Food Chemistry

Module 9. Cereals and cereal products

Lesson 24
PHYSICAL, CHEMICAL CHANGES DURING BAKING AND DETERMINATION OF GLUTEN AND STARCH CONTENT IN FLOUR

24.1 Introduction

Several stages can be distinguished during the changes from dough to a baked product. They are as follows

--- Enzyme active stage (from 30^oC to 70^oC).

---Stage of starch gelatinization. (From 55^oC to 70^oC)

---Stage of water evaporation

---Stage of browning and aroma formation.

These changes are different in the outer portion of the dough and in the interior of the crumb. This because in the oven since heat transfer occurs slowly in the dough, there is a steep temperature gradient inward from crust of the dough. The sequences of changes taking place during conversion of foamy texture of dough to spongy texture of bread and other product by baking at a temperature of 220^oC to 250^oC are as follows.

24.2 Chemical and physical changes

• When the dough is put in the oven, the rate of fermentation initially increases as heat is conducted through the dough. Upto 50^oC, yeast produces CO₂ and ethanol at an increasing rate. At the same time the viscosity of dough falls rapidly and reaches to minimum at about 60^oC. At the same time thermal expansion of gas within each cell result in rapid expansion of loaf volume, known as “oven spring”.

• As the internal temperature of the dough increases above 37^oC, activity of yeast decreases and gets inactivated at 54^oC. At the same time, beyond 60^oC, viscosity of dough again increases rapidly. This increase is caused by swelling of starch accompanied by release of amylose and also by protein denaturation. As the crumb starch gelatinizes at 65^oC, the α and β-amylase present will attack the starch. The amylolytic activity continues until their enzymes are inactivated at about 74^oC. A optimum amylolytic activity is desirable to limit the degradation of gelatinized starch to counteract staling of bread. At the same time the denatured protein, swollen and partially gelatinized starch forms a stable crumb network at about 74^oC. This transformation continues until the end of baking when the internal temp reaches to 93 - 100^oC. During this time gluten looses its tough and elastic state and becomes stiff and brittle.

• This stiffens the starch structure so that a firm elastic crumb is formed. The starch granules of crust surface gelatinize almost completely. This is specially the case when “oven humidity” is high; the resultant starch film produces a pleasing glaze. This also retards drying and settling of the crust and permits full expansion of dough.

• The above process results in tremendous increase in the tensile strength of the dough and the increase the presence of gas bubbles. Consequently the membrane gives way and becomes permeable, allowing H2O, CO₂ & ethanol to evaporate. This results in baking weight loss. The internal temperature never exceeds 100^oC but the outer temperature reaches nearly the oven temperature (~200^oC). Thus water evaporates more from the surface & the crust is formed. This results in weight losses during crust formation upto 8-14% of the fresh dough weight.

At high temperature to which the outer part of the dough is exposed. Starch degrades to dextrin, mono and disaccharide at 110^oC-140^oC. Caramalization & non-enzymatic browning also occur at ~140-150^oC providing the sweetness and colour to the crust. The roasted flavours developed at 150-200^oC.

In the crust heterocyclic compounds pyrroline and pyridine, as well as furanone and 2 and 3 methyl butanal are formed which are responsible for the roasty, malty and caramel flavour respectively in the products. The autoxidation products of linoleic acid, such as methional, and diacetyl are also involved in the aroma of the crumb.

24.3 Changes during Storage

Bread quality rapidly changes during storage. These changes are due to

1) Moisture adsorption- the crust loses its crispiness and glossiness.

2) The aroma compounds of freshly baked bread evaporates-resulting in loss of flavour.

3) Some of the very labile aroma compounds decrease rapidly on storage due to oxidation and other reactions.

4) The crumb structure also changes, although at a slower rate. The crumb becomes firm, its elasticity and juiciness are lost, and it crumbles easily. This is known as staling defect of crumb, which is basically a starch retrogradaton phenomenon.

24.4 Determination of Gluten Content in Flour

General: Determination of the most important indices, which determine the loaf volume of bread, is the gluten content of the flour and an increase in the loaf volume of bread is noticed with an increase in the gluten content of the flour. Gluten exhibits the properties of cohesion, elasticity and viscosity which are the combined characteristics of its two insoluble component proteins i.e., glutenin and gliadin.
For good bread flour, wet gluten content ranges between 30 to 36% and dry gluten content ranges between 10 to 12%.

Apparatus

1) Mortar
2) Glass rod
3) Hot air oven
4) Desiccator
5) Analytical balance

Principle: Gluten is separated out from the flour by washing the dough made using water. The albumins, globulins and other smaller proteins as well as starch are washed away with water leaving behind a cohesive, elastic and rubbery mass called crude wet gluten. The 65-75% of water present in crude wet gluten is dried out by drying at 100 °C for 24 h (or at 133 ± 2 °C for about 2 h) and weighed to get a value of dry gluten.

Procedure

1. Weigh 25 g of flour and add about 15 ml of water and stir with a glass rod.
2. Mix it into a smooth and tight dough using fingers, taking care that handling loss is minimum and all the material is mixed into the dough.
3. Immerse the dough ball into water for about 1 h to ensure proper hydration.
4. Remove the dough ball and place it on a piece of blotting silk cloth having an aperture of 0.5 mm or 150 mm.
5. Wash it with a gentle stream of water till the water passing through the silk cloth does not contain starch i.e. the water does not turn blue when a drop of iodine solution is added (or the wash water is clear from turbidity of starch).
6. Collect the residue on the silk cloth and make it free of water by rubbing between dry palms or by using a suitable press.
7. Round it and weigh as wet gluten.
8. Dry this wet gluten in an oven maintained at 100°C for 24 h (or break the wet gluten into pieces and dry in an oven maintained at 133 + 2 °C for 1-2 h).

Observations

1. Weight of sample taken = W₁g
2. Weight of wet gluten = W₂ g
3. Weight of dry gluten = W₃ g

Calculations

1. % wet gluten = ( W₂/W₁) x 100
2. % dry gluten = (W3/W1 ) x 100
   

24.5 Determination of Starch Content in Flour

General: Starch is the major component of wheat flour. In wheat flour, starch granules are embedded in protein matrix. The major role of starch is to act as a water sink and set the system through partial gelatinization. Starch is also responsible for staling phenomenon since amylose fraction retrogrades rapidly during initial cooling of bread loaves. Slow changes in the amylo-pectin fraction are implicated in the further firming of bread during storage. Some of the normal starches get damaged during milling stage. Moderate amount of damaged starch is advisable while presence of excessive damaged starch is quite harm

Apparatus

1. Conical Flasks
2. Funnel
3. Filter papers
4. Beakers

Reagents

1. Fehling A and B solutions
2. Methylene blue indicator
3. Concentrated HCl
4. Standard glucose solution
5. 50% NaOH
6. Phenolphthalein indicator

Principle:The flour is suspended in water and undissociated residue containing starch is allowed to hydrolyse in the presence of dilute HCl. The glucose produced is filtered out and titrated against Fehling A and B using Lane-Eynon method. Value of glucose obtained is multiplied by 0.9 to get value of starch content present in flour.

Procedure

1. Take 3 g flour sample in 50 ml cold water in a conical flask.
2. Stir it uniformly and keep it aside for 1 hr with occasional stirring.
3. Filter it and wash the residue with sufficient water.
4. Heat the undissolved residue for 2.5 hrs in 100 ml of 2.5% HCl solution in a flask equipped with a reflux condenser.
5. Cool, neutralize with NaOH and make up volume of 250 ml and filter it.
6. Fill the filtrate in burette.
7. Take 5 ml each of Fehling A and B solutions in a 250 ml conical flask. Add 20 ml of water and few pumice stones and bring to boil on burner.
8. Add into it sugar solution from burette until a faint blue colour remains.
9. Add 2-3 drops of methylene blue indicator and add sugar solution till red colour precipitates of Cu₂O is produced or obtained.
10. Record the volume sugar solution used for reduction of Fehling solutions.
11. Repeat the titration using standard glucose solution.
12. Calculate the sugar present in hydrolysate.
13. Convert the value of glucose to starch by multiplying with 0.9.

Observations

1. Weight of sample taken = W gm
2. Final volume of starch hydrolysate = V1 ml
3. Volume of standard glucose solution used = V2 ml
4. Volume of starch hydrolysate used = V3 ml

Calculations

% Glucose = V₃ x 2.5 mg glucose x V₁ x100

V₂ x 1000 x W

% Starch = % glucose x 0.9