenomenon. As it was concluded, calculation of the lycopene degradation rate constant from the mathematical model (6) or (7) and subsequent use of Eq. (8) was not sufficient to predict lycopene degradation during spray drying of Table 1 Experimental spray drying conditions No.

Ti (°C)

Qa (m3/h)

Qc (l/h)

To (°C)a

1 2 3 4 5 6 7 8

110 130 110 130 110 130 110 130

19.25 19.25 19.25 19.25 17.50 17.50 17.50 17.50

500 500 700 700 500 500 700 700

68.33 77.67 70.17 80.17 66.33 75.33 68.00 78.17

a

(0.29) (0.29) (0.29) (0.29) (0.58) (0.29) (0.00) (0.29)

Means of three replicates and standard deviation (in brackets).

90 Experiment

80

1 2 3 4 5 6 7 8

X (%, w.b.)

70 60 50 40 30 20 10 0 0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

t (s)

Fig. 9. Drying curves during each spray drying experiment (Table 1).

40 35 30 0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

t (s)

Fig. 10. Product temperature profiles during each spray drying experiment (Table 1).

tomato pulp. This was due to the fact that lycopene degradation in tomato products during thermal treatments is dependent on the presence of oxygen and light, in addition to product temperature and moisture content, and the extent of this dependence is strongly influenced by the product form. Cole and Kapur (1957) observed 25% apparent lycopene loss in the presence of oxygen and only 5–8% in the presence of carbon dioxide when heating tomato pulp at 100 °C for 2 h. Shi et al. (1999) studied the effect of tomato dehydration techniques on lycopene retention and concluded that after osmotic dehydration, total lycopene retention in tomatoes was greater than those using conventional air drying. A probable explanation was that the sugar solution keeps oxygen from the tomatoes and reduces the oxidation of lycopene in the tomato tissue matrix. According to Sharma and Le Maguer (1996), the apparent reaction rate constant values of lycopene degradation are lowest under vacuum and dark and highest under air and light at each temperature. In addition, Sharma and Le Maguer reported the effect of product form on the extent of lycopene degradation. When freeze- and oven-dried tomato samples were stored in closed containers at room temperature, the lycopene loss was higher in the freeze-dried samples (97%) than in the oven-dried ones (73.3– 78.9%) after 4 months of storage. The freeze-dried samples were more voluminous and fluffy in texture compared with the thin crust of sheets of oven-dried samples. Probably, exposure of freeze-dried fibers to air and light caused lycopene loss at a faster rate. The significant effect of drying method, and consequently of product form, on the various degradation reactions occurring during drying was observed for several products, in addition to tomatoes (Krokida, Maroulis, & Marinos-Kouris, 1998; Krokida, Tsami, & Maroulis, 1998). Thus, in spray drying, where the product has the form of droplets, the large surface exposed to air enhances lycopene oxidation and so, the lycopene loss is faster than in a concentration

A.M. Goula et al. / Journal of Food Engineering 74 (2006) 37–46

k0 ¼ a  k

ð9Þ

Based on the experimental data of lycopene retention, the value of the correction coefficient (a) for each spray drying experiment was determined using Eqs. (6)–(9). These values were significantly different (p = 0.05) between drying experiments. This should be expected since in a spray drying process the surface exposure to air is greatly influenced by the process conditions. Hawkes and Villota (1989) examined the folate degradation during spray drying of CMC-folic acid mixture and observed that, in addition to inlet and outlet air temperatures, major variables involved in folate stability included oxygen availability as influenced by particle size, which in turn was controlled by processing conditions such as feed rate and atomizer speed. In a previous work (Goula & Adamopoulos, 2005), the stability of lycopene during spray drying of tomato pulp under various operating conditions was studied. The extent of loss found to be influenced not only by inlet and outlet air temperatures and droplet moisture content but also by factors such as oxygen and light exposure, which in turn, being dependent on particle size, degree of aggregation, gas-to-feed flow ratio and air humidity, were controlled by processing conditions such as feed rate, initial feed solids concentration, drying and compressed air flow rate. According to Hawkes and Villota (1989), application of the traditional first-order rate equation and an Arrhenius correlation, based on the kinetic data collected from initial experiments, in combination with the processing variables used in spray drying, such as inlet temperature and atomizer speed, will provide a means of predicting a nutrient retention in a spray dried food. Multiple regression analysis was used to develop an equation predicting the effect of spray drying conditions on the coefficient a. The following equation was derived: a ¼
35203 þ 99  T i þ 0:111  ðR2 ¼ 0:998Þ

Qa
1:57  MSD2 Qf ð10Þ

where Ti is the air inlet temperature in K, Qa is the drying air flow rate in m3/h, Qf is the feed flow rate in m3/h and MSD the mean Sauter diameter of the atomized droplets in lm. The MSD value for each level of the atomizing agent flow rate was calculated by collecting the atomized droplets in a Petri dish, where the surface was covered with a layer of non-volatile silicone oil, examining the image obtained from a microscope and measuring the droplet size distribution (Goula & Adamopoulos, 2004). This empirical model will provide a means of predicting lycopene retention during spray

drying. Such models will most likely have to be tested in different types of spray dryers of larger scale, but a few simple test runs would allow for any adjustments that need to be made. Calculating the coefficient a from Eq. (10) and the rate constants from Eqs. (6), (7), and (9), the lycopene degradation during the spray drying experiments can be predicted (Fig. 11). As it can be drawn from Fig. 11, lycopene loss increases with an increase in drying and compressed air flow rate and air inlet temperature, effects that are associated with the influence of spray drying conditions not only on product temperature, but also on its moisture content and oxygen and light exposure (Goula & Adamopoulos, 2005). To compare the experimental and simulated values of final lycopene retention for all spray drying experiments, linear regression and paired t-test were applied. Graphic comparison between experimental and simulated values is shown in Fig. 12. When calculating the slope and intercept of simulated versus experimental values, no statistical differences (p = 0.05) were found from the theoretical values 1.00 and 0.00 respectively. In addition, the calculated t values were lower than the theoretical t values (p = 0.05). Therefore the null hypothesis was re-

1.00

Experiment

1 2 3 4 5 6 7 8

0.98 0.96

Ci/C0 (-)

process and a correction coefficient must be introduced in Eqs. (6) and (7):

0.94 0.92 0.90 0.88 0.86 0.84 0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

t (s)

Fig. 11. Simulated values of lycopene retention during each spray drying experiment (Table 1).

Ct/C0 (%) - simulated values

44

92 Experiment

1 2 3 4 5 6 7 8

90 88 86 84 84

86 88 90 92 Ct/C0 (%) - experimental values

Fig. 12. Experimental and simulated values of final lycopene retention for each spray drying experiment (Table 1).

A.M. Goula et al. / Journal of Food Engineering 74 (2006) 37–46

tained: the simulation data on lycopene preservation agreed well with the experimental data confirming the validity of the proposed corrected model for a spray drying process of tomato pulp.

4. Conclusions Lycopene content in tomato pulp with different moisture contents decreases during heating at different temperatures. The kinetics of lycopene degradation follows a first-order reaction with a reaction rate constant dependent on product moisture content, in addition to temperature. These effects can be expressed by a linear relationship between temperature, moisture content and natural logarithm of rate constant. In addition, there is a minimum reaction rate constant of degradation when the moisture content of tomato pulp is between 50 and 55%. The model giving lycopene degradation rate constant as a function of material temperature and moisture content in conjunction with the time–temperature–moisture content distribution in the product can accurately predict lycopene conversion rate during a concentration process of tomato pulp. However, in a spray drying process, where the product has the form of droplets, the large surface exposed to air enhances lycopene oxidation and so, the lycopene loss is faster than in a concentration process. Thus, a correction coefficient, expressed as a function of the spray drying conditions, must be introduced in the proposed model.

References AOAC (1980). Official methods of analysis of the association of official analytical chemists (13th ed., p. 538). Washington: Association of Official Analytical Chemists. Beetner, G., Tsao, T., Frey, A., & Lorenz, K. (1976). Stability of thiamine and riboflavin during extrusion processing of triticale. Journal of Milk and Food Technology, 39(4), 244–245. Bluestein, P. M., & Labuza, T. P. (1988). Effects of moisture removal on nutrients. In E. Karmas & R. S. Harris (Eds.), Nutritional evaluation of food processing (pp. 289–322). New York: Van Nostrand Reinhold. Bramley, P. M. (2000). Is lycopene beneficial to human health? Phytochemistry, 54, 233–236. Cole, E. R., & Kapur, N. S. (1957). The stability of lycopene. II. Oxidation during heating of tomato pulps. Journal of the Science of Food and Agriculture, 8, 366–368. Dumoulin, E., & Bimbenet, J. J. (1998). Mechanical, physical and chemical phenomena during food drying: Consequences on properties of dried products. In Proceedings of the 11th international drying symposium IDSÕ98, Halkidiki, Greece. Escher, F., & Blanc, B. (1977). Quality and nutritional aspects of food dehydration. In W. K. Downey (Ed.), Food quality and nutrition (pp. 297–322). UK, London: Applied Science Publishers. Franceschi, S., Bidoli, E., Vecchia, C., Talamini, R., DÕAvanzo, B., & Negri, E. (1994). Tomatoes and risk of digestive-tract cancers. International Journal of Cancer, 59, 181–184.

45

Goula, A. M., & Adamopoulos, K. G. (2003). Simulation of a spray drying process using computational fluid dynamics. In Proceedings of the symposium EUDryingÕ03, Heraklion, Crete, Greece. Goula, A. M., & Adamopoulos, K. G. (2004). Influence of spray conditions on residue accumulation––Simulation using CFD. Drying Technology, 22(5), 1107–1128. Goula, A. M., & Adamopoulos, K. G. (2005). Stability of lycopene during spray drying of tomato pulp. Lebensmittel-Wissenschaft und Technologie, 38(5), 479–487. Gould, W. A., & Gould, R. W. (1988). Physical evaluation of color. In Total quality assurance for the food industries (pp. 231–233). Maryland, Baltimore: CTI Publications. Hawkes, J. G., & Villota, R. (1989). Prediction of folic acid retention during spray dehydration. Journal of Food Engineering, 10(4), 287–317. Jayaraman, K. S., & Das Gupta, D. K. (1995). Drying of fruits and vegetables. In A. S. Mujumdar (Ed.), Handbook of industrial drying (pp. 669–687). New York: Marcel Dekker. Karel, M. (1975). Dehydration of foods. In M. Karel, O. R. Fennema, & D. B. Lund (Eds.), Principles of food science. Part II. Physical principles of food preservation (pp. 309–357). New York: Marcel Dekker. Karel, M. (1979). Prediction of nutrient losses and optimization of processing conditions. In S. R. Tannenbaum (Ed.), Nutritional and safety aspects of food processing (pp. 234–263). New York: Marcel Dekker. Khachik, F., Goli, M. B., Beecher, G. R., Holden, J., Lusby, W. R., Tenorio, M. D., et al. (1992). Effect of food preparation on qualitative and quantitative distribution of major carotenoid constituents of tomatoes and several green vegetables. Journal of Agriculture and Food Chemistry, 40, 390–398. Klaui, H., & Bauernfeind, J. C. (1981). Carotenoids as colorants and vitamin A precursors. UK, London: Academic Press, pp. 47–105. Krokida, M. K., Maroulis, Z. B., & Marinos-Kouris, D. (1998). Effect of drying method on physical properties of dehydrated products. In Proceedings of the 11th international drying symposium IDSÕ98, Halkidiki, Greece. Krokida, M. K., Tsami, E., & Maroulis, Z. B. (1998). Kinetics on color changes during drying of some fruits and vegetables. Drying Technology, 16(3–5), 667–685. Labuza, T. P. (1972). Processing and storage effects on nutrients in dehydrated foods. CRC Critical Reviews in Food Technology, 3, 217–240. Leniger, H. A., & Bruin, S. (1977). The state of the art of food dehydration. In W. K. Downey (Ed.), Food quality and nutrition (pp. 281–295). UK, London: Applied Science Publishers. Lovric, T., Sablek, Z., & Boskovic, M. (1970). Cis–trans isomerization of lycopene and colour stability of foam-mat dried tomato powder during storage. Journal of the Science of Food and Agriculture, 21, 641–647. Micozzi, M. S., Beecher, G. R., Taylor, P. R., & Khachik, F. (1990). Carotenoid analyses of selected raw and cooked foods associated with a lower risk for cancer. Journal of National Cancer Institute, 82, 282–288. Miki, N., & Akatsu, K. (1970). Effect of heat sterilization on the color of tomato juice. Nihon Shokuhin Kogyo Gakkai, 17, 175–181. Nguyen, M. L., & Schwartz, S. J. (1999). Lycopene: Chemical and biological properties. Food Technology, 53(2), 38–45. Noble, A. C. (1975). Investigation of the color changes in heat concentrated tomato pulp. Journal of Agriculture and Food Chemistry, 23, 48–49. OÕNeil, C. A., Schwartz, S. J., & Catignani, G. L. (1991). Comparison of liquid chromatographic methods for determination of cis–trans isomers of b-carotene. Journal of the Association of Official Analytical Chemists, 74, 36–42. Pan, Y. K., Zhao, L. J., Dong, Z. X., Mujumdar, A. S., & Kudra, T. (1998). Quality interaction in thermal dehydration of biological

46

A.M. Goula et al. / Journal of Food Engineering 74 (2006) 37–46

materials. In Proceedings of the 11th international drying symposium, Halkidiki, Greece. Quast, D. G., & Karel, M. (1972). Effects of environmental factors on the oxidation of potato chips. Journal of Food Science, 37(4), 584–588. Schierle, J., Bretzel, W., Buhler, J., Faccin, N., Hess, D., Steiner, K., et al. (1997). Content and isomeric ratio of lycopene in food and human blood plasma. Food Chemistry, 59, 459–465. Sharma, S. K., & Maguer, M. (1996). Le Kinetics of lycopene degradation in tomato pulp solids under different processing and storage conditions. Food Research International, 29(3–4), 309– 315.

Shi, J., Le Maguer, M., Kakuda, Y., Liptay, A., & Niekamp, F. (1999). Lycopene degradation and isomerization in tomato dehydration. Food Research International, 32(1), 15–21. Sokhansanj, S., & Jayas, D. S. (1995). Drying of foodstuffs. In A. S. Mujumdar (Ed.), Handbook of industrial drying (pp. 617–625). New York: Marcel Dekker. Southon, S. (2000). Increases fruit and vegetable consumption within the EU: Potential health benefits. Food Research International, 33, 211–217. Zanoni, B., Peri, C., Nani, R., & Lavelli, V. (1999). Oxidative heat damage of tomato halves as affected by drying. Food Research International, 31(5), 395–401.