Nitrate Extraction during Blanching of Spinach

Lebensm.-Wiss. u.-Technol., 30, 432–435 (1996)

Nitrate Extraction during Blanching of Spinach M. Gaiser, A. Rathjen and W. E. L. Spiess Federal Research Center for Nutrition, Institute of Process Engineering, Engesserstr. 20, D-76131 Karlsruhe, (Germany) (Received June 24, 1996; accepted August 30, 1996)

Blanching is suitable for reducing nitrate concentrations of vegetables. Ionic substances such as nitrate are extracted above 57 °C. At lower temperatures cell membranes retain their selectivity and are impermeable to solutes. Higher temperatures cause higher leaching rates with only slight differences between 80 and 90 °C. Nitrate extraction can be described by a model based on a fast washing-out of cell sap, followed by diffusional mass transfer through a limiting membrane. Temperature dependence of the diffusional transfer can be described by an Arrhenius relationship split into two areas of application: one in the temperature range from 60 to 80 °C and a second at temperatures higher than 80 °C. This model was used to calculate operating conditions in a continuously-working blancher running at steady state, and for batch processes.

©1997 Academic Press Limited Keywords: blanching; nitrate extraction kinetics; diffusion; spinach

Introduction Blanching, a thermal process preceding freezing or canning of vegetables, is necessary to inactivate shelflife limiting enzymes and to exhaust gas from the plant tissue (1). When carried out in water blanchers, a considerable loss of water soluble constituents by leaching may occur (2). Undesirable substances such as nitrate can therefore be reduced during this process. Nitrate is a precursor of nitrite or nitrosamines which may form intestinally (3). As the latter pose a potential risk for human health, the nitrate content of some vegetables is limited by law. Frozen spinach sold in Germany is not allowed to contain more than 2000 mg nitrate/kg spinach (4). Since raw spinach may contain much more, it is necessary to reduce higher levels to values below this limit. Blanching offers a solution to this problem. Many data describing leaching of water-soluble vegetable constituents during blanching have been reported (e.g. 5–8). Most of these, however, refer to peas and carrots and insufficient data is available for spinach (9, 10). In the present study the influence of water temperature on leaching of nitrate was investigated. A mathematical model was developed to describe this mass transfer in batch-wise and continuously working blanchers.

Material and Methods All experiments were carried out using fresh spinach

from a local farmers’ market. The spinach samples contained different nitrate concentrations varying between 53.1 mg/kg and 5532.3 mg/kg. The mean concentration was 1999.8 mg/kg with a standard deviation of 1411.4 mg/kg and a median concentration of 1648.0 mg/kg (60 samples examined). Prior to blanching, spinach samples were washed and weighed. Blanching experiments were conducted in shaken beakers (200 mL) at 60, 70, 80 and 90 °C in an agitated water-bath (frequency 120 min–1, amplitude 0.8 cm). The ratio of water : spinach was 120:10 (w/w). Samples were taken after 30, 60, 90, 120, 150 and 180 s, sterile filtrated (pore size 0.2 µm) and frozen. As nitrate contents varied greatly, experimental results were compared dimensionless. Original nitrate concentrations were calculated from concentrations in the blanching water after samples had been kept there for 90 min. As controls showed the nitrate concentration after this time to be constant, an equilibrium of nitrate contents of spinach leaves and blanching water was assumed to exist. Since nitrate has been reported (11) to be resistant to heat, including cooking temperatures, the values obtained can be assumed to be reliable. Water temperature inside the beakers was measured by a PT-100 device mounted in the lid. Nitrate was determined by HPLC (anion-exchange column Hamilton PRP-X100) and UV-detection. Some blanching experiments were carried out at 80 and 90 °C in a blancher with agitated baskets (frequency 100 min–1, amplitude 1.2 cm). During these experiments, samples were also taken after 4, 5, 7, 9, 15 and 30 min. In addition a plant cell membrane performance test was

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conducted in the same device by successively raising the temperature of the water-bath stepwise (10 °C) from 40 °C to 95 °C and measuring the electrical conductivity of the water. The ratio of water : spinach was about 1200:30 (w/w).

Results Plant cell membranes of fresh vegetables are not permeable to solutes as ions. Only water is able to penetrate intact cell membranes. During heat treatment, proteins in cell membranes are denatured above a certain temperature so that membranes become porous; cell membranes are then unable to maintain cell turgor. Solute containing cell sap leaves the cells. Membrane performance tests revealed that cells are permeable to ionic substances above 57 °C. Electrical conductivity in water-bath began to rise slowly from 55 °C and accelerated after 57 °C. A comparison of leaching curves at different water temperatures above 57 °C has shown the usual blanching temperatures of 80 °C and higher to be much more effective than lower ones. At water temperatures of 60 and 80 °C relatively low quantities of nitrate were extracted during the usual blanching time of less than 3 min, i.e. at 150 s: 4.5% at 60 °C and 7.8% at 70 °C. Extraction rates at 80 and 90 °C were much higher with 21% at 80 °C and 24% at 90 °C after 150 s. The differences in extraction rates at 80 and 90 °C were much lower than those between 80 and 70 °C. As water temperatures below 80 °C require extremely long blanching times to inactivate peroxidase sufficiently (1, 9), the low nitrate transfer rates in this range are of merely academic interest.

the leaching process this model was not suitable. A diagram of experimental data revealed a relatively sharp-edged curve of the extraction rate. For the following considerations the initial concentration was therefore assumed to be somewhat lower than the actual initial concentration of the raw material because of a quick loss of cell turgor after the first contact of cell tissue with hot water. After breakdown of the turgor, cell sap leaves the cells reducing the nitrate concentration of spinach at the beginning of the diffusional process while elevating the concentration of the blanching water: cSp,0 = (l–f)cSp,in + f·cw,in

(

cw,0 + l–f

mSp mw

)c

cSp,t =

(

exp[–β·a l +

msp mw

) t] (c

Sp,0

l +

– cw,0) + cw,0 +

mSp mw

cSp,0

mSp mw Eqn [1]

(See Nomenclature section for explanation of symbols.) Temperature is related to the pseudo-constant β.a. At higher temperatures, β.a increases. For the first 90 s of

mSp mw

cSp,in

Eqn [3]

Deff·S L·V

Eqn [4]

with L = 0.1 mm, S = 26.5 cm2 and V = 10.6 cm3. The extraction rate is assumed to depend on the temperature because of increased diffusion coefficients and more extensive cell membrane denaturation at higher temperatures. The temperature dependence of Relative nitrate concentration, cSp,t/cSp,0

The majority of leaching processes are described by diffusional models, using solutions of Fick’s laws in volumes of limited mass transfer. This is correct for slices of carrots (6) or other produce of homogeneous multilayer cell systems, but not in leafy vegetables consisting of only few layers with denser cells at the surface. Leaching from spinach can be regarded as mass transfer through one limiting membrane. Combining the kinetic equation with mass balances and integration one obtains:

+ f

where f = f(T) is a dimensionless temperature dependent washing-out factor. The curve reflecting data obtained from the extraction model fitted the experimental data much better (Fig. 1). Especially extraction within less than 2 min is predicted much better by using the extraction model. As this time period is typical of industrial blanching processes, the extraction model should be used to simulate nitrate extraction during blanching in industrial blanchers. The temperature-dependent kinetic parameters listed in Table 1 were determined by using the modified diffusional model. Diffusional coefficients Deff were calculated by means of the experimental values of β.a and approximated values of leaf surface S, volume of the plant sample V, and diffusional length L according to the relationship: β·a =

Mathematical Modelling

w,in

Eqn [2]

1

0.75

0.5

0.25

0

60 120 Blanching time, tb (s)

180

Fig. 1 Comparison of two diffusional models describing nitrate leaching at q = 90 °C. (s) = experimental data; (– – –) = diffusional model; (–––) = extraction model

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Table 1 Kinetic parameters to describe nitrate leaching during blanching of spinach

cSp,out =

β×a(10–3 m2/s) Deff (10–9 (m2/s))

Temperature (°C)

f/–

60 70 80 90

0.02 0.02 0.06 0.08

0.220 0.500 1.233 1.490

0.088 0.200 0.513 0.619

cSp,in·Zl + cw,in·Z2

Eqn [6]

N

(

with Z1 = (l–f) exp{–β·a·tb} l–

m ˙ Sp m ˙w

)+

Z2 = l–(l–f)exp{–β·a·tb} diffusion coefficients is described by the Arrhenius relationship:

{

Deff = D0 exp –

} R·T Ea

Eqn [5]

As this formula is suitable only for describing the increase of diffusion rates caused by greater Brownian molecular movement at rising temperatures, it is almost impossible to indicate a set of constants for the maximum diffusional coefficient D0 and activation energy Ea. This problem can be overcome by using constants valuable within certain temperature intervals (8). Figure 2 shows the Arrhenius plot for the coefficients presented in this paper. The parameters used in the temperature range of 60 to 80 °C are D0 = 2669 m2/s, and Ea = 86.0 kJ/mol, in the range of 80 to 90 °C D0 = 494.8 m2/s and Ea = 20.12 kJ/ mol. The latter are in good agreement with values reported in the literature (8). The error bars of the plots at 80 and 90 °C (Fig. 2) are calculated by means of the error bars of the extraction curves. For coefficients calculated at 60 and 70 °C no error bars were calculated because the data base for these temperatures was too small. As industrial blanchers work continuously, the batch model (Eqns 1–3) was adapted to continuous processes running at steady state. At preset temperatures and mass fluxes, linear relationships have been found to exist between the product’s nitrate concentration and the initial nitrate concentration of the raw material on the one hand, and the concentration of inflowing blanching water on the other.

Temperature, T (K) 363 1

353

343

333

Deff (10–9 (m2/s))

0.75 0.5

0.25

0.1

2.75

N = l–(l–f)exp{–β·a·tb}

m ˙ Sp m ˙w

+

m ˙ Sp m ˙w

Eqn [7] Eqn [8]

m ˙ Sp m ˙w

Eqn [9]

However, as continuous blanchers need a relatively long time to reach steady state (approximately 2 h), use of these equations is limited. Since spinach is not likely to show constant nitrate concentrations over hours, steady state might not be reached exactly. Moreover, spinach treated during this phase is surely below the limit if the expected steady state is under the limit too.

Nomenclature a β cSp,0 cW,0 cSp,in cW,in cSp,out cSp,t ct c0 D0 Deff f L mSp mw m ˙ Sp m˙w R S q T t tb V

Specific surface (surface related to volume) (m2/m) Mass transfer coefficient (m/s) Concentration in spinach at the beginning of diffusion (mg/kg) Concentration in water at the beginning of diffusion (mg/kg) Initial concentration in spinach (mg/kg) Initial concentration in water (mg/kg) Outlet concentration in spinach (mg/kg) Concentration in spinach at time t (mg/kg) Concentration at time t (mg/kg) Initial concentration at time t = 0 (mg/kg) Maximum diffusional coefficient (m2/s) Effective diffusional coefficient (m2/s) Dimensionless washing-out factor Diffusional length (mm) Mass of spinach sample (kg) Mass of blanching water (kg) Mass flux of spinach (kg/s) Mass flux of fresh water (kg/s) Gas constant (j (mol/K)) Surface area (cm2) Temperature (°C) Absolute temperature (K) Time (s) Blanching time (s) Volume (cm3)

References 2.8 2.85 2.9 2.95 3 Reciprocal temperature, 1/T (10–3 K–1)

Fig. 2 Relationship temperature

of

diffusional

coefficients

3.05

and

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3 COULTATE, I. P. Contaminants from food processing. Food Science and Technology Today, 9, 44–49 (1995) ¨ 4 Siebte Verordnung zur Anderung der Ruckstands-H ¨ ochst¨ mengenverordnung vom 22 February 1994. Bundesgesetzblatt, 1, 386 (1994) 5 PONNE, C. T., BAYSAL, T. AND YUKSEL, D. Blanching leafy vegetables with electromagnetic energy. Journal of Food Science, 59, 1037–1041 (1994) 6 VAROQUAUX, P., VAROQUAUX, F. AND TICHIT, L. Technical note: Loss of nitrate from carrots during blanching. Journal of Food Technology, 21, 401–407 (1986) 7 SWARTZ, J. B. Recycling of water in vegetable blanching. Food Technology, 33(6), 54–59 (1979) 8 GARROTE, R. L., SILVA, E. R. AND BERTONE, R. A. Losses by diffusion of ascorbic acid during water blanching of potatoe tissue. Lebensmittel-Wissenschaft und-Technologie, 19, 263–265 (1986)

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