1. Heat Damage
Heat, frost, drought and rain are environmental,
pre-harvest factors that can scarcely be altered by man. They affect not only
the yield of the cereals but also the quality, i.e. protein content and
properties, enzymes, mechanical properties of the grains etc.
Heat damage can
also be caused post-harvest by harsh drying conditions, either if the humidity
of a damp wheat is reduced by improper means or if a low Falling Number is to
be concealed.
Unsuitable transport or storage conditions may be reasons for
heat damage, including self-heating due to fermentation caused by the high
moisture content of stored wheat, and self heating following insect infestation
as the metabolic activities of the insects result in locally increased moisture
and thus fermentation.
Heat damage results in high Falling Numbers
(typically above 500 s, although such a high Falling Number does not
necessarily mean that the wheat is damaged).
If wheat with a very low Falling
Number has been heat treated, the resulting Falling Number may appear normal,
but sedimentation values will be down to 10 or 15 mL.
There will be difficulty
in extracting the gluten (crumbly gluten, running through the fingers or
sieves); extensibility will be reduced; the dough will be short; baking
performance will be impaired.
Measures taken to improve the baking performance are
blending with flour from sound wheat (if available), gluten softening (with
Lcysteine or enzymes), addition of vital wheat gluten with normal properties,
restoring of amylolytic activity with enzyme preparations, gluten stabilization
with salts and minerals and also with emulsifiers.
2. Bug Damage
Insects such as locusts are able to destroy the whole
harvest, but from the technical point of view the sunn pest caused by
Eurygaster beetles and the like (Eurygaster integriceps, Eurygaster maura,
Aelia acuminata, Aelia furcula, Aelia rostrata, Nysius huttoni) is a more
serious problem (Fig. 155).
When feeding on wheat, they inject their digestive
fluids into the green kernel and then suck off the liquefied contents.
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| Fig. 155 : Eurygaster integriceps (left) feeding on wheat, Aelia furcula (right) (source: University of Vermont, Entomology Research Laboratory, Burlington, VT, USA) |
The damage is hardly visible on the kernel itself; it
is only a tiny black spot surrounded by a zone of lighter colour. The weight of
the kernels is almost unchanged; shrinkage sometimes occurs, but not
necessarily.
The gluten index will be lower, but still close to normal; the
Swelling Index ("G" in the Alveograph) is lower; extracted gluten
liquefies upon resting; in the Farinograph, the dough shows very low stability
and a narrow curve; the dough is sticky and runny with low resistance; of
course the baking performance will be impaired; if the damage is very heavy,
even a bitter taste may occur.
The enzymes in the insect's saliva are mainly of a
proteolytic nature (no amylase reported so far). Bug damage therefore results
in flour with extremely soft gluten and a quality that is poor to useless for
bread baking.
The bitterness is caused by small protein fragments (peptides)
resulting from protein hydrolysis by the enzymes. Fig. 156 compares a dough made from
bug-damaged flour with a dough to which bacterial protease had been added.
The
insect enzymes seem to be very effective in causing gluten degradation. As
little as one percent of damaged kernels can make a wheat flour unsuitable for
bread making.
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Fig. 156 : Farinogram with flour from bug-damaged wheat (left)
and from wheat flour with bacterial protease (right)
|
Recommendations for improving bug-damaged wheat
include conditioning at increased moisture and temperature in order to destroy
the insect enzymes. But it appears rather difficult to selectively denaturize
the insect proteins without degrading the wheat protein as well.
Blending with
sound flour is another option, but the damaged part must be minimized. The
baker can improve the situation by reducing the dough resting times to a
minimum, which gives the enzymes less time to act on the gluten.
Furthermore,
the miller or the baker can use improvers that strengthen the gluten (ascorbic
acid, enzymes, phosphates), block the insect enzymes (acidic or alkaline
buffers, specific inhibitors), or improve the overall baking properties
(emulsifiers, vital wheat gluten).
There are also applications in which the
bug-damaged wheat can be used as it is, e.g. as flour for biscuits, crackers or
wafers.
3. Sprout Damage
The term sprout damage describes a condition caused by high humidity prior to harvest: instead of falling into their winter rest, the kernels start the growth cycle again.
In order to activate the energy stored in the form of starch, the synthesis of amylolytic enzymes is one of the first actions of the awakening kernel.
The enzymes can have dramatic effects on flour properties, depending on the severity of the sprout damage and thus the amount of enzymes formed.
Falling Numbers, gelatinization temperature and maximum Amylograph viscosity will be lower, insufficient water absorption may be observed as well as sticky doughs, a weak dough structure, excessive browning and a coarse pore structure.
Furthermore, the crumb structure has lower elasticity. On the other hand the baked goods tend to have a better taste, and the shelf-life (crumb softness) is also improved.
There are other causes of high amylase activity in grain: (a) frost on immature heads can result in increased residual levels of "green amylase", an enzyme that builds up during grain filling and disappears when the germination- type amylase is synthesized; and (b) late-maturity α-amylase (LMA), a genetic defect of some varieties which lack the ability to degrade the "green amylase" (Wrigley, 2004).
The miller can counteract the effects of sprout damage with lower extraction rates (higher enzyme concentrations in the aleuron layer) and by keeping the starch damage as low as possible (the amylase can only act on damaged starch).
The baker should increase the acidity with sour dough or acidifiers, prepare stiffer doughs, reduce the energy input during kneading, reduce the bench time (but increase the final proof), reduce the dosage of bread improvers with strong enzyme activity and slightly increase the amount of salt (sodium chloride) if possible.
In many cases, sprout-damaged flours require larger amounts of ascorbic acid. Emulsifiers and vital wheat gluten improve the dough stability, as usual. Enzyme preparations should be used with care.
Some fungal amylase can still be added, but it should be reduced as compared to sound wheat flour; hemicellulases can be used, but since the doughs from sprouted wheat flour tend to be softer, their dosage should also be lower.
In addition to these general recommendations, pH-buffering salt compounds can be used to shift the pH away from the optimum of the cereal enzymes and improve the water absorption of the flour components.
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| Fig. 136 : Volume yield (breakfast rolls) and Falling Number as affected by the addition of an alkaline buffering agent (Rowelit) to flour from sprout-damaged German soft wheat |
Fig. 136 shows the effect of an alkaline buffering agent on Falling Numbers and on baking volume yield. Although larger amounts of the additive increase the Falling Number further, there is a distinct optimum for the baking volume beyond which the volume decreases again. The same additive also has a clear effect on gelatinization, as the Amylogram shows (Fig. 157).
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| Fig 157 : Effect of Rowelit, an alkaline buffering agent, on the Amylogram (from bottom to top: 0, 50, 100, 150 and 200 g per 100 kg flour) |
4. Ropiness
Very small organisms are responsible for ropiness. The culprit, Bacillus mesentericus, the potato bacillus – a close relative of Bacillus subtilis, the hay bacillus – occurs almost everywhere.
It is present on the field and thus on the wheat, but it may also thrive on flour residues on the walls of the bakery or on the equipment. When it finds its way into the dough it survives baking and destroys the crumb during storage of the bread.
Warm, damp conditions promote the development of the micro-organisms, so its appearance is more frequent in summer. The dough shows normal baking properties and the fresh bread has a normal appearance.
But within a short time a fruity flavour develops in the bread, turns into sweet smell and finally becomes disgusting. Meanwhile, degradation of the bread crumb takes place, along with a yellowish-brownish discoloration. When the bread is broken, thin slimy strands are formed (Fig. 158).
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| Fig. 158 : Ropiness in wheat bread (Schünemann and Treu, 2002) |
The optimum growth conditions for B. mesentericus are 37 °C and pH 6. But it can grow in the range of 10 - 45 °C and a pH of 4.9 - 9.3. At 97 °C only 90% of the spores are inactivated within 2 h, and this temperature is far higher than that reached in the bread crumb.
Thorough cleaning of the wheat reduces the chance of the bacterium to get into the bread, because it only adheres to the surface of the kernels. The baker can reduce its survival by proper cleaning of the bakery and equipment.
If ropiness has occurred, the bakery and equipment should be disinfected, e.g. by washing with vinegar solution. Acidification is a means of controlling the growth of Bacillus mesentericus in the dough, for instance with sour dough or edible acids.
Antimicrobial substances such as acetic or propionic acid and their salts (acetates and propionates) are also very effective in suppressing the growth of the organisms. Their effect is improved at lower pH values, i.e. in the presence of other acidifying agents.
Since they affect taste and yeast growth, their dosage should be limited to a required minimum. The lack of volume yield caused by the antimicrobial agents can be compensated for by increased yeast levels and with flour additives such as enzymes or emulsifiers.
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