Food Biochemistry and Food Processing (2 edition)

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34 Rye Constituents and Their Impact on Rye Processing 659

residues. High molecular weight arabinoxylans with arabi-

nose:xylose ratios below 0.3 tend to be insoluble.

Rye arabinoxylans are said to have high water-holding ca-

pacities, with water-extractable and water-unextractable arabi-

noxylans able to absorb 11 and 10 times their weight of water,

respectively (Girhammar and Nair 1992a). These values were

typically determined by addition of arabinoxylan to flour and

measurement of the consecutive rise in Farinograph absorption.

However, it is not clear whether the results obtained by this

technique may be simply defined as “water-holding capacity.”

The mechanism behind water binding of discrete cell wall frag-

ments that contain water-unextractable arabinoxylans is bound

to be different from that of water-extractable arabinoxylans in

solution. It is also unclear whether the increased Farinograph

absorption is solely caused by the arabinoxylans. That water-

holding values obtained with the Farinograph method have to be

interpreted with care is clearly demonstrated by Girhammer and

Nair (1992b), who reported a water-holding capacity of 0.47 g/g

dry matter for rye water-extractable arabinoxylan, when deter-

mined by measuring the level of unfreezable water associated

with the water-extractable arabinoxylan using differential scan-

ning calorimetry. These authors stated that the water-holding

capacity was not related to the molecular weight of the ara-

binoxylan. Different milling fractions of rye showed different

water absorption capacities in relation to this total arabinoxylan

content (H ̈arkonen et al. 1997). ̈

Even at relatively low concentrations, water-extractable ara-

binoxylans are able to form highly viscous solutions in wa-

ter (Girhammar and Nair 1992b). Highly positive correlations

were found between the viscosity of a rye extract and its con-

tent of water-extractable arabinoxylans (Boros et al. 1993; Fen-

gler and Marquardt 1988a; Hark ̈ onen et al. 1995, 1997; Ragaee ̈

et al. 2001). For different rye milling fractions, extract viscosity

correlates with the content of water-extractable arabinoxylans

(Fengler and Marquardt 1988a, Glitso and Bach Knudsen 1999,

H ̈ark ̈onen et al. 1997). Water extracts from rye are more viscous

than those from other cereals (Boros et al. 1993, Fengler and

Marquardt 1988a), which can be ascribed to the higher concen-

tration of water-extractable arabinoxylans in rye than in other

cereals. Differences in viscosity are related to differences in the

structural features and molecular weight of water-extractable

arabinoxylans. Although the impact of un-, mono-, and disub-

stitution is unclear (Bengtsson et al. 1992b, Dervilly-Pinel et al.

2001, Ragaee et al. 2001), molecular weight seems a logical

determining parameter (Girhammar and Nair 1992b, Nilsson

et al. 2000, Ragaee et al. 2001, Vinkx et al. 1993). Arabinoxy-

lan conformation was also found to have a bearing on viscosity,

with a higher viscosity being associated with a larger radius of

gyration (Dervilly-Pinel et al. 2001, Ragaee et al. 2001). Viscos-

ity measurement conditions such as shear rate (Bengtsson et al.

1992b, Hark ̈ ̈onen et al. 1997, Nilsson et al. 2000) and temper-

ature, pH, and salt concentration (Girhammar and Nair 1992b)

also influence the viscosity of rye water-extractable arabinoxy-

lan solutions.

Water-extractable arabinoxylans undergo oxidative gelation

upon addition of hydrogen peroxide and peroxidase, resulting

in an increase in viscosity and eventually the formation of a gel

(Dervilly-Pinel et al. 2001, Vinkx et al. 1991). The phenomenon

is caused by the formation of covalent linkages through oxida-

tive coupling of ferulic acid residues esterified to arabinoxylan.

The aromatic ring and not the propenoic moiety of ferulic acid

is involved in the oxidative gelation (Vinkx et al. 1991). The

oxidative gelation capacity of arabinoxylans depends on sev-

eral parameters including the ferulic acid content, the molecu-

lar weight, and the structure of the arabinoxylans. Arabinoxy-

lan fractions with higher ferulic acid content, higher molecular

weights, and fewer disubstituted xylose residues have a higher

gelation potential (Dervilly-Pinel et al. 2001, Vinkx et al. 1993).

Dervilly-Pinel et al. (2001) stated that the intrinsic viscosity of

arabinoxylans is the main parameter governing gel rigidity.

Proteins

The total protein content in rye is influenced by both environ-

mental and genotypic factors and varies from 8.0 to 17.7% (see

Table 34.1). The protein content is higher in the outer layers than

in the inner layers of the rye kernel (Glitso and Bach Knudsen

1999; Nilsson et al. 1996, 1997a).

Protein Classification

Rye proteins can be separated by the classical Osborne fraction-

ation procedure. The albumins are extractable with water, the

globulins with dilute salt solutions, the prolamins with alcohol,

and the glutelins with dilute acid or alkali. Chen and Bushuk

(1970) found that the albumin, globulin, prolamin, and glutelin

fractions accounted for 34, 11, 19, and 9%, respectively, of the

total nitrogen content in a rye flour, with 21% of the protein re-

maining in the residue. Similar results were obtained by Preston

and Woodbury (1975) and Jahn-Deesbach and Schipper (1980),

except that the latter authors extracted a much higher proportion

of glutelins. Gellrich et al. (2003), however, extracted fewer al-

bumins and globulins and many more prolamins from rye flour.

In all cases, rye flour contained higher levels of albumins and

globulins and lower levels of prolamins and glutelins than wheat

(Chen and Bushuk 1970, Gellrich et al. 2003, Jahn-Deesbach

and Schipper 1980).

From a physiological point of view, rye proteins can be divided

into two main groups: the storage proteins and the nonstorage

proteins. The storage proteins are deposited in protein bodies

in the developing starchy endosperm and function solely as a

supply of amino acids for use during germination and seedling

growth. They include the prolamins, glutelins, and the proteins in

the Osborne residue. The nonstorage proteins include, amongst

others, enzymes and (in some cases) their inhibitors and corre-

spond with the Osborne albumin and globulin fractions.

Major differences are found in the amino acid compositions

of the proteins in the two groups. The nonstorage proteins con-

tain high proportions of lysine and arginine and low proportions

of glutamine/glutamate and proline, whereas the opposite was

observed for the storage proteins (Dexter and Dronzek 1975,

Gellrich et al. 2003, Preston and Woodbury 1975). No signifi-

cant differences exist in the amino acid composition of the pro-

teins from different rye varieties (Dembinski and Bany 1991,