Describing the latest research advances in the science and technology of hydrocolloids which are used in food and related systems, this book captures the presentations of leading scientists from the Gums and Stabilisers for the Food Industry Conference: Hydrocolloid Functionality for Affordable and Sustainable Global Food Solutions held in June 2015. Topics covered include sustainable and secure foods, healthy food products, innovative manufacture and formulation design as well as active packaging and edible films. Providing a fresh glance on food quality, it is a useful information source for researchers and other professionals in industry and academia and a reference for students of food science.
About the Author
Peter A Williams is Professor of Polymer and Colloid Chemistry and Director of the Centre for Water Soluble Polymers at the North East Wales Institute. Has published over 170 scientific papers and edited over 30 books. He is Editor-in-Chief of the international journal Food Hydrocolloids. His research is in the area of physicochemical characterisation, solution properties and interfacial behaviour of both natural and synthetic polymers. Recent work has been involved with the determination of molecular mass distribution using flow field flow fractionation coupled to light scattering, rheological behaviour of polymer solutions and gels, associative and segregative interaction of polysaccharides, development of polysaccharide-protein complexes as novel emulsifiers.
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Gums and Stabilisers for the Food Industry 18
Hydrocolloid Functionality for Affordable and Sustainable Global Food Solutions
By Peter A. Williams, Glyn O. Phillips
The Royal Society of ChemistryCopyright © 2016 The Royal Society of Chemistry
All rights reserved.
INVESTIGATION OF PECTIN-WATER INTERACTIONS: A PRACTICAL APPROACH
U. Einhorn-Stoll, E. Vasileva, T. Hecht and S. Drusch
Department of Food Technology and Food Material Science, Technische Universität Berlin, Königin-Luise-Straße 22, D-14195 Berlin
Pectin-water interactions are crucial for pectin production, transport, storage and application. Pectin is hygroscopic and during dissolution often lumps with a powder core and a gel-like surface are formed which are hardly to dissolve afterwards. Water can be sorbed to the powder also from the environment by gas diffusion during transport and storage or can be added by fluidization in order to induce better dissolution properties by agglomeration. A modified sorption and the capillary sucking method have been tested for several different types of demethoxylated pectins and a significant influence of the modification parameters on pectin-water interactions was found. In general, acidic treated pectins showed low water sorption by gas diffusion but high water uptake and delayed dissolution by capillary sucking. Enzymatically demethoxylated pectins, in contrast, sorbed more water from wet air but showed less water uptake by capillary sucking and started to dissolve earlier. The differences can be explained by the conditions during demethoxylation, especially pH and also enzyme type. The resulting more or less strong intermolecular interactions between neighboured demethoxylated pectin macromolecules were trapped during drying. They determined pectin molecular parameters and particle morphology and, thus, pectin water interactions. The results of the presented investigations are of general importance not only for pectins and their application but also for other comparable food hydrocolloids.
Water binding capacity, water uptake, water holding capacity, swelling capacity – all these terms describe the results of the examination of the interactions between hydrocolloids and water. Their scientific meaning is, however, not always clear.
Commercial hydrocolloids are sold in a dry state, mostly as powders, and have to be suspended or dissolved in water prior to any application in food products. Type and intensity of hydrocolloid-water interactions not only depend on the molecular and physico-chemical properties of the hydrocolloids but also on the water content in the system. Water uptake can be regarded as a step-wise process, starting at low water content with the sorption of water and formation of a monolayer on the surface of the powder particles by gas diffusion from wet air and condensation. With increasing water content the particle surface will swell and soften as described by Furmaniak. The subsequent steps, fluidization and complete dissolution, require a water surplus and sometimes also heating. Dissolution starts with intrusion of the solvent, followed by polymer disentanglement up to the final complete dissolution. It is necessary to differentiate between solubility (the amount of a sample that can be completely dissolved in a certain amount of solvent) and the process up to complete dissolution, which can be complicated by several factors.
Though the solubility of pectin is good, its dissolution may be difficult. Often the "fish-eye effect" occurs, which describes the formation of sticky and partly undissolved powder lumps as shown in Figure 1. The gel-like surface of the lumps can delay the transport of polymers from the powder to the liquid and considerably increases the dissolution time.
As a consequence of the different types of pectin-water interactions, the examination requires different methods to characterize the behavior of the product as far as possible. As outlined above, a wide range of methods exist, which often more or less empirically describe only one specific aspect with no correlations among each other. Typical methods for the characterization of pectin-water interactions are the determination of sorption isotherms, the capillary sucking method and DSC as well as special stirring tests for dissolution.
The aim of the paper is (I) to demonstrate possibilities and limitations of the investigation of pectin-water interactions from a practical point of view, (II) to present results, which have been achieved by the described optimized analytical methods and (III) to show some practical consequences for the industrial use of pectins.
2 METHODS AND RESULTS
2.1 Adaption of methods for routine testing of pectin-water interactions
The first method was a modification of the determination by sorption isotherms. The exact examination of a complete sorption isotherm is a time-consuming procedure, even in its automated form of dynamic vapor sorption (DVS). The endpoint of the sorption isotherm is determined at a maximum aw about 1. In order to simplify the procedure, this point may be chosen for a method that should be sufficient for many routine purposes. A defined amount of pectin powder is spread in a petri dish and stored in a small desiccator above distilled water for up to 80 h at constant temperature in a laboratory oven (Figure 2a,b). The water uptake by sorption (WUS) was determined according to the following equation:
WUS = mw – ms./ms (g/g)
with mw = mass of the wet sample and ms = mass of the dry pectin sample. Every determination was performed in triplicate.
The influence of temperature, sample mass/surface ratio and sample pretreatment (pre-drying time) was tested using an experimental design (Design expert 9, Stat Ease Inc.). The well-established effect of particle size on moisture sorption was confirmed, smaller particles sorbed significantly more water than large particles. Most of the water sorption took place during the first 8 h and differences between samples became nearly constant after 24 h (Figure 2 bottom). A prolonged sorption time did not lead to improved results.
All parameters strongly influenced the sorption behavior as can be seen from Figure 3. The water sorption significantly increased with increasing temperature because of improved particle surface softening and swelling. Prolonged pre-drying increased the water uptake, possibly due to an increase in hygroscopicity and, thus, in water uptake velocity. The sample mass to surface ratio proved to be very important, too. Water uptake was enhanced with increasing surface area available for sorption. For inhomogeneous samples, however, a critical sample mass is recommended in order to minimize deviations resulting from the behavior of individual particles. The sample mass is linked to the size of the petri dish in order to ensure the optimum ratio of mass to surface area.
In summary, the following experimental conditions were defined as optimum:
The pectin powder should be pre-dried at room temperature in a desiccator above P2O5 for one week.
0.150 [+ or -] 0.005 g sample should be exactly weighed and homogeneously distributed in a petri dish of 19 mm diameter.
Three dishes can be stored in one small desiccator (inner diameter 120 mm), filled with 100 ml water and equilibrated at 25 °C in a laboratory oven for 24 h. The desiccators with the samples have to be stored for exactly 24 h in the laboratory oven at 25 °C before the sample mass is determined again.
It is recommended to use always exactly the same desiccator. In case of a higher number of samples, a higher number of desiccators may be used and every sample should be placed once in every desiccator or samples must be randomized. All desiccators should be as similar as possible and should be evaluated for differences prior to the main trials. The use of large desiccators is not recommended due to difficulties in sample handling. Opening of the lid several times to get the samples in and out very rapidly led to severe deviations in the results. A temperature of 30 °C can be recommended for samples with a smooth and partly glassy surface to speed up surface softening and to get constant results in a short time period.
The second method used to test pectin-water interactions was the capillary sucking method (Baumann method) as modified for the examination of pectin by Wallingford and Labuza . They suggested placing a filter paper on top of the glass filter plate in order to reduce or at least to delay blocking of the glass filter plate by partly dissolved and swollen pectin particles. This allowed the application of the method for soluble pectin samples. For each measurement 10 mg of pre-dried pectin powder were distributed on the wet filter paper. It is of high importance to distribute the sample quickly in a thin but homogeneous layer on the filter paper. Otherwise the results of replicate measurements differ too much. The water uptake is determined in defined intervals until (I) the value in the capillary is constant for 10 min or (II) it starts to decrease because of partial dissolution of the pectin (end point criteria). The WUC is calculated by means of a calibration curve, considering the blank value (water uptake of the filter paper) as:
WUC = mw-mwb./ms (g/g)
with mw = water uptake of the sample, mwb = water uptake of the filter paper and ms = dry mass of the pectin sample. All measurements were performed at least in duplicate at 20 °C. The sample mass was reduced to 5 mg for those samples that sucked more water than contained in the capillary volume.
As a third tested method for the investigation of pectin-water interactions stirring tests for pectin dissolution were performed. The method has been described before but it was modified in the present study: The stirring velocity was increased from 500 to 800 rpm and the sample application had to be improved. As a result, the following procedure was defined: 0.150 g powder was applied in a ring-like area in the middle between the whirl and the glass beaker wall as suggested by  into 50 ml distilled water in a 100 ml glass beaker (low form) Alternatively, it was helpful to move the glass slightly from the center of the magnetic stirrer in order to form an asymmetric whirl as long as the pectin powder was added (Figure 4). The optimum stirring device (magnet) was a cylindrical stick (Ø 5 mm, 20 mm long).
Every 5 min the sample was checked visually against a black background. The dissolution was defined to be complete when no particles could be detected by a well-trained person. It is recommended to use a magnifying glass. It was tested, whether a photometer or turbidimeter would give more objective results, but this was too complicated because of sample loss and poor results. A viscosimeter control as recommended by Kurita or Wangrequires special equipment that will not be available in every company.
A strong connection between lump formation during application and dissolution time was found, initially formed "fish eyes" could hardly be dissolved afterwards. Lump formation was related to particle size as found also by Vasquez et al.: On the one hand smaller particles were sticking close together, they were able to hydrate and swell rapidly, but tended to form small particle lumps surrounded by a gel-like layer. On the other hand, small particles were able to dissolve faster than big particles, when homogeneously spread on the surface without lump formation. It is recommended, therefore, to apply the pectin powder through a sieve of appropriate pore size.
2.2 Results of pectin-water interaction tests for pectin samples with different degree of methoxylation and distribution of the free carboxylic groups
Modified pectins were prepared from one single commercial high-methoxyl citrus pectin (HMP) with 68 % degree of methoxylation (DM). The modified pectins (MP) were prepared by moderate (to DM 57 %) or strong (to DM 40 %) demethoxylation by treatment with HCI at pH about 1.5 (MP-A) or with fungal or plant-derived pectin methyl esterase (MP-F and MP-P) at pH 4.4 and 7.4, respectively. All results are described and discussed in detail in a recent publication but some examples clearly show the importance of choosing the appropriate examination method.
The sorption method revealed significant differences between the differently modified pectins (Figure 5). The two acidic treated pectin samples MP-A sorbed significantly less water from the air than the enzymatically treated pectin samples, independent on the DM.
The WUC values (Figure 6) were ten times higher than those of the WUS, reflecting the different character of pectin-water interactions in dependence on the water content of the system. Whereas the sorption method determines mainly the bound monolayer water on the surface, the capillary sucking method characterizes also swelling, surface softening and fluidization up to dissolution. Particles of acid-treated pectin samples had a smoother surface and stronger intermolecular interactions than those of enzymatically treated samples. As a consequence, the former samples needed more time for swelling and softening and did not dissolve in the capillary sucking tests even within 80 min. The more fibrous-like particles of the enzymatically treated pectin samples, which contained a considerable amount of sodium ions, bound water faster due to a larger surface and the existence of micro-pores. Swelling started earlier and samples dissolved rather quickly. The different behavior of the two pectin types was in agreement with observations during dissolution of pectin for gelation tests: Acid treated pectin samples of the same particle size were easier to suspend and formed less lumps but started to dissolve later than enzymatically treated pectins.
Excerpted from Gums and Stabilisers for the Food Industry 18 by Peter A. Williams, Glyn O. Phillips. Copyright © 2016 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
Chemical and Physiochemical Characterisation; Emulsions, Foams and Films; Encapsulation and Controlled Release; Health Aspects; Product Formulation;