J. Sci. FdAgric. 1975,26,1603-1608

Effect of Concentration Process on Cloud Stability of Reconstituted Lemon Juice

David Epstein and Shimon Mizrahi

Department of Food Engineering and Biotechnology, Technion, Israel Institute of Technology, Haifa, Israel

(Manuscript received 10 May I 9 74 and accepted 3 April 19 75)

The concentration process is a major factor in the low cloud stability of lemon juice reconstituted from a concentrate. A mechanism, based on the intermolecular bonding of pectin molecules, is suggested for this phenomenon, but actual proof of this has not yet been provided. It seems that the mechanism involved consists in reduction of the water activity below the critical level (inversely proportional to the pH of the system) necessary for maintaining the pectin in solution. This reduction favours an irreversible flocculation process through the intermol- ecular bonds of the pectin. This reduction favours an irreversible flocculation process through the intermolecular bonds of the pectin, thus resulting in the precipitation of the cloud particles trapped within the flocs.

1. Introduction

Lemon juice, an important ingredient of citrus beverages, is commonly marketed and stored in concentrate form. The obvious economic advantage of this practice is, however, offset by loss of the characteristic “cloud” of the juice, often observed shortly after reconstitution.

A well known phenomenon is the cloud-loss effect of lemon juice attributed to the demethoxylation of the pectin by pectin esterase (PE) (Bartholomewl). The extent of PE activity and the amount of pectin in lemon juice were reported by Rouse and Knorr.2 The enzymatic cloud-loss could be controlled by adequate heat treatment (Stevens et aZ.,3 Tressler and Joslyn4) or, at least, in the case of orange juice, by pectolytic enzymes capable of depolynierising pectic substances to soluble pectate (Baker and Bruemmer5).

However there are no data to indicate whether or not enzymatic activity or another mechanism (connected with the concentration process) is responsible for the cloud loss in reconstituted lemon juice.

The present study was undertaken in order to ascertain the role of the concentration process itself in the cloud loss of reconstituted lemon juice, in the absence of any PE activity.

2. Experimental The juice was extracted on an FMC in-line extractor (model 718), pasteurised for 30 s at 92 “C in an Alfa-Lava1 plate heat-exchanger (model PL-1-HB), concentrated

1603

1604 D. Epstein and S. Mizrahi

in a laboratory thermosyphon evaporator under vacuum (temperature 55-60 "C) at evaporation rate of 700 cm3/h (Berk and MizrahP), and reconstituted less than 2 days later to 7.5 "Brix. Both juice and concentrate were homogenised for 5 min, at maximum speed, on a Polytron disintegrator type 45/20D (Kinematica GmbH, Lucerne, Switzer- land). The samples, preserved with 1500 parts/l06 SO2 were stored in bottles. The pH was adjusted with HCl or NaOH as necessary.

Turbidity of the retained cloud was measured on a Klett-Summerson colorimeter equipped with filter No. 66.

Soluble pectin was determined on serum samples (see under Definitions, below) according to McComb and McCready.7 The degree of methoxylation was measured by saponification on the whole pectin content of the juice by the method of Hills et aL8

Effect of pectin precipitating solvent (i.e. acetone, ethanol and isopropanol) on cloud stability of lemon juice was measured by their addition to lemon juice and serum at different concentrations and determining the turbidity of the retained cloud. The results were corrected for the dilution effect of the added solvents on the turbidity.

2.1. Definitions Stable cloud is defined as the portion of suspended particles retained in suspension after 10 min centrifugation at 360 g , using an M.S.E. laboratory centrifuge with a 50 ml conical tube.

Serum is defined as the supernatant after 10 min centrifugation at 18 400 g , using a Sorvall centrifuge (model SS-34).

Water mole fraction (WMF) was calculated as follows :

where Nw and N8 are the number of moles of water and solutes respectively. Since lemon juice contains relatively small amounts of sucrose and minerals, a negligible error is involved in simplifying the calculation by considering the total solids as if com- posed only of citric acid and hexose sugars (MW 180). Moreover, in such a procedure the error caused by the high molecular weight (MW) sucrose was compensated by the low MW minerals. The following is an example of the method of calculation : The lemon juice sample was of 7.5"Brix and 5 % acidity (citric acid anhydrous). According to Stevens and Baierg the corrected Brix value was 8.5" of which 3.5" was attributed to the sugar fraction. I t can be shown that when this juice sample is concentrated to 40"Brix, each 100 g portion of the concentrate would contain 26.7, 18.7 and 54.6 g of acid, sugar and water respectively. The water mole fraction was calculated as follows (Eq. 1):

54.6/18 WMF= 54.6/18 + 26.7/192 + 18.7/180

The organic solvents, when added to the lemon juice, were in fact an additional solute to those already present in the aqueous solution. Therefore another term, the number of moles of organic solvent, was added to the denominator of equation 1.

Cloud stability of lemon juice 1605

3. Results and discussion

The results summarised in Table I indicate that the reconstituted juice, prepared from a 39 "Brix concentrate, exhibited a considerable loss of cloud (compared with the original single strength juice) without any change in soluble pectin content or in degree of methoxylation. This circumstance rules out pectin hydrolysis as the res- ponsible factor : in fact, the hydrolytic reaction was suppressed by the pasteurisation treatment and by minimising the time interval between concentration and reconstitution.

The results in Table 2 indicate that the cloud loss is recoverable by homogenisation, provided the treatment is applied to the reconstituted juice; homogenisation of the concentrate failed to yield a stable cloud after reconstitution, especially at dilution to 1 "Brix.

The effect of the degree of concentration on cloud stability is shown in Table 3; cloud loss occurs mainly in the 40-45 "Brix interval, and changes are very abrupt.

Table 1. Effect of concentration on cloud stability" ____________________

Soluble Saponified methoxyl Turbidity pectin groups

Sample (Klett units) ( %I (ml NaOH 1 N)

Original juice 325 0.05 3.9 Reconstituted juice 185 0.045 4.0 Standard error 2 0.003 0.03

a Results are average of 6 replicates.

Table 2. Effect of homogenisation on cloud stability

Turbidity Cloud stability Sample (Klett units) at 1 "Brix

Original juice 400 Stable Reconstituted juice 140 Unstable Reconstituted and homogenised

juice 355 Stable Reconstituted juice from

homogenised concentrate 250 Unstable

Table 3. Effect of degree of concentration on cloud stability

Degree of Turbidity concentration at 7.5 "Brix Cloud stability

("Brix) (Klett units) at 1 "Brix

8 310 14 280 18 215 22 270 31 268 40 260 44 140 50 62

Stable Stable Stable Stable Stable Transition Unstable Unstable

1606 D. Epstein and S. Mizrah

The effect of pectin-precipitating solvents (isopropanol, ethanol and acetone) was found (Figure 1) to be similar to that of the soluble solids during the concentration process: clarification of the juice occurred at a water molar fraction of about 0.92, indicating that the cloud loss in all cases is mainly effected by the reduction of the water activity. Actually, this is a “salting out” phenomenon where the cloud particles precipitate when solute concentration reaches a certain level.

Figure 1. Effect of reduced water activity on cloud stability. x , concentration; +, isopro- panol; A, ethanol; 6, acetone.

Water rnoior fraction

Reduction of solvent activity, of polymer solution by addition of solutes or proper solvents, is a commonly used method for the precipitation and fractionation of a polymer according to molecular size. The method is based on decreasing of solvent availability to the polymer molecules, so that their extent of solvation is reduced, to a level where they are no longer stabilised in solution and thus flocculation sets in. For high molecular weight species the flocculation and precipitation process is initiated at higher solvent activity compared with low molecular species of the same polymer. It seems reasonable to assume that in the case of lemon juice also the properties of the soluble polymer, namely pectin, as well as of the suspended particles will determine the value of the critical water activity level for the initiation of the precipitation process.

The similarity of the above mentioned phenomena due to the reduced water activity effect justifies the simulation of the concentration process by adding ethanol to lemon juice.

Cloud stability of lemon juice 1607

The results of this technique of adding ethanol to lemon juice and to its serum are summarised in Table 4. Considerable changes of turbidity take place both in the juice and in the serum within the same range of ethanol concentration. It seems therefore that the flocculation process is mainly caused by the soluble pectin.

Table 4. Effect of ethanol admixture on cloud stability of juice and serum

Amount of ethanol (% w/w>

0 5

10 15 20 25 30 35 40 45 50 55 60

_ _ _ _ ~

Turbidity of centrifuged juice

(Klett units)a

Turbidity of centrifuged serum

(Klett units)a

355 3 60 345 335 335 264

82 33 20 30 39 12 12

85 84 84 84 86 70 35 35 15 20 14 12 12

a Corrected for dilution with ethanol.

On the basis of the data referred to, a hypothesis may be put forward for the mecha- nism responsible for the cloud loss. Below a certain range of water activity, the extent of hydration of the pectin molecules is decreased to a level where intermolecular bonding sets in. The resulting bonds (mainly of the hydrogen type) cause flocculation of the pectin, which is precipitated together with cloud particles trapped (either mechanically, or actively through their own pectin) in the flocs. The flocs, stabilised by the intermolecular bonds, do not redissolve when the water activity is once again increased by the reconstitution of the juice. Only considerable mechanical dis- integration of the flocs results in a stable cloud. The same process is not applicable to the concentrated juice, having water activity below the critical range, probably due to reflocculation of the disintegrated particles.

The effect of pH on the amount of ethanol required to reach the critical water activity of a lemon juice is shown in Figure 2. As expected lower pH favours the formation of intermolecular hydrogen bonding by decreasing electrostatic repulsion through the association of the charged carboxyl groups. Thus, the amount of ethanol needed to obtain flocculation is somewhat lower, implying, that cloud loss will com- mence at a correspondingly lower degree of concentration (in this specific experiment by about 3 "Brix lower at pH 2.0 than at pH 2.4). Therefore, within the critical Brix range, pH may play an important role in the cloud loss of reconstituted lemon juice.

1608 D. Epstein and S . Mizrahi

Figure 2. Effect of added ethanol on cloud stability at different pH. 0, 2.0; A, 2.2; x , 2.4.

Ethono! concentroticn i%)

It should be noted that in certain cases, where water activity of the concentrate is above the critical level, some damage to cloud stability may be caused by a high degree of concentration in the limited area of the evaporator.

References 1 . Bartholomew, E. T. The Lemon Fruit 1965, University of California Press. 2. Rouse, A. H.; Knorr, C. C. Citrus Industry 1970, 51, (6), 16. 3. Stevens, J. W.; Pritchett, D. E.; Baier, W. E. Fd Technol. 1950,4,469. 4. Tressler, D. K.; Joslyn, M. A. Fruit and Vegetable Juice Processing Technology 1971, 2nd ed.,

p. 139, Westport, Conn., AVI Publishing Company. 5. Baker, R. A.; Bruemmer, J. H. J. ug. Fd. Chem. 1972,20,1169. 6 . Berk, Z . ; Mizrahi, S. Fruchtsaft Zndustriel 1965, 10, 71. 7. McComb, E. A.; McCready, R. M. Anal. Chem. 1952,24, 1630. 8. Hills, C. H.; Ogg, C . L.; Speiser, R. Znd. Eng. Chem. Anal. Ed. 1945, 17, 507. 9. Stevens, J. W . ; Baier, W. E. Ind. Eng. Chem. Anal. Ed. 1939, 11, 447.