FPE20206-2019 - Chapter 1 - Food Material Properties - Theory.pdf - Chapter 1 Food Material Properties Theory Contents 1 Food Material Properties 1-2

FPE20206-2019 - Chapter 1 - Food Material Properties - Theory.pdf

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Chapter 1 Food Material Properties Theory Contents 1 Food Material Properties ............................................................................................................................................ 1-2 1.1 Introduction ........................................................................................................................................................... 1-2 1.2 Phases and states ................................................................................................................................................. 1-2 1.3 State diagrams ...................................................................................................................................................... 1-7 1.4 Chemical potential, concentration and activity ...................................................................................... 1-9 1.5 The sorption isotherm: properties of the product ............................................................................. 1-14
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FPE20306 Food Process Engineering 1-2 1 Food Material Properties 1.1 Introduction Food products are very complex products. They do not interact with their environment in a simple way. We therefore need to look more closely at some essential properties. They are critical in the treatments discussed in the next chapters: drying, dehydration, frying, crystallisation and more. The first item is to use state diagrams to estimate the states in which different phases in a product may be. This influences not only the product properties but places also limits to the methods that we have to prepare them. A state diagram relates the physical state of components, or mixtures of them to their overall composition. A sorption isotherm, the second item to discuss, relates the water activity, which is the affinity of a product to moisture, to its composition. It thus tells us more about the driving forces in the product for interaction with a (moist or dry) environment. We will first discuss phases and states of phases, and then come to the composition of state diagrams, and the difference with phase diagrams. 1.2 Phases and states In many processes used for the preparation of food products, products change state. This means that they go from solid to liquid (dissolution, melting), liquid to solid (freezing), or from a liquid into a vapour and a solid (drying). Food products are very complex materials, since the main components in a food, proteins, carbohydrates and fats do not mix. They are therefore dispersed into each other, and the way that they are dispersed is a major determinant of the quality of the food. Each major component makes up a phase . Each phase may contain many components; some ingredients may be dispersed or dissolved into more than one phase: flavours are often dissolved in the aqueous and in the oily phases of a product. However, macromolecular components such as proteins, high-molecular weight carbohydrates, such as starch, pectin, pentosans, xylans, etc., do not mix with other high-molecular weight components. Examples are - Mayonnaise: this contains an aqueous and an oil phase, the aqueous phase contains egg yolk and some vinegar. - Ice cream: a dispersion of ice crystals, solidified milk fat and air bubbles, in a concentrated solution of sugar. - Bread: a complex mixture of a protein phase (the gluten) and a carbohydrate phase (the starch), which both are the continuous phase surrounding a dispersed gas phase (the pores in the bread). - Cheese: a continuous phase of casein, interspersed with fat, and which may contain gas bubbles (the ‘eyes’) .
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