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General Overview of Food Hydrocolloids

1.1 Introduction to the World of Hydrocolloids

The term ‘hydrocolloid’ is derived from the Greek hydro ‘water’ and kolla ‘glue’.

Hydrocolloids are colloidal substances with an affinity for water. From a chemical point of view, they are macromolecular hydrophilic substances. Some of them are water soluble and form colloidal solutions others are only able to swell in water and can be dispersed by means of shear forces. Hydrocolloids produce viscous solutions, pseudo-gels, or gels in water. The heterogeneous group consists of polysaccharides and proteins.

Hydrocolloids are used in technical and regulated applications to thicken and to stabilize formulations. In processed foods, they are ubiquitous – no other group of ingredients contributes more to viscosity, texture, and body like hydrocolloids do.

Hydrocolloids are not really emulsifiers because, mostly, they do not have the characteristic linkage of lipophilic and hydrophilic groups in the molecular structure. The molecules are too big and complex in size and therefore are not flexible enough to cover the interfaces being formed during homogenization of oil–water mixtures fast enough to create a long-term stable emulsion with sufficiently small droplet diameter. However, these thickeners can stabilize emulsions by increasing the viscosity of the water surface or by interaction with surface-active substances. Some hydrocolloids like gum Arabic or non-ionic products such as methylcellulose (MC), HPMC (hydroxypropylcellulose), or propylene glycol alginate (PGA) reduce the surface tension and exhibit limited emulsifying properties [1].

In accordance to their origin and way of manufacturing, hydrocolloids can be classified in four different groups:

1. hydrocolloids purely isolated from plants (without chemical modification);
2. hydrocolloids obtained by fermentation;
3. plant-derived hydrocolloids that are chemically modified;
4. hydrocolloids from animals.

According to their botanical origin and their function in the plant organism, naturally occurring vegetable hydrocolloids can be divided into [1]:

• exudates (protective colloids being deposited on wounds):

  acacia gum/gum Arabic, tragacanth, karaya gum, ghatti gum;

• seed flours (reserve polysaccharides):

  guar gum, locust bean gum, tara gum, tamarind seed gum;

• extracts from land plants and marine algae (scaffolding substances):

  pectins, agar, alginate, carrageenan, starches, cellulose, furcelleran, larch gum.

Additionally, there are [1]:

• microbial or bacterial polysaccharides:

  xanthan, dextran, curdlan, scleroglucan, gellan, pullulan;

• modified polysaccharides:

  propylene glycol alginate, amidated pectin, modified starches, cellulose derivatives;

• proteins of animal origin:

  gelatine, caseinates.

Figure 1.1 presents an overview of globally used food hydrocolloids. There are also other substances available and in use, but several of them are restricted to local use and, depending on availability and legislation, are not in industrially-produced applications. Please always check the relevant legislation before using one of these stabilizers, gelling agents, or thickeners.

Figure 1.1 Overview of food hydrocolloids used globally.

The individual substances are described in subsequent sections. Information on their origin, manufacturing, structure, properties, and handling are provided. For the most used products there are overview tables; Tables 1.1–1.9 below give a quick orientation.

The individual cellulose-based substances are then described in more detail. Overview tables for selected cellulose derivatives are given in Section 1.6 (Tables 1.10–1.14).

1.2 Plant Extracts

1.2.1 Agar

Raw Material, Harvesting, and Manufacturing

Agar is a structure-building component of the cell wall of red algae (Rhodophyceae). Gelidium, Gracilaria, and Pterocladia species especially serve as a source of raw materials. The main producing countries are Japan, United States of America (California), Chile, and Spain, on whose rocky shores they occur. Agar was discovered in 1658 in Japan.

The red algae are harvested and extracted under pressure with hot water (100–130 °C (212–266 °F)) at pH 5–6. The extract is purified by filtration or centrifugation and subsequently bleached with calcium hypochlorite. To isolate the agar, the extract is frozen and, after thawing, the remaining gelatinous residue is dried. More recently, the water is squeezed out by means of a high-pressure press, and the remaining water is removed by drying. The annual production involves about 55 000 MT of dried seaweed to manufacture 7500 MT of agar [1,2].

Chemical Structure

Agar is a heterogeneous polysaccharide composed of the monomeric substances D-galactose and (3 → 6)-anhydro-L-galactose (Table 1.1). The main component, which is responsible for the strong gelling ability, is so-called agarose. Additionally, a small amount of a slightly acidic polysaccharide, the non- or only weakly gelling agaropectin, is present. Agaropectin also contains sulfate ester, glucuronic, and pyruvic acid groups. Agarose is a neutral, linear galactan. The D-galactose and 3,6-anhydro-L-galactose monomers are linked alternately by α-(1 → 3)- and β-(1 → 4) bonds with each other. Owing to the high anhydrogalactose portion and the absence of sulfate ester groups, which strengthen the hydrophobic character, agar is a good gelling agent that is independent of cations [1].

Table 1.1 Characteristics of agar.

• Galactose and anhydro-galactose;
• low sulfate content (<4.5%, mostly 1.5–2.5%)

• Tannic acid can inhibit the gelling process;
• gum karaya reduces gel strength of agar gels;
• proton scavengers like potassium iodide, urea, guanidine, sodium thiocyanate, and so on block the gelling process and prevent agarose gel formation

Gelation of agarose from aqueous solutions is assumed to occur...