Dr. Hurley’s Magical Milk Show IAVAT STUDENT BRANCH Dr. Hurley’s Magical Milk Show Walter Hurley Professor Animal Sciences Crystal Allen PhD Student Animal Sciences

The Main Milk Components Water Lactose Lipid Proteins Minerals Vitamins Others IAVAT STUDENT BRANCH

Each component can be viewed as to its : Biochemical & physicochemical properties Mechanisms of component synthesis Importance to the nursing young Importance as a food source for humans Factors that affect component variability IAVAT STUDENT BRANCH

Milk is suppose to be easily digested – perishable For the pig, the milk is usually no more than an hour and a half old from synthesis to digestion: Gland is emptied by piglet, starts refilling with milk (milk synthesis & secretion) Milk accumulates in the gland for ~45 min (suckling interval) Milk is ingested by piglet, ~15-20 seconds (milk ejection) Milk digested by piglets stomach and intestine Piglet suckles again 45 min later IAVAT STUDENT BRANCH

Dr. Hurley’s Magical Milk Show IAVAT STUDENT BRANCH Dr. Hurley’s Magical Milk Show These experiments will demonstrate: Milk phases. Preparation of butter from cream. Precipitation of milk protein with acid. Enzymatic hydrolysis of milk protein. You will conduct a series of experiments with milk that demonstrate the various fractions and phases of milk.

Physicochemical Properties of Milk Milk is: an emulsion of fat globules floating in the plasma phase of milk (skim milk), which in turn is a suspension of casein micelles suspended in the milk serum phase (whey), which in turn is composed of all the water soluble components (lactose, whey proteins, etc) Milk viewed through a microscope Lower magnification Higher magnification IAVAT STUDENT BRANCH

IAVAT STUDENT BRANCH Cream Skim Milk fat, some protein Experiment A: Observe and compare thickness of the cream layer in the large tubes. What is the composition of each layer? Cream Milk fat, some protein Skim Proteins, lactose, water IAVAT STUDENT BRANCH

IAVAT STUDENT BRANCH Processing of milk from the store: Skim Raw Milk Cool to 4 C Cream 40% fat Centrifuge 60 C Cream 12% fat Skim Homogenize Standardized milk Pasteurize Packaging IAVAT STUDENT BRANCH

IAVAT STUDENT BRANCH Pasteurization: Ensures safety and enhances shelf life of the milk. Time & Temperature “low” pasteurization: 15 seconds at 72 C or 15 sec at 75 C for homogenized milk “high” pasteurization: 15 sec at 85 C “ultra-high temp” [UHT milk]: 2 sec at 140 C or 3 sec at 135 C Consequences of pasteurization: Kills bacteria Inactivates some enzymes (from bacteria or from the milk) IAVAT STUDENT BRANCH

IAVAT STUDENT BRANCH Homogenization: Prevents the formation of cream layer in stored milk. High pressure forces liquid through a narrow opening Fat globules are deformed and disrupted Must be proper ratio of fat and milk protein so that the protein covers the surface of the smaller droplets so that they will not re-form larger droplets IAVAT STUDENT BRANCH

Experiment B: Butter from cream Observe consistency of cream in the container cream. Vigorously shake the container until butter is formed. Observe consistency of the butter compared with that for cream (step 1). What is the consistency of the cream initially and after butter is formed. What happened to the milk components for the butter to form? Butter Butter “milk” IAVAT STUDENT BRANCH

Physicochemical Properties of Milk Milk is: an emulsion of fat globules floating in the plasma phase of milk (skim milk), which in turn is a suspension of casein micelles suspended in the milk serum phase (whey), which in turn is composed of all the water soluble components (lactose, whey proteins, etc) Milk viewed through a microscope Lower magnification Higher magnification IAVAT STUDENT BRANCH

Milk Composition Breed Variability Milk fat concentration varies among the breeds of dairy cattle lowest % fat - Holstein highest % fat - Jersey and Guernsey IAVAT STUDENT BRANCH

IAVAT STUDENT BRANCH Species Variability Cream Cheese = 44% fat, 6% protein, 45% water Fat percentage is the most variable component among species. Fat % IAVAT STUDENT BRANCH

Experiment C: Precipitation of casein with acid Swirl the milk in the flask marked Skim and note the consistency of the film of milk that stays on the wall of the flask. Slowly add an acidic solution drop-wise to flask. Swirl the flask after each drop. Count the drops. Observe when a precipitate forms (on the sides of the tube). Allow precipitate to settle. What are the two layers called that are formed by this process? What milk component is precipitated under these conditions? Why does this component precipitate under these conditions? Skim milk Whey What are the components of the two layers? Curds IAVAT STUDENT BRANCH

Experiment D: Enzymatic hydrolysis of milk protein (casein) to form a curd. Gently shake contents of the tube marked Whole to remix the milk phases. Dump the contents of one tube marked Whole into the tube marked Rennin, and dump the other tube marked Whole into the tube marked Pepsin. Seal the tubes with parafilm, mix and hold in your hand to warm. Observe each tube every few minutes for hardening of the curd. Turn the tubes upside down to determine when the curd has formed. What is the consistency of each curd? What happened to the milk for the curds to form? Pepsin digested Rennin digested IAVAT STUDENT BRANCH

scanning EM of casein micelle Casein is the major protein in cow milk Total milk protein = 80% casein + 20% whey proteins Casein is secreted as a micelle (granular) structure The casein micelle also contains: the casein proteins calcium and phosphorous PO3- Ca++ PO3- Ca++ PO3- PO3- Casein protein IAVAT STUDENT BRANCH scanning EM of casein micelle

IAVAT STUDENT BRANCH Casein micelle Perhaps >1000 varieties of cheese Started >8000 years ago Micelle structure IAVAT STUDENT BRANCH

IAVAT STUDENT BRANCH Cheese Making Heating of milk-promotes bacterial growth Inoculation- buttermilk/yogurt contains bacteria which acidifies (ferments) the milk Rennin-enzyme digests casein (milk protein), causing coagulation Curds & Whey-liquid whey separates from coagulated milk Pressing-gives shape, squeezes out whey Curing-bacteria acts upon curds, changing flavor and texture of cheese IAVAT STUDENT BRANCH