Here, process water content can be very different from actual sample water con- tent. In the studies of Faubion and Hoseney (29) using the Warner–Bratzler method, the shear strength fell by a factor of 3 for starch and by a factor of 2 for flour with increasing extrusion moisture content from 17 to 24%. Owusu- Ansah et al. (39) observed the breaking strength of extrusion cooked corn starch to decrease with increasing extrusion moisture content. Bhattacharya and Hanna (41), using the Kramer shear press, found the shear strength of extrusion cooked waxy and nonwaxy corn that had been dried to 1–2% moisture to decrease with increasing extrusion moisture content from 15 to 45%. Martinez-Serna and Vil- lota (43) found that ‘‘breaking force’’ showed a maximum or minimum in the extrusion moisture range 17–25%, depending on the manipulation of whey pro- tein isolates (WPI) blended with corn starch. Brittleness increased with increasing extrusion moisture content in all cases of WPI modification. Cohesiveness of rehydrated extrudates decreased with increasing extrusion moisture content, ex- cept for acidic and alkaline WPI, which showed the opposite trend.
Hutchinson et al. (68) found that the flexural modulus and strength of wheat starch and wheat flour extrudates decreased with increasing density, caused by increasing the sample water content (Fig. 7).Attenburrow et al. (25) found that compressive modulus and strength decreased with increasing RH in the range 33–75% for sponge cakes. Seymour and Hamann (89) used a shear cell for single or multiple sample testing. The force per unit mass and the work to failure in- creased with increasing water activity, aw, from 0 to 0.65, indicating greater toughness. Experiments by Zabik et al. (90) using a Kramer shear press, a texturo- meter cell, and a breaking test were carried out on cookies conditioned to different RH. Breaking strength was the most sensitive mechanical measure, showing a decrease with increasing RH from 52 to 79%. Katz and Labuza (91) used breaking
and puncture tests and texture profile analysis to test snacks equilibrated to aw
from 0 to 0.85. The initial slope of force–deformation curves (kilograms per millimeter), and the compression work and cohesiveness from texture profile analysis decreased with increasingaw. Similarly, Sauvageot and Blond (47) found that the initial slope decreased with increasingaw, particularly aboveaw⫽0.50.
Loh and Mannell (92) used the General Foods Texturometer with cereal flakes and milk contained in a cup and compressed with a plunger. They showed a decrease in texture retention, defined as the peak force relative to that for a dry sample, with time in milk.
Studies by Attenburrow et al. (73) have monitored the changing acoustic emission amplitude of crushing starch and shown a marked reduction in intensity with increasing moisture content. Nicholls et al. (72) reported the decrease of acoustic events with increasing relative humidity when crushing extruded gran- ules of gluten wheat starch and waxy maize starch (Fig. 14).Seymour and Ha- mann (89) also found a general decrease in the acoustic emission parameters, sound pressure and sound intensity, with increasing water activity.
B. Protein
The effect of protein enrichment was investigated in studies on extruded crisp- breads by Antila et al. (18). Increasing the wheat content increased the breaking force more than did protein enrichment. Faubion and Hoseney (93) found that the Warner–Bratzler shear strength and the flexural strength of wheat starch de-
Figure 14 Acoustic emission produced when crushing extruded granules as a function of conditioning relative humidity:䊊gluten,䉭wheat starch,䊐waxy maize starch. (From Ref. 72 with kind permission from Academic Press.)
Figure 15 Shearing and breaking strengths of extruded wheat starch as a function of gluten addition:䊉shearing strength,䊊breaking strength. (From Ref. 93 with kind permis- sion from American Association of Cereal Chemists.)
creased as gluten was added, up to 15% (Fig. 15). Interestingly, soya protein isolate had the reverse effect up to the 10% level. Mohamed (23) found that adding soya protein up to 25% to corn grits decreased the compressive strength.
Matthey and Hanna (42) studied whey protein concentrate (WPC) addition to corn starch of different amylose content as an extrusion cooking feedstock. The shear strength increased with increasing amylose content, but no significant effect of WPC was observed. They commented that WPC could not be added in excess of 20% because of decreased expansion and low sensory scores. Martinez-Serna and Villota (43) used native and modified whey protein isolates (WPI) blended with cornstarch before extrusion cooking. They considered that the breaking force was influenced by two factors, the level of expansion and the strength of the cell walls, which is consistent with variations on the foam model described earlier [Eq. (1)]. Whey proteins decreased the expansion relative to cornstarch alone.
They found the ‘‘brittleness’’ to be least for the alkaline WPI—corn starch mix- tures increasing for the esterified and acidic WPI and highest for the native WPI with starch. ‘‘Cohesiveness’’ was least for the native WPI relative to the modified WPI mixtures. They concluded that disulfide bonds accentuated toughness and inelasticity in extrudates while predominance of hydrophobic interactions made extrudates more brittle and less cohesive.