Masters Theses

Date of Award

5-2001

Degree Type

Thesis

Degree Name

Master of Science

Major

Food Science and Technology

Major Professor

Jochen Weiss

Committee Members

Greg Hulbert, John Mount

Abstract

Pectin, a gel forming plant polysaccharide, is primarily composed of linear polymers of D-galacturonic acids. The carboxyl group of pectin is esterified to various degrees with methanol and this forms the basis for classification of pectin into highmethoxyl (>50% esterified) and low-methoxyl (<50%) pectins. A major source for commercial pectin for food and pharmaceutical uses is apple pomace. It has been shown that ultrasonic pretreatment of apple pomace can increase the yield of pectin by more than 15%. At the same time, little is known about how ultrasound can affect the physicochemical properties of pectin (e.g. gelation). The main objectives of the research were to (1) Determine the effect of pH, calcium concentration, calcium-pH interaction and temperature on the gelation properties of pectin (to find a optimum gel) and (2) Study the effect of ultrasonic processing (sonication time and power) on the texture and appearance of pectin gels. Structural changes during gelation of pectin solutions were followed by measuring the dynamic rheological properties (storage modulus (G'), loss modulus (G") and complex viscosity (η*)) as a function of time at various oscillation frequencies using a rotational rheometer (MCR-300 Paar Physica, NJ). UV-VIS spectra of pectin gels were measured using a spectrophotometer (Hewlett Packard HP 8452A). The process of network formation leading to pectin gelation was faster at lower pH conditions and the resulting gel strength was significantly higher. Addition of 0.05 g CaCl2 / g of pectin increased the pectin gel strength by more than 10 times. Calcium and pH interacted in a unique way in determining the gel strength of pectin. As the pH of the system increased, a higher amount of calcium was required to form a strong gel. Gel strength was directly related to the setting temperature in the range of 0-50°C. More rigid gels were formed at higher setting temperature, which was attributed to the increase in hydrophobic interactions. Ultrasonically pretreated pectin formed slightly weaker gels with increasing sonication time and power. The time dependence of phase angle δ, which is defined as arctan (G'/G') was used to extract information about the gelation kinetics. The gelation kinetics of sonicated pectin was considerably slower that the kinetics of gelation of pectin that was not treated with ultrasound. However, the gels formed from ultrasonically pretreated pectin were much clearer as indicated by absorbance data (50% less turbid). Ultrasound has both negative and positive effects on the physicochemical properties of pectin. Although ultrasound increases the yield of pectin it weakens the strength of the gels. Ultrasound on the other hand improves the optical properties of the gels since transparent gels are usually more desirable in the food industry than opaque ones. Further research is currently planned to evaluate the potential of ultrasound to be used as an alternative processing method to physically modify hydrocolloids such as pectin.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS