Food Biochemistry and Food Processing (2 edition)

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656 Part 5: Fruits, Vegetables, and Cereals

ring contains amylose and probably less-ordered amylopectin

(Morrison 1995). The semicrystalline growth ring consists of

alternating crystalline and amorphous lamellae. The crystalline

lamellae are built from amylopectin double helices, whereas

the amorphous lamellae contain the branch points of the amy-

lopectin side chains (Andreev et al. 1999, Gallant et al. 1997).

Evidence has been provided that the crystalline and amorphous

lamellae of the amylopectin are organized into larger, more or

less spherical blocklets (Gallant et al. 1997).

The structure of starch can be studied at different levels by

several techniques such as optical microscopy, scanning elec-

tron microscopy, transmission electron microscopy, and X-ray

diffraction. X-ray diffraction provides information about the

crystal structure of the starch polymers, but also about the rel-

ative amounts of the crystalline and amorphous phases. Cereal

starches have an A-type X-ray diffraction pattern, while B-type

and C-type X-ray diffraction patterns are characteristic for tuber

and root starches and for legume starches, respectively. C-type

starches are considered to be mixtures of A-type and B-type

starches. In general, rye starches show A-type X-ray diffraction

patterns, but some researchers (Gernat et al. 1993, Schierbaum

et al. 1991) also reported a significant B-type portion.

Starch Physicochemical Properties

The physicochemical gelatinization, pasting, gelation, and ret-

rogradation properties of starches are important parameters in

determining the behavior of cereals in food systems.

When a starch suspension is heated above a characteristic

temperature (the gelatinization temperature) in the presence

of a sufficient amount of water, the starch granules undergo

an order-disorder transition known as gelatinization. The phe-

nomenon is accompanied by melting of the crystallites (loss

of X-ray pattern), irreversible swelling of the granules, and

solubilization of the amylose. Gelatinization can be studied

by measuring the loss of birefringence of the starch granules

when observed under polarized light. The end point and the

range of temperatures at which gelatinization occurs can be

determined. For rye starch, birefringence end point tempera-

tures of 59.6◦C (Klassen and Hill 1971) and 58.0◦C(Liiand

Lineback 1977) were reported. These birefringence end point

temperatures were somewhat lower than those observed for

wheat starches by the same authors. Another technique often

used to study gelatinization-associated phenomena is differen-

tial scanning calorimetry (DSC). DSC enables determination of

onset, peak, and conclusion gelatinization temperatures, gela-

tinization temperature ranges, and gelatinization enthalpy. The

gelatinization temperature is a qualitative index of crystal struc-

ture, whereas gelatinization enthalpy is a quantitative measure

of order. The DSC technique also measures the temperature and

enthalpy of the dissociation of the amylose-lipid complexes. Rye

starches show an onset of gelatinization at rather low temper-

atures. Gudmundsson and Eliasson (1991) reported onset gela-

tinization temperatures of 54.8–60.3◦C, Radosta et al. (1992) of

53.5◦C, and Verwimp et al. (2004) of 49.2–51.4◦C. Rye starch

has somewhat lower gelatinization temperatures and enthalpies

than wheat starch (Gudmundsson and Eliasson 1991, Radosta

et al. 1992, Verwimp et al. 2004). Lower dissociation enthalpies

of the amylose-lipid complexes for rye starch than for wheat

starch were observed by Gudmundsson and Eliasson (1991) and

Verwimp et al. (2004), indicating lower levels of lipids in rye

starch.

When starch suspensions are heated beyond their gelatiniza-

tion temperature, granule swelling and amylose and/or amy-

lopectin leaching continue. Eventually, total disruption of the

granules occurs, and a starch paste with viscoelastic properties

is formed. This process is called pasting. Upon cooling, gelation

occurs. During gelation, at sufficiently high starch concentra-

tions (>6%, w/w), the starch paste is converted into a gel. The

gel consists of an amylose matrix enriched with swollen gran-

ules. During gelation and storage of starch gels, retrogradation

occurs. This is a process whereby the starch molecules begin to

reassociate in an ordered structure, without regaining the original

molecular order. Initially, starch retrogradation is dominated by

gelation and crystallization of the amylose molecules. In a longer

time frame, amylopectin molecules recrystallize. Rye starch ex-

hibits a unique, typical swelling behavior with a high pasting

temperature (79–87◦C according to Schierbaum and Kettlitz

1994; 75◦C according to Verwimp et al. 2004), a stable con-

sistency during cooking, and a high viscosity increase during

cooling (Schierbaum and Kettlitz 1994, Verwimp et al. 2004). It

has lower pasting temperatures than wheat starch (Schierbaum

and Kettlitz 1994, Verwimp et al. 2004). Fredriksson et al. (1998)

and Gudmundsson and Eliasson (1991) showed that rye starches

retrograde to a lesser extent than starches from wheat or corn and

ascribed this to differences in the structure of the amylopectin

polymers.

Nonstarch Polysaccharides

Rye contains, on average, 13.0–15.0% nonstarch polysaccha-

rides, which serve mainly as cell wall components and the ma-

jority of which are arabinoxylans (Table 34.1). Other nonstarch

polysaccharide constituents of rye are mixed-linkedβ-glucan,

cellulose, arabinogalactan peptides, and fructans (Table 34.1).

The variation in nonstarch polysaccharide content and compo-

sition is significantly influenced by both harvest year and rye

genotype (Hansen et al. 2003, 2004). Most of the nonstarch

polysaccharides are classified as dietary fiber because of their

resistance to digestion by human digestive enzymes. In view of

the major importance of arabinoxylans in rye, these components

will be further discussed in detail.

Arabinoxylan Occurrence and Structure

Arabinoxylans consist of a backbone of 1,4-linked β-d-

xylopyranose residues, unsubstituted or mono- or disubstituted

with singleα-l-arabinofuranose residues at the O-2- and/or O-

3-position. To some of these arabinose residues, a ferulic acid

moiety is ester bound at the O-5 position. In rye, as in other

cereals, two types of arabinoxylans can be distinguished: water

extractable and water unextractable. Water-unextractable arabi-

noxylans, representing approximately 75% of the total arabi-

noxylan population of the rye kernel, are unextractable because

they are retained in the cell walls by covalent and noncovalent in-

teractions among arabinoxylans and between arabinoxylans and