Food systems, heterogeneous

Ice cream serves as a wonderful (and tasty) example of a complex, dynamically heterogeneous food system. A typical ice cream mix contains milk or cream (water, lactose, casein and whey proteins, lipids, vitamins, and minerals), sucrose, stabilizers and emulsifiers, and some type of flavor (e.g., vanilla). After the ingredients are combined, the mix is pasteurized and homogenized. Homogenization creates an oil-in-water emulsion, consisting of millions of tiny droplets of milk fat dispersed in the water phase, each surrounded by a layer of proteins and emulsifiers. The sucrose is dissolved in... 

Differences in mobility of various components (e.g., starch, sucrose, water) within a food system (e.g., a cookie), as well as the inherent heterogeneity of many food systems (e.g., crust versus crumb of a cookie), suggest the need to measure more than an average Tg for a system. Ruan and Chen (1998) proposed the creation of a Tg map to capture the distribution of Tg values within a food system. Since conventional techniques used to measure Tg do not have the capacity at the present time to provide spatial information, Ruan and Chen (1998) suggested the use of MRI, as a function of temperature, to produce a Tg map. ... 

McClements, D.J., Decker, E.A. (2000). Lipid oxidation in oil-in-water emulsions impact of molecular environment on chemical reactions in heterogeneous food systems. Journal of Food Science, 65, 1270-1282. 

Many of these food systems are dispersions that belong in the realm of colloids. Colloids are defined as heterogeneous or dispersed systems that contain at least two phases—the dispersed phase and the continuous phase. Colloids are characterized by their ability to exist in either the sol or the gel form. In the former, the dispersed particles... 

Since food systems usually contain a heterogeneous mixture of proteins and polysaccharides differing in chemical nature, conformation, chain rigidity, size, and shape of molecules, degrees of hydrolysis, denaturation, dissociation and aggregation, information on model systems containing two biopolymers is not sufficient to interpret phase behaviour and functionality of real dispersed food systems. 

In virtually all food systems, there are numerous small particulates (dust, etc.) or surfaces (vessel walls, etc.) that catalyze formation of nuclei by providing a portion of the energy needed to form the interface. Thus, in most situations relevant to food manufacturing, homogeneous nucleation does not occur and nucleation based on a heterogeneous mechanism dominates. 

Lecithin interaction with other food ingredients. Food systems are usually heterogeneous mixes of components, in which the interaction of ingredient classes... 

In addition, water availability as discussed in terms of water mobility here may only be relevant at the local level where the microorganisms are located. If the system is so efficiently heterogeneous that water molecules cannot rapidly exchange among different domains or locations, the mobility effect is only local. NMR parameters can be manipulated to discriminate various water populations and hence the method can be made robust for studying water availability in a given heterogeneous food system. This would be... 

This selective review, which deals primarily with the chemical modification of soy proteins, is further limited to nondestructive chemical reactions which alter physical and biochemical properties of importance in food systems. Soy protein products have been modified by various chemical reactions including (a) treatment with alkalies and adds, (b) acylation, (c) alkylation and esterification, and (d) oxidation and reduction. In most instances these reactions have been applied to heterogeneous protein mixtures containing nonprotein impurities, and often to proteins of unknown prior history. Nonetheless, these reactions indicate that protein functional properties of value in food fabrication can be altered significantly through reaction with chemical reagents. It is recognized that chemically modified proteins must be critically evaluated for food safety. 

It is clear that for some heterogeneous food systems, at least two optima exist, as indicated in the uppermost curve shown in Fig. 5, which is taken from Rockland and Nishi (1980). This curve represents the relationship between a and the integrated resultant relative stability based on the summation of a series of independent and/or interdependent chemical reactions, which are characterized diagrammatically in the figure. This figure presents an updated diagrammatic summary of a and its major effects upon some chemical, enzymatic, and microbiological properties of foods. 

Quaternary structure of proteins in food systems is important for a number of reasons. Firstly, many food proteins are large and oligomeric in nature, composed of either identical or heterogeneous subunits. Examples include p-lactoglobulin (dimer), avidin... 

Objective Evaluation of Color. In recent years a method has been devised and internationally adopted (International Commission on Illumination, I.C.I.) that makes possible objective specification of color in terms of equivalent stimuli. It provides a common language for description of the color of an object illuminated by a standard illuminant and viewed by a standard observer (H). Reflectance spectro-photometric curves, such as those described above, provide the necessary data. The results are expressed in one of two systems the tristimulus system in which the equivalent stimulus is a mixture of three standard primaries, or the heterogeneous-homogeneous system in which the equivalent stimulus is a mixture of light from a standard heterogeneous illuminant and a pure spectrum color (dominant wave-length-purity system). These systems provide a means of expressing the objective time-constant spectrophotometric results in numerical form, more suitable for tabulation and correlation studies. In the application to food work, the necessary experimental data have been obtained with spectrophotometers or certain photoelectric colorimeters. 

NMR spectroscopy is one of the most widely used analytical tools for the study of molecular structure and dynamics. Spin relaxation and diffusion have been used to characterize protein dynamics [1, 2], polymer systems[3, 4], porous media [5-8], and heterogeneous fluids such as crude oils [9-12]. There has been a growing body of work to extend NMR to other areas of applications, such as material science [13] and the petroleum industry [11, 14—16]. NMR and MRI have been used extensively for research in food science and in production quality control [17-20]. For example, NMR is used to determine moisture content and solid fat fraction [20]. Multi-component analysis techniques, such as chemometrics as used by Brown et al. [21], are often employed to distinguish the components, e.g., oil and water. 

A disperse system is defined as a heterogenous, two-phase system in which the internal (dispersed, discontinuous) phase is distributed or dispersed within the continuous (external) phase or vehicle. Various pharmaceutical systems are included in this definition, the internal and external phases being gases, liquids, or solids. Disperse systems are also important in other fields of application, e.g., processing and manufacturing of household and industrial products such as cosmetics, foods, and paints. 

Heat storage in biological systems is much the same as for the previous material classes. For example, the heat capacity of biological fluids, which are heterogeneous suspensions and mixtures, can be approximated by Eq. (4.47). In instances where the biological fluid or food substance is an aqueous-based suspension or solution of fat and solids, the specific heat (heat capacity per unit weight in J/g K) can be approximated as... 

I suggest that variable plant chemistry, by restricting resource availability and focusing the activities of herbivores on a few tissues, promotes compromises between food-finding and risks from natural enemies which are not readily countered by most insects. The spatial and temporal heterogeneity which appears to be common in forest trees is the most important part of the tree s defensive system, and is the only way a plant s chemical defenses can remain effective over evolutionary time. This variable impact on natural enemies may be more important in regulating consumption than any single factor can be. 

Incorporation of the appropriate kinds of (nano)particles into heterogeneous mixed biopolymer systems may offer new opportunities for the generation of novel structures having interesting physicochemical properties. In particular, the concept of particle structuring at liquid-liquid interfaces may have future possibilities in food-based biopolymer-based delivery and encapsulation technology. 

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