Lipids

TA Documents

Department of Nutrition Science

Purdue University

Purdue griffin

Post Lab Discussion

Lab Preparation 

Lipids

End of lab discussion

Experiment 1 - Odors

- short chain fatty acids have unpleasant odors

 
Lard    - stinks; mostly contains oleic (18:1), palmitic (16:0), stearic (18:0) and linoleic (18:2).

- has not been highly refined

Hydrogenated vegetable oil

- hydrogenation process improves the oxidative stability of fats

- does not smell

Refined corn or cottonseed oil:

- refining removes substances that impart undesirable flavor, color, keeping quality or odor (free fatty acids, phospholipids, pigments, e.g., carotenoids, carbohydrates, proteins, water, tannins, phenols) - refining process: can not use metal machinery = metal ions would catalyze oxidative rancidity.

stearic (18:0) or palmitic (16:0) acid:

- both are in lard

- also found in butterfat, cocoa butter, beef and mutton tallow

butyric (4:0) or caproic (6:0) acid:

- stinks the worst because the short fatty acids are volatile

- in butterfat

olive oil:

- a highly monounsaturated oil (high amounts of oleic acid 18:1)

- is difficult to refine, so it has some odor

butter:

- buttermilk like smell

- contains some butyric (4:0) and caproic (6:0) acid which is what smells when butter goes rancid

- also has a lot of palmitic (16:0) and oleic (18:1) acid

margarine:

- made from vegetable oils which are unsaturated, e.g., corn oil

- has been hydrogenated, so should have little odor

- high in oleic (18:1) and linoleic (18:2) acids

marine oils:

- high degree of unsaturation

- therefore, more susceptible to oxidation => smell

- made up of long chain polyunsaturated fatty acids (with up to 6 double bonds)

sucrose esters:

- smell sweet

- sucrose molecules that have been esterified with one to eight fatty acids

sucrose polyesters:

- have more fatty acids

- can not be digested

- have 0 calories

- used as fat substitute, e.g. Olestra

Experiment 2 - solubility, SG and RI

Solubility:

- lipids are most soluble in nonpolar solvents, e.g. toluene

 

Polarity

Most polar

Slightly polar

Least polar

Water, alcohol

Chloroform

Toluene, hexane

Lipids least soluble

Lipids slightly soluble

Lipids most soluble

Each lipid has a characteristic SG (specific gravity) and RI (refractive index)

RI (refractometer): Increases with unsaturation (double bonds)

Increases with chain length

Useful for measuring the degree of hydrogenation of oil in industry

SG (hydrometer): Density increases with double bonds, the molecules are more densely packed

----------------------------------------------High RI----------------- High SG

soybean- linolenic acid group 18:3------------1.4735   ----------------0.927

and linoleic group 18:2

corn- linoleic acid group 18:2-----------------1.4730-------------------0.922

cottonseed- linoleic acid group 18:2----------1.4729--------------------0.917

peanut- oleic-linoleic acid group -------------1.4691-------------------0.915

18:1, 18:2       

olive- oleic-linoleic acid group-----------------1.4679-------------------0.918

18:1, 18:2

safflower- oleic-linoleic acid group-----------1.4620-------------------0.918

18:1, 18:2

lard- animal fat group; highly sat.--------------1.452    ----------------------?

-----------------------------------------------low RI-------------------low SG

 

Above taken from p. 101, Lee, Basic Food Chemistry

Questions p. 57

1. In which solvents did the oil dissolve? Why?

The lipids were most soluble in toluene because it is a nonpolar solvent.

 
2. What quality of oil is demonstrated by specific gravity?

Density of oil is reflected by the specific gravity (unitless) = ratio of density of an oil / density of water.

Density increases with an increase in the number of double bonds (unsaturation) because the molecules are more densely packed.

 
3. Relate the structure of the various oils to their refractive index.

As unsaturation increases and length of chain increases, the refractive index increases. Soybean oil has the highest refractive index because it is the most unsaturated. Also, as refractive index increases, the specific gravity increases.

 
Experiment 3 - Water Absorbing Capacity (WAC)

As the amount of MG (monoglycerides) and DG (diglycerides) increases, WAC increases

Presence of charged molecules (proteins) - increases WAC

Fats must have WAC to hold water in foods that need to emulsify water and fat (baked goods)

e.g. cake mix: type of fat makes difference - polar lipids help to emulsify

 
1. Hydrogenated shortening-has polar lipids, should be the highest in WAC, when processed add MG and DG

2. Butter -proteins from milkfat are charged and can hold water;

phospholipids, PO4 charged and can hold water

3 .Soft butter-can hold more water than regular margarine because of MG

and DG added - have more OH groups

4. Margarine-has some OH groups but not as much as soft butter,

holds less water than soft butter

5. Lard-no water holding capacity

 

Questions p. 57

1. What factors influence the emulsifying capacity of fats?

Emulsion capacity depends on the polarity of a lipid, MG and DG, charged proteins.

 
2. Why is the emulsifying capacity of fat important?

It is important for product moistness, especially for things like baked goods, e.g., cakes..

  
Experiment 4 - Plasticity of Fats

 Plasticity => ease with which you can deform a substance

- in the case of creaming ability of fats => ability of fats to incorporate air by whipping

- creaming ability is related to crystal size: smaller they are => the better creaming ability

- more air => lighter => less densely packed => lower specific gravity

 
Sugar is used to beat with fat to incorporate the air.

1. hydrogenated shortening: holds air most; small crystals; best creaming

low specific gravity - not lumpy

2. soft margarine/ butter :about the same size crystals, crystals larger than hydrogenated shortening

3. margarine: large crystals

4. lard: worst creaming ability - makes soggy; low plasticity; high specific gravity

 

Questions p. 58

1. What determines the creaming ability of a fat?

Crystal size determines it: small crystals => best.

 
2. Why is the creaming ability of a fat important?

Creaming ability influences the texture (heavy or light) of a food.

 
Experiment 5 - Fat bloom in chocolate

Crystals

- relatively smaller crystals are desired to hold fat under surface of the chocolate

- to be stable, chocolate needs smaller b-3 V (beta) crystals so they don't rise to surface and form white fat bloom on top (b -3 VI).

- larger, unstable seed crystals are formed in rapid cooling (step 2, p.59)

- heat => rapid cool => destroys crystal structure (beta-3 V), forms crystals which float to top and give fat bloom by recrystallizing into larger, more stable beta crystals (b -3 VI).

- therefore, raise and lower temperature of chocolate slower to prevent fat bloom. Generally, fat bloom is due to temperature abuse of chocolate.

 
Questions p. 59

1. Compare the surface color and appearance of the samples and the unmelted square.

The chocolate with fat bloom should have a white layer on the top while the unmelted square should be all brown.

 
2. Describe why the specific treatments affected gloss as you observed it.

See above.

Smoke point

 
Hydrolytic rancidity:

Triglyceride => glycerol + FA ==> acrolein (causes smoke)

Acrolein formation

 High contents of MG and DG will lower smoke point (takes less energy to get rid of 1 FA or 2 FA than 3 FA - triglyceride)

 
Questions 

1. Why is the smoke point of lipid an important consideration in food preparation?

Because if the smoke point is reached, the product is only a few degrees below the flash point.

 
2. How is the smoke point affected by the addition of monoglycerides?

MG and DG lower the smoke point 4-5o C (takes less energy to get rid of 1 FA or 2FA than 3 FA).

Experiment 6 - Oxidative Rancidity

Oxidative rancidity

- cannot visualize directly

- add carotene which changes from yellow/orange to colorless when oxidized

- rendered fat is used (bacon fat; can't use commercial lard as it contains BHA or BHT)

- cooler, darker => less oxidation

- catalysts = heat, light, metals (Cu, Fe); BHA - allows itself to be attacked before lipids in the food are.

Phenolics, quinones, and BHA

- plant phenolics - naturally do what BHA is formulated to do

- saturated salt solution - somewhat of an antioxidant (the particles slow down the reaction

1 a. no oxidation (dark)

b. oxidation (light = catalyst)

c. less oxidation than at room temperature (no heat or light)

d. some oxidation (heat present)

2 a. no oxidation (control)

b. Cu => catalyst - oxidizes the fat

c. Hemoglobin has Fe => catalyst - oxidizes fat

(this is why we refrigerate meats, among other reasons)

d. BHA - antioxidant

e. salt solution - particles slow down the oxidation reaction

f. plant phenolics - naturally occurring antioxidant- donates H to free radical and stops the propagation of oxidation

Top

Lipids

Lab preparation

Experiment 1 - Odors and Physical State of Lipids and Fatty Acids

Get lipids out - they should be on the bottom shelf behind the right chalkboard. Some of the lipids are supplied by the storeroom.

Do NOT take the butyric acid out of the can. It really smells bad.

 

Experiment 2 - Solubility, specific gravity and refractive index

The vegetable oils are supplied by the storeroom

Make sure there is enough of the organic solvents chloroform, toluene and methanol - located in the cabinet under the second hood. Get solvents out and put in hood. Need about 150 ml of each/lab

Place 3 graduated cylinders in the hood to measure the solvents.

Get out large test tubes - they are in the cabinet underneath the front right bench.

Get out the refractometer, Kimwipes, disposable pipettes and water squirt bottle.

Get out waste cans for the lipids (these may be supplied by storeroom)

 Experiment 3 - Water Absorbing Capacity

Did not use the powdered shortening as one of the fats

Make sure that there are enough mixing bowls in the lab. If not get additional ones from the storeroom.

If we don't have enough burettes, use pipettes to deliver the stream of water.

Get pipettes out- they are located in the drawers under the Spec 20s.

Experiment 4 - Plasticity of Fats

The sugar is under the front left bench-make sure there is enough in the bin.

Make sure to follow the procedure for determining the specific gravity (found in the back of the lab book in the equipment guide, page 126)

Make sure that the metal, 1 cup, measuring cups are available for the specific gravity determination

 

Experiment 5 - Fat Bloom in Chocolate

This experiment may not work due to lack of time for fat to bloom. Try doing it as a demonstration by preparing it beforehand. (We did get a little bit of fat bloom when we tried the procedure the day prior to lab)

Adjust water bath in the back of the room near the texture analyzer to 38-42o C to melt the chocolate.

Follow the procedure carefully!


Experiment 6 - Oxidative Rancidity

Get out disposable petri plates - they are under the seed volume apparatus.

Prepare the BHA (or BHT), hemoglobin, saturated salt and copper sulfate solutions.

There are some already prepared in the solution cabinet but check for mold or other objects in the bottles that shouldn't be there. If so, make new solutions.

Get out filter papers - in the back of the room in the drawer under the right window or in right front bench in drawer.

Prepare rendered fat (lard) and carotene mixture.

Carotene is in the freezer in the desiccator.

Melt fat and add beta-carotene dissolved in chloroform.

Prepare in a glass container.

Keep warm as to not allow mixture to resolidify.

(This mixture oxidized when stored overnight. Follow directions carefully. May need to be prepared the day of lab).

Turnip greens, onions and potato prepared by Jan in the storeroom and are found in the refrigerator.

Calibrate incubators (get thermometers from storeroom and put in incubators to check actual temp)--the incubators may already be labeled.

One under the water bath - 60o C.

Silver one under ovens - 40o C.

Get out forceps (Teflon-coverd ones)--in left front bench in drawer closest to windows. If can not find, cover regular forceps with white tape on the end because we don't want the metal to come in contact with the solution.

Top

Home | Lab Documents | TA Lab Documents

Web Author: James R. Daniel, Ph. D.

Last Updated: