Lipids comprise a set of compounds that are soluble in organic solvents and marginally soluble or insoluble in water. Although this is a very broad definition, lipids contain a vast number of organic substances. However, the majority of lipids from plant and animal origin are made up of glycerol esters of fatty acids, i.e. monoacylglycerides (MAGs), diacylglycerides (DAGs) and triacylglycerides (TAGs). Depending on their composition and melting temperatures, lipids are referred to as fats (solid lipids) or oils (liquid lipids).
Lipids may either be used in foods as hidden constituents (i.e., ice creams, cheese, cookies, etc.) or as the major visible form (i.e., butter, lard, ghee, etc.). Although dietary lipids have been in the spotlight due to associated negative human health implications including obesity, cancer, cardio-vascular disease and metabolic syndrome, they do play a crucial role in human nutrition. They provide calories, essential fatty acids and act as carriers for fat soluble vitamins. From a structural standpoint, fats increase the palatability of foods, providing essential characteristics in foods such as texture and functionality. Both texture and functionality are influenced by the chemical composition of the triacylglycerols, their crystalline structure and microstructure, as well as their melting and solidification behavior.
A great deal of work examining the effect of lipids on the physical properties of foods has been carried out. Lipids are extremely useful and desirable in structuring foods, largely through the formation of a fat crystal network. The mechanism of network formation is complicated and the resultant structural properties are influenced by many factors .
The microstructural organization of fat crystals has a major impact on the mechanical properties of the food. The importance of hierarchies in the structural organization is stressed in an attempt correlate microstructure to macroscopic properties. The above figure depicts the hierarchies in fat crystal networks. Past work has focused on lipid composition, polymorphism and solid fat content to interpret the mechanical strength of the network while ignoring the microstructure. However, the microstructural level of a fat crystal network (i.e., 0.25-200 μm) greatly influences the hardness of the fat. It was first noted by van den Tempel that this level of structure has a large effect on macroscopic rheological properties of the fat network. deMan (1964) and deMan and Beers (1987) furthered the understanding of the effect of microstructure on macroscopic hardness when examining milk fat. At this time it was demonstrated that the microstructure had a great deal of importance on the spreadablility of chemically interesterified and enzymatically interesterified milk fat and milk fat-canola oil blends. This study indicated that there were factors other than SFC that influence the elasticity of fats. It is important to note that the composition, polymorphism, solid fat content and microstructure of triacylglycerols all play a crucial role in the structure of lipid based products. The solid-like properties of the fat are derived from the hard stock triacylglycerols including saturated- and trans-fatty acids.
Most commonly, saturated and trans fatty acids are used to provide structure to lipid based food products. Although the structure they confer on products is desirable, and indeed required, in many products, both types of fatty acids have been shown to deleteriously influence human health. For example, when healthy individuals were fed diets enriched with trans- and saturated-fatty acids a reduction in the high-density-lipoprotien (HDL), the cardioprotective form of cholesterol, was observed. In addition, trans-fatty acids increase concentrations of lipoprotein A. Both the reduction in the high density lipoprotein (HDL) and elevations in lipoprotein A increase the risk of ischemic heart disease leading to increased risk of myocardial infarctions. At the same time, it is important not to generalize all saturated fatty acids as detrimental. Stearic acid does not increase low density lipoprotein (LDL) or decrease HDL; there is no adverse health effects of this fatty acid as it pertains to lipoprotein metabolism . Polyunsaturated oils (i.e., linoleic, docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA)) have beneficial health implications due to the fact that they do no elevate levels of LDL in the blood.
Another health implication for the use of trans- and saturated-fatty acids is metabolic syndrome. It is estimated that 234 million people will be suffering from metabolic syndrome worldwide (Roche, 2005). Metabolic syndrome comprises several risk factors including: impaired insulin sensitivity, dyslipidaemia, abdominal obesity and hypertension. Metabolic syndrome leads to a high risk of subsequent development of type 2 diabetes mellitus, cardiovascular disease and premature death. The estimated trans fat intake in the United States is 2.6% of their total energy and 7.4% of their energy from fat. Negative health implication associated with the intake of trans and saturated fats may be reversed by altering the intake of these fatty acids and replacing them with polyunsaturated fats. It is estimated that replacement of 5% of our daily intake of energy from saturated fats with either equivalent energy from carbohydrates or monounsaturated fatty acids would be associated with a decreased risk of CVD in the range of 22 to 37%.
For semisolid fat products (i.e., ice cream, margarine, and chocolate), the solid lipids, which normally exist as a 3-dimensional (3D) colloidal fat crystal network, determine the physical properties of the product. Upon crystallization, hardstock triacylglycerols aggregate to form fat crystals which appear to aggregate, in a similar fashion as colloidal gels, to form clusters. These clusters aggregate into flocs and finally weak links between the flocs form in the final macroscopic network, as portrayed in this figure (Figure produced by A.G. Marangoni, Univeristy of Guelph).
Heat, mass and momentum transfer conditions have significant effects on the final microstructure and resultant macroscopic physical properties (i.e. hardness, yield stress and compressibility) of fat products. For example, many studies have shown that the hardness of a fat crystal network is directly correlated to hardness determined by sensory analysis, and thus the sensory perception of the food product.
Soft, plastic materials,
including fats, have different levels of structure present and each level
influences the macroscopic properties of the material (Tang and Marangoni,
2006). The rheological properties of fats are the result of the combined effects
of solid fat content (SFC), polymorphism, and fat crystal network
microstructure, including the shape, size, area fraction and the distribution
pattern of the fat crystals. Since it is the
hardstock triacylglycerols (TAGs) that are responsible for the network
structure, it is often difficult or impossible to eliminate these ingredients
to improve the health aspects of the product, without sacrificing some of the
characteristic properties of the product. For example the TAG profile is responsible
for the narrow melting range of chocolate and the broad melting profile of