Date of Award:

1995

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Nutrition, Dietetics, and Food Sciences

Department name when degree awarded

Nutrition and Food Sciences

Advisor/Chair:

Donald J. McMahon

Abstract

It has been shown that iron binds strongly to the proteins in milk, and our aim was to determine whether or not this binding was affected by lowering pH in the manufacture of yogurt. Iron-protein complexing was studied using two different techniques. 1) Skim milk was fortified with 10 mg iron/100 ml and the pH of the milk was adjusted to 6.7, 6.2, 5.8, 5.3, 4.5, and 4.0. The milk was fractionated by ultracentrifugation at 52,000 x g for 60 minutes. The pellets and serum were then analyzed for iron, calcium, and phosphorus content by inductively coupled plasma spectroscopy. SOS-PAGE gels were used to determine protein profiles in the pellets and serum. 2) Yogurt was made from milk fortified with FeCl3, iron complexed with casein, and iron complexed with whey proteins. Small samples of the yogurt were then freeze-dried on carbon coated grids and examined by transmission electron microscopy at 80 KV.

Affinity of iron for milk proteins was independent of pH. Iron fortification of milk did not cause loss of calcium or phosphorus from casein micelles. Electron spectroscopic imaging (ESI) showed that iron was bound to casein when yogurt was fortified with FeC13 or iron-casein complex. When fortified with iron-whey protein complex, the iron was distributed throughout the non-micellar portion of the yogurt.

To determine effects of iron on yogurt quality, low-fat (2%) and nonfat iron fortified yogurt was made with three sources of iron: FeCl3, iron complexed with casein, and iron complexed with whey protein, at three levels (10, 20, 40 mg/kg). Iron content and lipid oxidation were determined over one month of storage at 4°C.

Iron fortification had no effect on the rate of fermentation by the lactic cultures. There was no significant increase in oxidation levels between iron-fortified yogurt and unfortified yogurt (P > .05). No differences in the appearance, mouth feel, flavor, and overall quality ,between iron-fortified yogurt and unfortified yogurt were detected in consumer sensory analysis. Our study showed that high quality iron-fortified yogurt could be manufactured without added food safety risks.

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