Volume 130, 5 October 2015, Pages 405–419
Review
Inulin, a flexible oligosaccharide I: Review of its physicochemical characteristics
- Under a Creative Commons license
Open Access
Highlights
- •
- Inulin's size distribution strongly influences its physicochemical behavior.
- •
- Processing history of inulin has a large impact on its physicochemical behavior.
- •
- Inulin is a versatile carbohydrate with many applications in food and pharma.
Abstract
Inulin, a fructan-type polysaccharide, consists of (2→1) linked β-d-fructosyl residues (n = 2–60), usually with an (1↔2) α-d-glucose end group. The applications of inulin and its hydrolyzed form oligofructose (n = 2–10)
are diverse. It is widely used in food industry to modify texture,
replace fat or as low-calorie sweetener. Additionally, it has several
applications in other fields like the pharmaceutical arena. Most notably
it is used as a diagnostic agent for kidney function and as a protein
stabilizer. This work reviews the physicochemical characteristics of
inulin that make it such a versatile substance. Topics that are
addressed include morphology (crystal morphology, crystal structure,
structure in solution); solubility; rheology (viscosity, hydrodynamic
shape, gelling); thermal characteristics and physical stability (glass
transition temperature, vapor sorption, melting temperature) and
chemical stability. When using inulin, the degree of polymerization and
processing history should be taken into account, as they have a large
impact on physicochemical behavior of inulin.
Keywords
- Physical;
- Chemical;
- Carbohydrate;
- Polysaccharide;
- Oligofructose;
- Polymer
1. Introduction
Inulin was discovered over two centuries ago by Rose (Fluckiger & Hanbury, 1879) and since then its presence in many plants became apparent (Livingston, Hincha, & Heyer, 2007).
Some examples of plants containing large quantities of inulin are
Jerusalem artichoke, chicory root, garlic, asparagus root, salisfy and
dandelion root (Kaur & Gupta, 2002).
More commonly consumed vegetables and fruits containing inulin are
onion, leek, garlic, banana, wheat, rye and barley. Daily intakes have
been estimated to range from 1 to 10 g per day in the Western diet (Coussement, 1999 and Van Loo et al., 1995). The average American diet contains between 1.3 and 3.5 g of inulin per day, with an average of 2.6 g (Coussement, 1999).
The European consumption of inulin appears to be substantially higher
at 3–11 g per day, which is below reported tolerances of at least
10–20 g per day (Bonnema et al., 2010 and Carabin and Flamm, 1999). Inulin has also been used safely in infant nutrition (Closa-Monasterolo et al., 2013).
This has led to the American Food and Drug Administration to issuing a
Generally Recognized As Safe notification for inulin in 1992 (Kruger, 2002). Inulin is also used pharmaceutically, most notably as a diagnostic agent for the determination of kidney function (Orlando et al., 1998 and The editors of Encyclopaedia Brittanica, 2015).
Over
the past decades, a lot of research has been done showing that inulin
is a versatile substance with numerous promising applications. Several
reviews have been published on inulin, its characteristics and
functionality in food (Boeckner et al., 2001, Kelly, 2008, Kelly, 2009 and Seifert and Watzl, 2007) and pharma (Imran, Gillis, Kok, Harding, & Adams, 2012).
This review aims to provide an overview of the relevant physicochemical
properties of inulin, which make it such a useful excipient in food and
pharma.
1.1. Chemical structure
Inulin,
depending on its chain length, is classified as either an oligo- or
polysaccharide and it belongs to the fructan carbohydrate subgroup. It
is composed of β-d-fructosyl subgroups linked together by (2→1) glycosidic bonds and the molecule usually ends with a (1↔2) bonded α-d-glucosyl group (Kelly, 2008 and Ronkart et al., 2007a).
The length of these fructose chains varies and ranges from 2 to 60
monomers. Inulin containing maximally 10 fructose units is also referred
to as oligofructose (Flamm, Glinsmann, Kritchevsky, Prosky, & Roberfroid, 2001).
In food, oligofructose is more commonly used a sweet-replacer and
longer chain inulin is used mostly as a fat replacer and texture
modifier (Kelly, 2008).
Both inulin and oligofructose are used as dietary fiber and prebiotics
in functional foods. Its longer chain length makes inulin more useful
pharmaceutically than oligofructose.
Before
processing, the degree of polymerization of inulin depends on the plant
source, time of harvest, and the duration and conditions of
post-harvest storage (Kruger, 2002, Ronkart et al., 2006b and Saengthongpinit and Sajjaanantakul, 2005).
Processing itself also has a great influence on degree of
polymerization of the obtained product as will be discussed in Section 1.2. Table 1
provides an overview of the structure and size of some carbohydrates
frequently used in the pharmaceutical arena. The structures of a
selection of those carbohydrates are shown in Fig. 1.
