Characterizing porous food systems using fundamental measurements of ultrasound, rheology, and thermal analyses

Kelly Anne Ross, Purdue University

Abstract

Texture is an important parameter that dictates the acceptability or unacceptability of foods. The texture of porous foods is strongly affected by their structure in terms of bubble/pore size distribution and porosity. Consequently, there is need for an effective means of evaluating the textural properties of porous foods. The measurement of the glass transition temperature (Tg) has been noted in the literature to be a viable method of obtaining an index of texture. Dynamic mechanical thermal analysis (DMTA) is commonly used in food science research as a method of obtaining T g measurements. This work has shown that the level of porosity affects the measurement of Tg. Most likely the effect of porosity on measuring the Tg is an artifact of the DMTA test. Therefore, the effect of porosity should be acknowledged when discussing Tg measurements. Since the characterization of bubbles or pores in porous food systems is an effective means of determining the textural quality, another aim of this work was to develop methods capable of characterizing bubbles/pores. Agar gel systems were subjected to different mixing conditions to impose different bubble class sizes into the gels. A fundamental ultrasonic spectroscopic analysis was performed and indicated that ultrasonic attenuation behavior could successfully determine the dominant class sizes of multiple bubbles in agar gels. Moreover, ultrasound attenuation behavior may be useful in determining the spacing distance between bubbles/pores in a system, which may also given an indication of the structure of the system. Microscopy coupled with conventional image analysis was used as a method to validate the ultrasound bubble sizing data. The two-point correlation function, which as its roots in physical statistics, was studied as an alternative method for conventional image processing and was used to measure bubble/pore size, porosity, and spacing between bubbles/pores. Good agreement was found between the ultrasound analysis, conventional image analysis, and the two-point correlation function. An interesting tangential result of this work was the effect of mixing speed. Mixed agar gels were stronger than the no mix agar gels even though the no mix agar gels exhibited negligible porosity (i.e. no bubbles). Differential scanning calorimetry (DSC) indicated that gels mixed at different conditions underwent key thermal transitions at different temperatures. Work was performed on different wheat doughs subjected to different mixing conditions; conventional rheology and ultrasound measurements can be used to determine the effect of mixing on the rheological characteristics of wheat doughs. From all of this, it appears that the fundamental ultrasound measurements namely, velocity and attenuation, have the potential to be used as an on-line quality control technique in food processing operations.

Degree

Ph.D.

Advisors

Campanella, Purdue University.

Subject Area

Food science

COinS