Two-stage water blanching of asparagus

l'~llu T T E R W O II~IE I N E M

A

R T H N N

0140-7007(94)00007-7

h~t. J. REJrig.Vol. 18, No. 3, pp 14,~ 152,1995 CopyrightI~ 1995ElsevierScienceLid and IIR Printed in Great Britain.All rights reserved 0140-7007~95.$l0.00+ .(10

Two-stage water blanching of asparagus R. L. Garrote, E. R. Silva and R. A. Bertone Instituto de Tecnolog}a de Alimentos (FIQ-UNL), Ciudad Universitaria, Casilla de Correo 428, (3000) Santa Fe, Argentina

R. Re Intec (Conicet-UNL), G~Jemes 3450, (3000) Santa Fe, Argentina Received 20 July 1992; revised 2 September 1994

Asparagus, Argentuille variety, was water blanched for different times at 76, 88 and 100 °C, in order to study thermal inactivation of peroxidase in asparagus tips and stalks. A two-stage blanching method, in which the asparagus stalk was initially immersed in the water bath for an interval after which the tip was immersed, was also evaluated. At 100 °C and at a blanching time between 50 and 60% of that used for the stalk, the peroxidase activity of the asparagus tip was similar to that of the asparagus stalk. A procedure was developed to predict residual peroxidase activity in asparagus stalks and tips during two-stage water blanching at 100 °C. Predicted values agreed well with experimental results.

(Keywords:vegetable;asparagus; blanching;design;temperature;peroxidase)

Blanchiment fi l'eau et en deux 6tapes des asperges On a soumis des asperfes de la varibtb Aryentuille ~ difJ~rentes durbes de blanchiment, h 76, 88 et I00 °C, dans le but d'~tudier l'inactivation thermique de la peroxydase dans les pointes et les tiyes. On a bgalement bvaluk une mbthods de blanchiment en deux ~tapes, qui consiste h immerger d'abord les tiges dans un bain d'eau pendant un certain temps, puis fi immerger les pointes. A 100 °C et h une duroc de blanchiment correspondant h 50 et 60% de celle utilis~e pour les tiges, I'activitb de la peroxydase des pointes d'asperges a #tb similaire h celle des tiges. On a mis au point une procbdure pour pr~voir l'activitb de la peroxydase rbsiduelle dans les tiyes et pointes d'asperges au cours du blanchiment h l'eau en deux btapes. On a obtenu une bonne corrblation entre les valeurs prbvues et les rbsultats expbrimentaux.

(Mots cl6s: 16gume;asperge; blanchiment; conception; temp6rature; peroxydase)

Canned and frozen asparagus, green or white, are products of high value. The international market for asparagus is very important, the tonnage being approximately 170000. Although canned asparagus is more important in volume, frozen white asparagus is being produced in increasing amounts. It is usual during the processing of canned white asparagus to perform the blanching operation in two stages, so that the tip receives a shorter thermal treatment than the stalk. Consequently the delicate structure of the tip is protected. The main objective of blanching in the processing of frozen vegetables is to inactivate enzymatic systems that might cause deterioration of the product during frozen storage. Peroxidase inactivation is an indication of good blanching practice, because this enzyme is the most resistant to thermal treatments 1'2. Blanching white asparagus in two stages to obtain the same residual peroxidase activity in the tip and in the stalk has not been previously studied. The objective of this paper was to investigate peroxidase activity in the different parts of the white asparagus, to determine its thermal inactivation in tip and stalk at 76, 88 and 100 °C, and to evaluate a simple method of estimating blanching

148

times or residual peroxidase activities during two-stage water blanching at 100 °C.

Materials and methods Raw material

The Argentuille variety of asparagus was obtained from the zone of Rosario, Argentina. Asparagus spears were sorted, washed, cut to a length of 12.5 cm and sized. In order to know the relation of weight of tip (3 cm) or stalk (9.5 cm) to total weight of whole asparagus (12.5 cm), the corresponding determinations of weight of tips and stalks were made for the different sizes of asparagus (base diameters equal to 0.95, 1.10, 1.20, 1.30, 1.40 and 1.60 cm). The sized asparagus used in the one- and two-stage blanching tests had a base diameter of 1.10cm and a weight of 10.52 g. The specific weight of asparagus was determined by weighing asparagus in air and in water. Blanching operation

Blanching runs were performed in a pilot plant water blancher, with a capacity of 30 dm 3 of water, heated by

Two-stage water blanching of asparagus

Nomenclature

Blot number Initial peroxidase activity in asparagus (AU g- 1) Cf Final peroxidase activity in asparagus (AU g - 1) d Equivalent diameter of asparagus (cm) Dr Decimal reduction time at reference temperature (min) Ea Activation energy (kcal mol- 1) fh Slope index of heating curve (min) F, Average inactivating value of blanching cooling process in asparagus (rnin) F~ Inactivating value of blanching-cooling process in the asparagus centre (min) F). Inactivating value of blanching cooling process in the 2 position of asparagus (min) Bi

Ca

stainless steel electric resistance elements with a temperature controller (_0.5°C) and agitation by compressed air. For the one-stage blanching tests, 15 asparagus spears were placed in a stainless steel basket with lid, completely immersing it in the water for the various blanching times at the prescribed temperature. After blanching, the asparagus was chilled in agitated water at 3 °C for the same time as used in the blancher. Blanching temperatures were 76, 88 and 100°C. Blanching times used were l, 2, 3 and 4 min at 76 and 88°C and 1.0, 1.5, 2.0 and 2.5 min at 100°C. For the two-stage blanching at 100°C, the asparagus was also placed in a stainless steel basket of special design; the asparagus (15 spears) was first immersed to a depth of 9.5 cm from its base, blanching the asparagus stalks for a prescribed time; finally, the whole asparagus spears were completely immersed, blanching the tips and finishing the stalks; chilling was at 3 °C in agitated water. Kinetic data of peroxidase thermal inactivation (D, and /=a) were determined at the three working temperatures following Lund et al. 3 and Garrote et al. 4 methodology. Peroxidase activity determination

Peroxidase activity, expressed as AU g-1 (absorption units per gram), was determined in the tips, in the stalks and in the whole fresh asparagus 5. Once blanched, the whole asparagus were cut at 3 cm from the tip, obtaining lots of stalks and tips; residual peroxidase activities were determined in each of these lots. For the blanching in two stages, a similar methodology was followed. In all cases three replicates were obtained. Temperature profiles

In order to predict blanching times theoretically for asparagus it was necessary to determine temperature profiles during blanching at 100°C for different times. Temperature was measured by placing a Cu-constantan thermocouple in the centre of the whole asparagus spear at 7 cm from its base, connecting it to a Leeds and Northrup potentiometer. Assuming that the asparagus is a finite cylinder, with an equivalent diameter calculated from its volume, and using the analytical solution for

h Jh Jc Rg Tc Th Ti T, Ts Tw t t~ th

Asparagus height (cm) Intercept coefficient of heating curve Intercept coefficient of cooling curve Universal gas constant (kcal mol- 1 K - 1) Cooling temperature Heating temperature Asparagus initial temperature ('~C) Reference temperature (°C) Temperature of blanching bath (:C) Temperature of cooling water (~C) Time (min) Cooling time (min) Heating time (min)

x

= G/Ci

z

Slope index of thermal inactivation time curve

p

('c)

Thermal diffusivity of asparagus (m 2 s- 1) Specific weight (kg m 3)

unsteady heat conduction6, thermal diffusivities during heating, ~, that minimized the variances were determined 7. Four tests were performed and a mean thermal diffusivity was determined. The same thermal diffusivity was used for heating and cooling of asparagus. Results and discussion

Asparagus specific weight was 960 kg m-3. The weight relation of tip to whole asparagus was 15.64% ( + 0.83 % at 99% confidence level) considering all asparagus sizes, from 0.95 to 1.60 cm base diameter. Average peroxidase activity was 6.67 UA g-1 in the whole fresh asparagus, 7.68 AU g- 1 in the tip and 6.40 AU g- ~ in the stalk. The weight relation of tip to whole asparagus shows that asparagus becomes thinner as the tip is reached; in addition, the asparagus tip structure, characterized by a lot of small leaves making an extended surface, suggests that heat transfer in the tip would be faster than in the stalk and, consequently, thermal inactivation of peroxidase might be greater in the tip than in the stalk during one-stage blanching. Fiyures 1 and 2 show data confirming that peroxidase in the tips is inactivated more quickly than in the stalk at any condition of time-temperature used. It is seen that at 76 and 88 °C peroxidase inactivation is not important, at least at practical times used in industry. At 100 °C a high thermal inactivation is obtained with short blanching times. For example, at a blanching time of only l min, residual peroxidase activity was 6.62% in asparagus tips and 38.67% in stalks. At 2.5 min, it is possible to get a significant inactivation of peroxidase in the stalk (residual activity of 4%), while in the tips residual activity was very low (0.84%). Experimental data reveal that at 100 °C and at different blanching times, the residual peroxidase activity in the stalks is 5-7 times greater than the activity in the tips. In order to obtain equal residual peroxidase activity in both the tips and stalks, blanching time for the tips would be lower than that for the stalks. Consequently, two-stage blanching would seem to be a feasible way of improving the quality of frozen white asparagus.

149

R. L. Garrote et al.

100

variety and this size of stalk, the blanching time for the tips was between 50 and 60% of the blanching time for the stalks in order to get almost equal residual peroxidase activities. It may be concluded that blanching in two stages inactivates peroxidase in similar proportions in tips and stalks of white asparagus, thus protecting the delicate structure of the tip and consequently improving the final quality of the frozen a s p a r a g u s .

50 r = - 08745

(3 fi)

o

10

88 ° ~ 9 7 4 4

Estimating residual peroxidase activity during the blanching-cooling of asparagus

o_

° ~ O o c

[12

1

-09877 ,

05

I

i

t

i

I

2

3 Blonching time (rain.)

0

4

Residual peroxidase activity in asparagus tips during blanching of whole asparagus. Asparagus weight: 10.52 g Figure I

Several authors have studied the prediction of blanching times for different foods 7-~°. Because the range of asparagus sizes is very wide, it is difficult to predict blanching times or residual peroxidase activity in a simple and practical way. We have used two procedures to estimate the residual peroxidase activity during the two-stage water blanching of asparagus. The first procedure is a reference analytical

Figure 1 Activi/~ de la pero.vydase r~;siduelle dans les pointes des asperqes au cours du blanchiment de I'asperqe °ntis;re. Poids de I'a.v~er#e 10,52 #

100 - -

I00

76 °C r =-08595

J

76 °C

.

r= 08751

I

+

5O

+

° 69

>

o~ 5o

~ >_

7

~J

~

IO

2 ~

5

r = - 09439 u)

QE

CE

1

o

--_

I 1

L___

I 2

,

I

3

,

09816 1

I 4

= 0--

BIonching time ( min. ) Figure 2 Residual peroxidase activity in asparagus stalks during blanching of whole asparagus. Asparagus weight: 10.52 g Figure 2 Actil,it~ de la pero.~3'dase r~siduelle dans les tiges des asperges au tours du blanchiment de I 'asper.qe enti~re. PoitL~"de l'asper.qe 10.52 ~t

Figure 3 shows residual peroxidase activity as a function of blanching time and temperature in whole asparagus. The values obtained are similar to those in Figure 2, which is reasonable because weight and residual peroxidase activities in the tips are significantly lower than in the stalk. Consequently, asparagus blanching was studied in two stages at 100°C. Table 1 shows the blanching times used in each asparagus part and the final residual peroxidase activities obtained. At a blanching time of 1 min, the peroxidase activity in the stalk is still very significant; the combination of 1.5 min exposure in the tip and 2 min in the stalk shows a reasonable residual peroxidase activity in the stalk and a low activity in the tip; exactly the opposite result is obtained when the combination is 1 min blanching time in the tip and 2.5 rain in the stalk. At blanching times of 1 min in the tip and 2 min in the stalk, or 1.5 min and 2.5 min respectively, equal residual peroxidase activities were obtained (5% and 1.65% respectively). For this asparagus

150

I + 1

i

i 2

E

Blenching

time

i 5

~

I 4

(rain.)

Figure 3 Residual peroxidase activity in the whole asparagus as a function of blanching time. Asparagus weight: 10.52 g Figure 3 Actirit~ de la peroxydase r~siduelle duns les asperges entig,re.~ en Jbnction de la dur~;e de blanchiment. Poids de I'asperge 10,52 g

Residual peroxidase activity in asparagus stalks and tips during the two-stage water blanching at IO0'C for various times Tableau 1 Actil:it~;de [a pero+vydase r~siduelle dans les liges et poilltes Table !

d'a,sper,qes at+ tours d'un bhmchiment h I'eau, en deux L,tapes, h IO0~'C et pour diff2;rentes durbes Residual

Blanching time in each part

peroxidase activity

(min)

(%)

Tip Stalk

1.0 1.0

4.92 42.48

Tip Stalk

1.0 1.5

5.02 19.42

Tip Stalk

1.5 2.0

1.63 4.92

Tip Stalk

1.0 2.5

5.02 1.65

Asparagus" part

"Asparagus weight = 10.52 g

Two-stage water blanching of asparagus equivalent diameter

100

v/p d= 8O

60

2

20-

o

I

I

I

[

I

2 3 Time (rain)

4

5

Figure 4. Temperature profile of the asparagus centre during blanching-cooling. T~ = 100°C; T~ = 23,50°C; Tw = 3°C; blanching time = 2.5 min. Asparagus weight: 12,60 g. Equivalent diameter = 1.156 cm

Figure 4 Profil de temperature du centre de I'asperqe au tours du blanchiment r<[hfidissement. T, = 100 ':C," T i = 23,50°C; T w = 3°C; durbe de blanchiment 2.5 ,tin; poid~' de I'asperge 12,60 g," diambtre bquivalent 1,156 cm

model in which x = Cf/Ci is estimated as

:exp,_2 303( f) exp[_< (1 L Rg Th (

Dr

t. + ~

Ea

1

i)]

7~a

dt

1

where Th and T~ are obtained from the solutions of the differential equations of unsteady heat conduction, supposing Bi = oo in heating and cooling; the average residual peroxidase activity ~ is obtained by numerical integration (Simpson method) over the whole finite cylinder volume. The second procedure uses the equation ~ = 10-eJD% and estimates F~ by the Stumbo Formula~: F~ = F~ + Dr log

[D ~+ IO'91(F~- F~!] D~

(2)

This formula has been verified by Garrote et al} 2 for estimating the degradation of nutrients and enzymes in thermally processed foods, and is quite convenient for its simplicity. For application of Equations (1) and (2) it is necessary to know ~, Dr, E~ or z, fh,Jh andj¢. Experimental values for thermal diffusivity of asparagus were 1.45, 1.55, 1.57 and 1.45× I0 - v m zs -~, with a mean value of 1.505 x 10 7 m 2 s - i (see Figure 4). For Dloo~c and E a we determined 0.783 min and 24.33 kcalmol -~ (z= 24.45 °C) respectively; these kinetic parameters are similar to those determined for broccoli peroxidase (D~oooc = 0.616 min and E, = 25 kcal mol- 1) by Singh and Chen 1°. fh, Jh andj~ were estimated by regression, considering the experimental or theoretical temperature profiles in the centre of the asparagus during blanching and cooling. The assumptions made for predicting residual peroxidase activity during the two-stage water blanching of asparagus at 100 °C were the following. 1. Asparagus is a finite cylinder of height 12.5 cm and

4 × Weight (g) (g cm---~x~ x ~-.5 cm

2. Initial peroxidase activities in tips and stalks of fresh asparagus are equal. 3. We define the blanching time for the stalk at 100 °C as the immersion time for which the stalk reaches an established residual peroxidase activity; the blanching time for the tip is the immersion time for which the tip reaches the same residue of peroxidase activity as the stalk. The time at which the tip is immersed in the water bath is the difference between the blanching time required for the stalk and the tip to reach the same residual peroxidase activity. 4. The blanching time for the stalk is estimated as if it were the blanching time for the whole asparagus; this restriction is not important as experimental results have shown that both peroxidase activities after any blanching time are similar. Blanching time for the tip is estimated as a blanching time for the stalk (considering that the whole asparagus is being blanched) in which its residual peroxidase activity is 6.48 times greater than in the tip. For example, if we estimate blanching times for a residual peroxidase activity of 2% in both parts, we first calculate the blanching time for the stalk to obtain a peroxidase residue of 2%; for estimating the blanching time for the tip we multiply 2% x 6.48 = 12.96% and we estimate the time necessary for obtaining this residue in the stalk, which will be the time that the tip has to be immersed in the blanching bath in order to get a residue of peroxidase of 2%. For different asparagus sizes and blanching times, we experimentally found that the relation of residual peroxidase activity in the stalk to that of the tip was 6.48 _ 2.10 (at 95% confidence level); this ratio is greater as residual peroxidase activity is lower. 5. Cooling time is equal to blanching time.

Table 2 shows predictions made for residual peroxidase activities in asparagus stalks. Percent deviations, according to Heldman ~3, between experimental and predicted values using Equations (1) and (2) were 13.36 and 22.73% respectively. Table 3 shows the estimated two-stage blanching times for the tips and stalks and compares them with experimental values for mean residual peroxidase activities (Table 1) of 4.97 and 1.64%. It is seen that estimates of blanching times for stalks are very good, Table 2 Theoretical and experimental residual peroxidase activity in asparagus stalks during the two-stage water blanching Tableau 2 A ctivitb de la pero.~vdase rbsiduelle thborique et expbrimentale dans les tiges d'asper,qes au tours d'un bhmchiment [ll 'eau en deux ~;tapes Residual peroxidase activity (%) Blanching time (min) 1.0 1.5 2.0 2.5

Experimental

Analytical reference model

Stumbo formula

42.48 19.42 4.92 1.65

44.95 17.54 5.20 1.33

44.08 14.72 4.34 1.18

Ts = 100 C ; T~ = 22 '~C; T, = 3 JC. Asparagus weight: 10.52 g. Equivalent asparagus diameter - 1.056 cm

151

R. L. Garrote et al. Table 3 Theoretical and experimental blanching times for tips and stalks during the two-stage water blanching of asparagus Tableau 3 Durbes de blanchiment thboriques et expbrimentales pour les pointes et les tiges au cours d'un blanchiment ~ l'eau et en deux ~tapes des asperges

Blanching time in each asparagus part (min) Residual peroxidase activity (%) 4.97 1.64

Experimental

Analytical reference model

Stumbo formula

Tip

1.0

1.198

1.175

Stalk

2.0

2.020

1950

Tip

1.5

1.716

1.642

Stalk

2.5

2.425

2.367

Ts = 100 ° C ; T i = 22 °C; Tw = 3 °C. Asparagus weight = 10.52 g. Equivalent asparagus diameter: 1.056 cm

Table 4 Predicted blanching times for tips and stalks during the two-stage water blanching of asparagus of different sizes, to obtain a 2% residual peroxidase activity Tableau 4 Dur~es de blanchiment pr~vues pour les pointes et les tiges cours d'un blanchiment ~t l'eau et en deux btapes d'asperges de tailles diffbrentes, dans le but d'obtenir une activitb de la peroxydase rbsiduelle de2%

Blanching time for tips may be estimated as t (blanching time for tip, min)= 0.70 x /stalk AS frozen white asparagus is marketed in no more than five sizes, it is believed that the quick and simple procedure described for estimating blanching times in a two-stage blanching process will give good results, and the final quality of the finished product will improve.

Acknowledgements The authors acknowledge the financial support of the Secretaria de Estado di Ciencia y Tecnologia de la Repfiblica Argentina (Programa Nacional de Tecnologia de Alimentos). They also gratefully acknowledge the technical assistance of Adriana Avalle.

References 1

art

Blanching time (min) Stalks Asparagus equivalent diameter (cm)

2 3

Tips

4 Analytical reference model 2.354 3.246 4.733

Stumbo formula

Analytical reference model

Stumbo formula

2.291 3.160 4.855

1.632 2.354 3.518

1.560 2.336 3.560

5 1.056 1.450 2.000

T, = 100 °C; T~ = 22 °C; Tw = 3 ~C

while those for tips are overestimated by between 14 and 19% (Equation (1)) or 9.33 and 17% (Equation (2)). Several estimations of blanching times for tips and stalks needed to obtain a 2% residual peroxidase activity during two-stage water blanching at 100°C of white asparagus of various sizes were made, as shown in Table 4; in general, it is seen that the blanching time for the tips is approximately 70% of that for stalks. A simple equation, which satisfies predictions made with Equation (1) in Table 4, was developed by regression, to estimate the blanching time for the stalk as a function of the equivalent diameter of asparagus: t (blanching time for stalk, min)= -0.3584 + 2.5318 x d (equivalent asparagus diameter, cm); r = 0.9988

152

6

7 8 9 10 11 12

13

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