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Kastner, H., Einhorn-Stoll, U., & Senge, B. (2012). New Parameters for the Examination of the Pectin

Gelation Process. In Gums and Stabilisers for the Food Industry 16 (pp. 191–197).

H. Kastner, U. Einhorn-Stoll, B. Senge

H. Kastner, U. Einhorn-Stoll, B. Senge

Department of Food Technology and Food Chemistry, Technische Universitaet Berlin,

Koenigin-Luise-Strasse 22, D-14195 Berlin, Germany

1 INTRODUCTION

Industrial pectins are mainly obtained from by-products of the citrus or apple juice industry

by acidic extraction. They are used as gelling or thickening agents in order to improve the

texture of food products. Pectins are branched polysaccharides with a backbone of

galacturonic acid and neutral sugar side chains, present in the plant cell walls. The

galacturonic acid molecules are partly esterified with methanol. Materials with more than 50

% methoxylated carboxyl groups (degree of methylation DM > 50 %) are named as high-

methoxylated pectins (HMP) and those with less than 50 % are low-methoxylated (LMP).

The typical DM for commercial use is about 60 to 77 % for HMP and about 25 to 40 % for

intermolecular ionic junction zones between smooth regions of neighbored chains (egg-box

model).1,5

Fundamental knowledge of the material and structuring properties of pectins, especially of

their gelling behavior, is of crucial importance for their application in foods as well as for

the configuration of technological processes. The gelation temperature of pectins depends

not only on their botanical source, manufacturing conditions and molecular and material

properties but also on the gel preparation procedure and cooling conditions. There are many

experimental studies of the complex pectin gelation process as transition from sol to gel as

well as of the “gel point”, the temperature at which the material properties change from

liquid to solid. Sometimes, however, it can be difficult to define a clear gel point because the

process, the gel point was frequently defined as cross-over of storage modulus G´ and loss

modulus G´´ (tanδ = G´´/G´ = 1). This method was developed initially for chemical

gelation.8 In most food products, however, physical gelation via junction zones occurs and

sometimes no clear gel point can be determined for pectin gels, for instance because of pre-

gelation.9,10,11 Therefore, a modified method examined the point of intersection as a function

of frequency,3,8 where the gel point as tanδ = 1 might be defined only in case it is

independent on the frequency.11 But also this method is not always exact and, therefore,

sometimes the described point is named as “apparent gel point”.3 Other indicators of the gel

formation were described by a strong decrease of tanδ12,13 or using the structure development

rate SDR = dG´/dt during cooling14. Nevertheless, the gel point determination is not

2 MATERIALS AND METHODS

2.1 Materials

High or low methylated non-standardized commercial citrus pectins were obtained from

three pectin companies. For data protection reasons an anonymous pectin declaration 2A,

3B, etc. had to be made. Gelling parameters and the DM of the samples are given in Table

3, further molecular characteristics and the relating determination methods are presented in

pH (Table 1). All experiments were made at least in duplicate for each pectin gel.

2.2.1 HMP sucrose gel preparation: Dry pectin powder was dissolved in demineralised

water. While stirring, the mixture was heated quickly to boiling. The sucrose was added and

the HMP-sucrose mass was reduced to a defined value. Afterwards, the pH of the mass was

reduced degraded by addition of tartaric acid solution.

2.2.2 LMP sucrose gel preparation: Demineralised water, citric acid solution and sodium

citrate solution were mixed in a steel pot and the dry pectin powder was added while stirring.

The mixture was heated quickly to boiling, sucrose was added and the mixture was boiled

again. The calcium chloride dehydrate solution was added and while stirring the LMP-

sucrose mass was reduced to a defined value. The detailed information of the ingredients of

2.2 Rheological measurements

Oscillation measurements of pectin-sugar model gels were carried out on a Physica MCR

301 (Anton Paar). The applied geometry was a double gap rotational cylinder CC27/P1 with

peltier cylinder temperature system TEZ 150P. After gel preparation, about 15 ml of the

sample was transferred immediately into the pre-heated rheometer (105 °C) and cooled to

20 °C (HMP) or 10 °C (LMP) with a cooling rate of 1 K/min. The sample was coated with

silicone oil and the cylinder was closed with a special lid in order to avoid evaporation. The

The USA-sag method was implemented by the IFT committee for pectin standardisation.

This method is rather empirical but frequently used for routine or quality tests in the pectin

industry, yet.

The rest of the hot pectin-sugar solution, prepared as described above, was filled into three

special glasses and stored at 25 °C for 24 h before measurement. For an accurate result, the

average gel properties of the three gel cones had to be within the limits shown in Table 2.

A Lab850 pH-meter with a special penetration electrode from Schott Instruments was used

for the determination of the pH in the gels after Ridgelimeter measurements. Moreover, the

soluble solid SS was determined in the gel using a refractometer (Schmidt and Haensch).

temperature (GP) as cross-over of G´ and G´´, the initial structuring temperature (IST) and

critical structuring temperature (CST) as shown in Figure 1 from the first derivation of the

storage modulus (dG´/dt = structure formation velocity) using Origin 8.1 software. The IST

is defined as the temperature at which the structure formation velocity was different from 0

for the first time. The CST is the extrapolated temperature for the first strong increase of the

structure formation velocity. In contrast to the GP, these two structuring temperatures could

be detected for any pectin we have ever investigated. Moreover, from the structure formation

velocity curve often different structuring phases could be identified.

The details of the new defined parameters are shown in Figure 1.

Figure 1 Evaluation of the structure formation during cooling of an HMP-gel

The rheological studies were made using different but constant preparation conditions for

HMP and LMP, respectively, (Table 1) and allowed the comparison of the gelling processes

and gel properties. Sometimes, the rheological measurements gave no clear setting point as

intercept of G´ and G´´, causing uncertainty for identifying the exact gelling temperature.

Especially in these cases, the new structuring parameters IST and CST allowed a better

description and comparison of the structure formation process.

The structure formation velocity curve dG´/dt characterised not only the gelling kinetic but

also different phases in the gelation process by alterations of the structure formation velocity.

In order to investigate the influence of manufacturing conditions on the gelling process, two

HMP (2A and 3A) and two LMP (2C and 3E), respectively, with similar DM but from

different companies were compared. Differences in the start of the structuring process and

structuring velocity were observed (Figure 2b and 2c, Table 3). As shown in Figure 2b, there

was an earlier increase in dG´/dt of the 2A gel, with structure formation temperatures GP,

IST and CST about 9 K higher than in the 3A gel. The same was found for the two LMP

with a difference > 15 K. This means that the gelling properties of pectins were strongly

dependent on their processing conditions and that the DM is one indicator but not the only

parameter for evaluating the gelling properties.

An increase or decrease of structure formation velocity dG´/dt can indicate different gelling

showed the complexity of the polymer properties and their influence on the pectin gel

characteristics, depending on the pectin manufacturing process. The newly defined initial

structuring temperature IST and critical structuring temperature CST were, beside the GP,

suitable parameters in order to describe the incipient gel structuring process for both types

of pectins. Even samples with no clear GP could be evaluated this way. Therefore, IST and

CST have proved to be valuable complementary parameters and can help to understand the

structure formation processes more detailed. They can be used for optimizing the pectin

production process as well as the pectin application in food products.

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6 IFT Committee, Food Technology, 1959, 8, 496.

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