Human Nutrition 314

Banana Bread Analysis

Jackie Geitz

When presented with this project at the beginning of the quarter, millions

or recipes circulated through my head. I salivated thinking of all the possibilities.

Should I do chicken cutlets, macaroni and cheese, or maybe a vegetarian dish?

Until one day I made banana bread, and I thought to myself, how do all of these

ingredients create this decedent 9x4 loaf of pure heaven? How do sugar, butter,

eggs, bananas, milk, cinnamon, flower, baking powder, baking soda and a dash

of salt react to create such flavorful springy goodness? Challenge accepted.

The Ultimate Banana Bread Recipe

1 cup granulated sugar

8 tablespoons (1 stick) unsalted butter, room temperature

2 large eggs

3 ripe bananas

1 tablespoon milk

1 teaspoon ground cinnamon

2 cups all-purpose flour

1 teaspoon baking powder

1 teaspoon baking soda

1 teaspoon salt

Before an apron is even put on, the oven must be preheated to 325°F.

Preheating the oven creates an overshoot of temperature. Once placed in the

oven, this overshoot of heat will cause flour, egg, and milk protein coagulation

(which be explained later on) on the outside, evidently locking the outside

structure of the bread. Preheating allows the initial increase of CO2 to produce

and expand once placed in heat. Then, grease a 9 x 5 x 3 inch loaf pan.

Greasing the pan will increase the cake volume by enabling the batter to slide of

the sides of the pan. Now aprons can be applied. To start, wet and dry

ingredients will be mixed separately, and then combined, a method referred to as

the muffin method. This recipe requires creaming sugar and butter in a large

mixing bowl. Sugar contributes a variety of functions to the end product of

banana bread besides providing sweetness, such as enhancing browning,

delaying gelatinization of starch, or increasing elevation of the coagulation

temperature of eggs.

A couple things to remember about sugar are it’s ability to hinder excess

gluten formation, along with it’s hygroscopic property, enabling it to attract and

retain water or moisture from surrounding environments while lowering the water

activity of the food, locking in moisture (Reynolds 62). When combined with

butter, the 16% water that is present in the butter will gravitate to the sugar

molecules and be trapped. These sugar molecules will not dissolve in the fat, but

rather dissolve in the 16% water supplied by the fat. In addition to creating a

moist environment, the sugar will also prolong bacterial growth by competing with

microorganisms for water, an essential component to bacterial growth (Reynolds

36). Without this hygroscopic property, the end product would come out drier

because the water would evaporate quicker as a consequence of higher water

activity. The nature of sugar’s irregular shape traps in air molecules around it.

The 80% milk fat from butter will coat these trapped air molecules, further locking

them into the mixture. This allows more air to become incorporated than if this

step was skipped. Later on, I’ll find this trapped air will serve as a physical

leavening agent later on when it will help the dough to rise upon heating.

Aside from coating air molecules, the saturated fat from the butter will also

serve as a great mechanism of heat transfer. Because there is a high saturated

fat level in butter, the melting point will increase since there are more saturated

single bonds in the fatty acids from the triglycerides of the lipid. Butter will supply

a majority of the fat content in this recipe. In correspondence, the fat from the

butter will provide flavor and tenderness in the end product by shortening gluten

strands. Crystallization will be prevented by the presence of this fat from the

physical coating of the granules. Continue beating until the mixture is creamy and

fluffy. Once this is achieved, it’s time for eggs to be added.

Eggs are a huge player in the success of this recipe for a multitude of

reasons. First, let’s look at the fundamentals. Upon deconstructing the alkaline

egg white, one can find it’s made up of 88% water, the rest of the composition

containing proteins. It’s pH floats around 7.6-7.9. Egg yolks, however, have 49%

water composition, the other 51% holding lipids, lipoproteins, cholesterol, and in

particular, the emulsifier lecithin. The pH of egg yolk is slightly acidic around 6.0-

6.2. When the egg is incorporated into the butter sugar mixture, the hygroscopic

sugar will attract the egg’s water. Hydrolysis of present triglycerides (into glycerol

and three fatty acids) found in fat will also take place. Although not stated in the

recipe, eggs should be room temperature at the point of incorporation. A room-

temperature egg will have decreased surface tension allowing the egg whites of

the eggs to whip easier and to a greater volume (Brown 232).

Incorporating the eggs is a crucial role in producing desired bread, given

that they have so many functions. Adding eggs one at a time while mixing allows

ample time between additions for a slight layer of egg protein to stretch around,

and stabilize all the air molecules inside the fat-sugar mixture (Reynolds 225) to

make a stable emulsion with the butter and sugar. This thorough mixing ensures

there’s an even distribution of egg proteins throughout the compound. While

mechanical beating continues, the proteins (such as ovalbumin, ovotransferrin

and ovomucoid found in the egg white) contributing to tertiary structure of the egg

will partially denature. Cystine amino acids linked by disulfide and hydrogen

bonds will break apart through beating, uncoiling the protein, and will give rise to

sulfhydryl and hydrogen groups. The whole egg proteins will stabilize the present

air bubbles by coating them in a protein film (Reynolds 225). This film is from the

egg’s fine cell proteins stretching and trapping many air bubbles. Later addition of

heat will instigate these egg proteins to coagulate around the expanding air

molecules, solidifying the batter and acting as a leavening agent. Decreased

potential volume of this soon to be batter falls responsible to both the sugar and

fat, which lessens the beaten egg volume. But wait there’s more!

Emulsification also occurs in this creaming stage when dealing with the

butterfat that’s insoluble in the water added by the eggs. Lecithin found in the egg

yolk is comprised of phospholipids. These phospholipids have a charged portion

that is attracted to fats (end with phosphate), while the other end (fatty acid) is

drawn to water. As water from the whole egg is released into the batter and

incorporates with the present fat, the lecithin will prevent the fat and water from

separating, which in turn will stabilize, thicken (Brown 230) and bind the batter.

Emulsifying agents, also known as surfactants, reduce liquid surface tension to

increase wetting and blending ability, as we will see upon further addition of

remaining ingredients. When the egg-sugar-butter mixture appears to be uniform

or consistent, set aside, and collect milk, bananas, and cinnamon.

Combining this triad of flavors initially calls for mashing the ripe (brown)

bananas with a fork. First, how did these ripe bananas get so brown and soft,

and why use them? Please first direct your attention to the phenolic compounds

that are accountable for the browning of ripened fruit, including bananas (Brown

275). When these phenolic compounds are combined with room temperature

oxygen and a polyphenol oxidase enzyme, enzymatic browning arises. In this

enzymatic browning process, the polyphenol oxidase enzyme oxidizes the

phenolic compounds, resulting in a brown pigmentation of the banana peel. Now,

please direct your attention to pectin molecules found in bananas. Upon

climacteric ripening (ripening after harvest), pectinase enzymes break down

these pectin structures into more hydrophilic pectin, creating a softer banana

quality (Brown 274). During climacteric ripening, there is a decrease in

chlorophyll, a softening of flesh, and a development of pleasant flavors (Reynolds

141). As this banana has ripened, the organic acids responsible for the fruits

initial bitter flavor decrease (Brown 273), implementing a better recognition of the

fructose sugars. Aromatic compounds and organic acids contribute to this fruity

treat’s flavor. Furthermore, ethylene gas is emitted, which only further improves

ripening. For these reasons, over-ripened bananas will have an enhanced

“banana-y” flavor.

Fruits generally have an acidic pH, and bananas are no exception. With a

pH of 4.6, bananas will partially function as an acid in this recipe (Brown 273).

Other functions include instigating oxidizing agents (baking soda), adding flavor,

and adding liquid (water). Mashing the bananas with a fork is done simply to

break it down, ultimately releasing intercellular water and decreasing remaining

rigidity making incorporation of other ingredients feasible. Structurally, these

bananas contain 5% starch, 80% fructose sugars (Brown 278), little fiber and

protein, and have a water content around 85%. Although the quantity of starch is

small, acid hydrolysis will occur where released water and the fruit acidity will

break the starch molecules down into monosaccharides. As these starch

molecules break down from acid hydrolysis and agitation, the sweetness will

increase because the cellulose and little starch in the fruit is being converted to

sugar. Next, milk is added to this fragrant fruity goo.

The casein protein found in milk denatures and coagulates when blended

with the acid from the banana. The whey protein will coagulate later when

exposed to heat. Not only does milk contain these two main proteins, but it also

has a slightly acidic pH of 6.6 (Brown 200), fat, the lactose disaccharide, and

87% water. At this stage, milk keeps us in suspense, only divulging that it gives

flavor, is a surfactant, and contributes water. It will reveal it’s magic later on in the

cooking process. Aromatic cinnamon is also incorporated, providing flavor, and

being a ground spice, acts as a surfactant as well (Reynolds 115). As soon as

the dynamic trio is combined to your liking, set aside and move on to the dry

ingredients.

Gather flour, baking powder, baking soda and salt. In a separate bowl, mix

together all ingredients to ensure a uniform mixture. No reactions will take place

at this point seeing as no liquid is available to penetrate the flour starch granule

and flour protein or to react with baking soda and baking powder to emit CO2.

Salt is important for flavor purposes, but that’s not all. It can also aid in

denaturation, (Reynolds 106) prolong staling by competing with microorganisms

for water and lowering water activity, (hygroscopic tendency), and once

combined with gluten and water, will strengthen gluten protein in flour. However,

in this recipe, salt is generally added for flavor purposes. Finally, after joining all

these dry ingredients together, we are ready to combine wet and dry components

together.

The long awaited anticipation of marrying these liquids comes to an end

as the banana compound is stirred into the butter-sugar-egg mixture. Stirring is

preferred at this step over mechanical beating in order to lessen agitation and to

maintain the stabilized structure made by the egg proteins. We do not want to

beat the eggs more for fear of breaking the thin protein film surrounding the air

molecules. We’ve segregated these ingredients into two wet groups up to this

point in order to separate acidity and to prevent premature dissolving of sugar,

allowing the air bubbles trapped by the irregular shaped sugar granules to be

stabilized by the egg proteins first, creating a preferred stable emulsified airy

texture. Combination with the liquid from the milk and banana will cause the

sugar to dissolve into the mixture.

Reactions in this step are limited. Slightly lowered pH of the batter induced

by the bananas acidity will improve the volume slightly by helping protein cross-

links form, promoting additional stabilization of the egg structure. Milk will

increase the coagulation temperature of the egg proteins by increasing the

protein concentration. Acidity from the bananas may be low enough to coagulate

the casein proteins, which commonly coagulate by acid. However, judging by the

consistency of the banana-milk mixture, the absence of curdles indicates there is

no coagulation.

Stir just until combined to restrict the indispensable accumulated air

molecules from escaping. At this point, you will add the dry ingredients.

Reactions begin as soon as the flour hits the liquid. All-purpose white flour, such

as the one used in this recipe, is composed of only 90% starch and 10% protein

as a result of the grain being stripped of the germ and bran. Hydration of the flour

proteins glutenin and gliadin with water (present from the banana, butter, egg,

and milk) stimulates the formation of gluten. Glutenin will help form disulfide

bonds between proteins. These flour proteins have relatively low protein content,

thus having low glutenin and gliadin (responsible for absorbing water) levels,

which cause less water absorption. Addition of water will hydrolyze the starch

molecules.

Fats and sugars in the dough will decrease the amount of gluten formed.

Sugar will compete for water with the starch granule, decreasing the amount of

gluten, yielding more tender dough. Fats and milk solids (from the butter) will

coat the gliadin and glutenin strands, ultimately shortening the formation of gluten

strands, also yielding a tender product. This is important because we want the

bread to come out flaky and tender. If too much gluten is formed, more strength

and elasticity of the gluten will develop, causing the bread to be tough and

chewy.

Emulsification will occur again while mixing, this time using milk proteins

as an emulsifier to improve texture. The phospholipids’ membrane proteins of

casein and whey function the same way and lecithin does. Emulsifiers will evenly

distribute fat throughout the dough, as we’ll see in the finished product. Milk will

also help retain moisture in the bread because of its high water content (87.4%)

and having lactose sugar that is hygroscopic, lowering the water activity.

Lowered water activity prolongs staling by not allowing water to easily evaporate

out of the product.

When stirring, air bubbles will appear on the surface of the dough.

Chemical leavening agents, baking soda and baking powder, are to blame for

this reaction. Baking powder, which is simply baking soda and a dried acid,

dissociates upon contact with water by hydrolysis. The calcium phosphate plus

sodium bicarbonate of baking powder will react with water to produce the

gaseous CO2. Alkaline baking soda is not quite the same. Being comprised of

only sodium bicarbonate, baking soda needs water and an acid to react with in

order to produce an acid-complex that intensifies pleasurable flavor. This acid-

complex is the product of baking soda neutralizing excess acid from other

ingredients. Getting rid of these extra acids brings out the flavor of the

ingredients. In addition to this acid-complex, CO2 is also emitted. Stirring

disperses the CO2 all the way through the dough. Baking powder and baking

soda, also classified as oxidizing agents, help reform gluten cross-links to secure

the dough structure while baking.

The baking soda reaction takes place instantly once water and acid are

introduced from the acidic flour, banana, and baking powder where as baking

powder reacts with only water instantly, and then again later when exposed to

heat. Bananas provide necessary acidity for these leavening agents, along with

the flour and egg yolk. It’s very important that as much CO2 is contained inside

the batter to receive optimal leavening, creating a light and fluffy bread. Extra

baking soda will increase the leavening of the baking powder by also providing

CO2.

Fats and proteins in this mixture will soon inhibit gelatinization of starch

granules by delaying hydration, decreasing the thickness (viscosity), and lower

gel strength (Reynolds 83). Hydration will be delayed as hygroscopic sugar

competes for water with the starch granule. Proteins and fats will coat the

granule, slowing the absorption process of water, and get in the way of the re-

association of dissolved amylose molecules.

Stop mixing as soon as flour disappears in order to protect the batter from

losing any essential CO2. Excessive or prolonged stirring will slowly eliminate the

CO2 air cells incorporated in the batter, making the final bread flatter by reducing

the amount of CO2 the bread has to be leavened by. Gluten formation will also

increase as mixing continues reason being that mixing will help glutenin and

gliadin from the flour protein to unnecessarily associate. To avoid the risk of

having a tough and chewy banana bread, mixing should stop as soon as

possible. Air cells stabilized by the emulsion must also be maintained, and

excessive mixing could disrupt the structure by breaking the protein film, allowing

the air to escape. The surfactants discussed earlier, such as milk proteins and

the egg yolk’s lecithin, will allow the surface tension of the dough to lessen and

facilitate blending/wetting. Pour batter into pre-greased loaf pan. As for the next

step, I hope you have your oven mitts handy.

Place the pan in the oven for one hour. During this long hour, reactions

are taking place. First, while air is a poor conductor of heat, the metal pan is a

great conductor. The circulation of air will heat the metal pan faster than the

actual dough. Once the pan gets hot enough, the egg, flour, and milk proteins in

contact with the pan will coagulate before the inside, creating and locking the

outside structure. At random, the lone sulfhydryl and hydrogen groups of cytosine

from the egg albumen, whey protein, and flour protein will rearrange and link

together by means of disulfide and hydrogen bonds to form a new linkage with

another protein or within one protein; this process is known as coagulation.

Oxidative agents such as baking powder and baking soda, aid in cross-link

formation.

Heat induced denaturation and coagulation are a part of the baking

process. Casein milk protein will not coagulate solely upon heat because it lacks

an adequate amount of amino acids that have sulfur to form the disulfide bonds.

Therefore, Casein coagulates by acid. Denaturation and coagulation will happen

throughout the entire batter. Denaturation changes the function of proteins by

changing its tertiary structure. At 140°F, the egg white will coagulate, and at

149°F the egg yolk will coagulate. When the batter reaches 158°F, the egg white

and egg yolk will firm. Protein coagulation gives strength and structure to the

banana bread, allowing it to hold its shape. Hydration of proteins with the present

water facilitates a gel formation, which is a complex network of protein strands

that trap the water. This gel formation, or gelation gives a firm structure. Once the

structure of egg, gluten and other proteins stabilize and strengthen, the bread will

stop rising, turning the soupy mixture into a solid.

When the batter reaches a moderate heat of about 310°F, the

carbohydrates (sugar, banana, starches from flour) carbonyl groups and the

protein’s (milk, flour protein, eggs) amino groups present will react with each

other, forming a Maillard reaction. Consequences of this reaction are that the

dough will form brown coloration, flavor is generated, and aromatic compounds

will be released into the air, giving your kitchen a mouth-watering smell

(Reynolds 65). Sugar enhances this browning by adding pure sugar to react with

proteins and heat for the Maillard reaction. Baking soda and the egg whites also

enhances the browning of the Maillard reaction because their alkaline nature

increases the pH. Bananas are affected by heating as well. The hydrolysis of

pectin will convert into pectic acid. Pectic acid will cause the banana texture to

become more mushy and soft (Brown 275).

In the first 10 minutes of baking, oven springing will occur, which is the

rapid expansion of the batter as an effect of the expanding gases. As the batter

heats up, water from the eggs, butter, banana, and milk will turn to steam that will

expand the bread as it, itself, expands, creating the dough to rise. Increased heat

of the CO2 and the air molecules will cause them to enlarge. The emulsification

created earlier in creaming that trapped air molecules now comes into play. The

protein and gluten surrounding these air molecules will stretch (especially gluten)

along with the expanding gas and eventually coagulate, forming air cells in the

bread as well as causing the bread to rise. The heat along with the sodium

aluminum sulfate (SAS) present in baking powder will initiate its second reaction.

SAS will generate sulfuric acid upon heating, and this sulfuric acid will react with

the baking soda to give off additional CO2 to provide addition volume in the

bread.

Heating continues, and the fats in the batter will melt. Moisture present in

the fat, like butter, will turn to steam and expand the bread. The starch granules

in the flour will begin gelatinization first in response to heating. Gelatinization of

starch differs from the gelation of protein. In this stage, starch granules will begin

to swell as water is soaked in. The starch granule structure will start to

disorganize. Some soluble amylose molecules in the starch are extracted and

blended into the surrounding water. While still heating, pasting of starch will

begin after gelatinization. This process carries on the swelling of the starch

granule, extraction of amylose, and structure disorganization. However, during

this process, the starch granule eventually bursts, releasing amylose and

amylopectin starch molecules. Addition of water will hydrolyze the starch

molecules. Sugar, fats, and proteins delay this hydration of water.

The top of the bread will brown because it’s exposed to the hot circulating

air, intensifying the Maillard reaction browning. Cracks will form on the top as

CO2 continues to heat and escape after the protein coagulation has completed,

breaking through the top of the bread. Stick a toothpick in at the end of the hour,

to check that coagulation has occurred successfully. When the toothpick comes

out clean the bread can be removed and placed somewhere to cool for 15

minutes. Then invert bread onto a rack to cool completely.

Cooling is a critical role of the baking procedure. During the first 15

minutes of cooling, cooking the amylose from the starch comes to an end after

two steps. First, gelation or gel formation of starch (different from gelation of

protein) transpires once the product is removed from the heat source. Previously

dissolved amylose will slowly begin to recollect, molding a disordered structure.

Amylose will form complex networks by binding with another amylose or an

amylopectin branch. Further cooling allows a more organized structure to form,

where the network of amylose and amylopectin will trap water during

retrogradation. Amylose bonds continue to form, eventually pulling together tight

enough to push water out of its gel. This is known as synersis, also a part of

retrogradation. This water trapping retains moisture in the bread.

While fat melted, it combined with amylose molecules in the batter,

forming an inclusion compound, ultimately hindering gel formation and

retrogradation. Sugars, fats and proteins decrease gel formation described

previously. Sugar caramelization gives the top crust its rich deep brown color. In

this case, thermal degradation is the result of the top sugars being heated above

their melting point. A lower pH setting as an outcome of the acidic flour, banana,

and baking powder promotes browning and aromatic volatiles. After a few

minutes, the bread retracts from the pan. The air molecules will shrink and

contract upon cooling. This will evidently shrink the bread itself, and it will pull

away from the pan.

Continued cooling will structurally solidify and set protein structures, starch

structures, solidified fats, as well as secure intercellular water, increasing rigidity.

Initial removal of bread has a softer quality because there’s more air in the bread

that hasn’t yet contracted, and there’s still steam that hasn’t evaporated out.

Gaseous water trapped in the bread will condense while cooling, and be

reincorporated back into the bread by formation of hydrogen bonds. Presence of

proteins and carbohydrates help in binding water, keeping moisture. Residual

heat (especially from eggs) will continue to cook the proteins, and bread in

general. Inverting the bread will allow the bottom of the bread to thoroughly cool,

and prevent the proteins and starches from further stretching by cooking more.

Prohibiting this extra stretching of proteins and glutens prevents the banana

bread from becoming tough, chewy, and dry. Once completely good, take a slice,

and enjoy!

Learning the biochemistry components that contribute to make this

decadent, moist, and flavorful bread makes it that much more enjoyable and

rewarding when eating. Comprehending the functions of each ingredient and

purpose of each step while baking builds a fundamental base of knowledge that

near guarantees successful desserts every time, no matter what the recipe.

Developing this understanding and awareness of food science only fuels my

enthusiasm and hunger to continue cooking.

Directions of Baking Process

1. Preheat the oven to 325 degrees F. Butter a 9 x 5 x 3 inch loaf pan.

2. Cream the sugar and butter in a large mixing bowl until light and fluffy. Add the

eggs one at a time, beating well after each addition.

3. In a small bowl, mash the bananas with a fork. Mix in the milk and cinnamon.

4. In another bowl, mix together the flour, baking powder, baking soda and salt.

5. Add the banana mixture to the creamed mixture and stir until combined. Add

dry ingredients, mixing just until flour disappears.

6. Pour batter into prepared pan and bake 1 hour to 1 hour 10 minutes, until a

toothpick inserted in the center comes out clean.

7. Set aside to cool on a rack for 15 minutes. Remove bread from pan, invert

onto rack and cool completely before slicing.

Sources:

Brown, A. Understanding food, principles and preparation. 3. Wadsworth Pub Co,

2007.

Dr. Reynolds. Fundamentals of food: Lecture and laboratory notes. Department

of Human Nutrition. 2011-2012.