Food Biochemistry and Food Processing (2 edition)

BLBS102-c05 BLBS102-Simpson March 21, 2012 12:2 Trim: 276mm X 219mm Printer Name: Yet to Come

98 Part 1: Principles/Food Analysis

biomaterials in the intercellular fluid play a buffering role in

keeping the pH constant. The body fluids are very complicated

buffer solutions, because each conjugate pair in the solution has

an equilibrium of its own. These equilibria plus the equilibrium

due to the self-ionization of water stabilize the pH of the solution.

Buffer solutions abound in nature: milk, juice, soft drinks, soup,

fluid contained in food, and water in the ocean, for example.

Hydrophilic and Hydrophobic Effects

Thehydrophilic effectrefers to the hydrogen bonding, polar-

ionic and polar–polar interactions with water molecules, which

lower the energy of the system and make ionic and polar sub-

stances soluble. The lack of strong interactions between water

molecules and lipophilic molecules or the nonpolar portions of

amphiphilic molecules is called thehydrophobic effect,aterm

coined by Charles Tanford (1980).

When mixed with water, ionic and polar molecules dis-

solve and disperse in the solution, whereas the nonpolar or hy-

drophobic molecules huddle together, forming groups. At the

proper temperature, groups of small and nonpolar molecules

surrounded by water cages form stable phases calledhydrates

orclathrates. For example, the clathrate of methane forms sta-

ble crystals at temperatures below 300 K (Sloan 1998). The

hydrophobic effect causes the formation of micelles and the

folding of proteins in enzymes so that the hydrophobic parts

of the long chain huddle together on the inside, exposing the

hydrophilic parts to the outside to interact with water.

Hydrophilic and hydrophobic effects together stabilize three-

dimensional structures of large molecules such as enzymes, pro-

teins, and lipids. Hydrophobic portions of these molecules stay

together, forming pockets in globular proteins. These biopoly-

mers minimize their hydrophobic surface to reduce their inter-

actions with water molecules. Biological membranes often have

proteins bonded to them, and the hydrophilic portions extend to

the intra- and intercellular aqueous solutions. These membrane-

bound proteins often transport specific nutrients in and out of

cells. For example, water, amino acid, and potassium-sodium

ion transporting channels are membrane-bound proteins (Gar-

rett and Grisham 2002).

Hydrophilic and hydrophobic effects, together with the ionic

interaction, cause long-chain proteins calledenzymesto fold

in specific conformations (three-dimensional structures) that

catalyze specific reactions. The pH of the medium affects the

charges of the proteins. Therefore, the pH may alter enzyme

conformations and affect their functions. At a specific pH, some

enzymes consist of several subunits that aggregate into one com-

plex structure in order to minimize the hydrophobic surface in

contact with water. Thus, the chemistry of water is intimately

mingled with the chemistry of life.

During food processing, proteins are denatured by heat,

acid, base, and salt. These treatments alter the conformation

of the proteins and enzymes. Denatured proteins lose their life-

maintaining functionality. Molecules containing hydrophilic and

hydrophobic parts are emulsifiers that are widely used in the food

industry.

Hydrophilic and hydrophobic effects cause nonpolar portions

of phospholipids, proteins, and cholesterol to assemble into mi-

celles and bilayers, or biological membranes (Sloan 1998). The

membrane conformations are stable due to their low energy, and

they enclose compartments with components to perform biolog-

ical functions. Proteins and enzymes attached to the membranes

communicate and transport nutrients and wastes for cells, keep-

ing them alive and growing.

Hard Waters and Their Treatments

Waters containing dissolved CO 2 (same as H 2 CO 3 ) are acidic

due to the equilibria

H+(aq)+HCO− 3 (aq)↔H 2 CO 3 (aq)
H 2 O+CO 2 (g)

HCO− 3 (aq)↔H+(aq)+CO^23 −(aq).

Acidic waters dissolve CaCO 3 and MgCO 3 , and waters con-

taining Ca^2 +,Mg^2 +,HCO 3 −,andCO 32 −aretemporary hard

waters,as the hardness is removable by boiling, which reduces

the solubility of CO 2. When CO 2 is driven off, the solution

becomes less acidic due to the above equilibria. Furthermore,

reducing the acidity increases the concentration of CO 32 −,and

solids CaCO 3 and MgCO 3 precipitate:

Ca^2 +(aq)+CO^23 −(aq)↔CaCO 3 (s)

Mg^2 +(aq)+CO^23 −(aq)↔MgCO 3 (s).

Water containing less than 50 mg/L of these substances is consid-

ered soft; 50–150 mg/L moderately hard; 150–300 mg/L hard;

and more than 300 mg/L very hard.

For water softening by thelime treatment, the amount of

dissolved Ca^2 +and Mg^2 +is determined first; then an equal

number of moles of lime, Ca(OH) 2 ,isaddedtoremovethem,

by these reactions:

Mg^2 ++Ca(OH) 2 (s)↔Mg(OH) 2 (s)+Ca^2 +

Ca^2 ++2HCO− 3 +Ca(OH) 2 (s)↔2CaCO 3 (s)+2H 2 O.

Permanent hard waterscontain sulfate (SO 42 −), Ca^2 +,and

Mg^2 +ions. Calcium ions in the sulfate solution can be removed

by adding sodium carbonate due to the reaction:

Ca^2 ++Na 2 CO3↔CaCO 3 (s)+2Na+.

Hard waters cause scales or deposits to build up in boilers,

pipes, and faucets—problems for food and other industries. Ion

exchange using resins or zeolites is commonly used to soften

hard waters. The calcium and magnesium ions in the waters are

taken up by the resin or zeolite that releases sodium or hydrogen

ions back to the water. Alternatively, when pressure is applied

to a solution, water molecules, but not ions, diffuse through the

semipermeable membranes. This method, calledreverse osmo-

sis, has been used to soften hard waters and desalinate seawater.

However, water softening replaces desirable calcium and other

ions with sodium ions. Thus, soft waters are not suitable for

drinking. Incidentally, calcium ions strengthen the gluten pro-

teins in dough mixing. Some calcium salts are added to the

dough by bakeries to enhance bread quality.