FAT RICH DAIRY PRODUCTS

Module 8. Quality of butter

Lesson 26
RHEOLOGY OF BUTTER

26.1 Introduction

Rheology is defined as the science of the deformation and flow of matter. The most important aspect of rheology deals with the relationship between force and time on one hand and deformation and flow of the material on the other.

Butter is an example of plastic materials that is upon application of force, the product tends to yield or undergo permanent deformation. Butter texture is a critical factor in determining functionality and consumer acceptance. It influences spreadability, taste, mouthfeel, appearance, and its suitability for various uses. Spreadability is by far is the most significant in relation to the table use of butter. Several methods have been developed to determine the spreadability of butter in particular and texture in general.

Most of the tests used to characterize butter rheology are empirical in nature and designed to imitate sensory perceptions for quality control operations. These are based mainly on the principles of penetrometry, extrusion, and sectility and involve large deformations which break down the material’s structure. The response of the material to an applied stress provides an index for some consistency parameter which is generally used to regulate a step in the butter making process.

Butter is a multiphase emulsion, consisting of fat globules, crystalline fat, and an aqueous phase dispersed in a continuous oil phase. The textural properties of butter are closely linked to their fundamental structures.

26.2 Butter Structure

Fat exists in butter in form of liquid fat and also in the form of intact globules. Liquid fat makes the continuous phase in butter. Water droplets and air cells are dispersed in this continuous phase. Some amount of fat is present as crystals. The number and size of the fat crystals greatly depend on the temperature and the temperature history. A considerable part of crystalline fat may be inside the fat globules because during churning, liquid fat is extruded from the globules, mainly by spreading over the air bubbles. But there are also crystals outside the globules and these aggregate to a continuous network and may grow together to form a solid structure, which is mainly responsible for the butter firmness. The microstructure of butter is shown in figure 26.1.

Fig. 26.1 Butter structure

26.2.1 Rheological properties of butter

The rheological properties of butter, to a large extent, are determined by the physical state of milk fat. Hence, the consistency of fat can be modified in several ways like by changing the temperature of cooling and working, and by changing the mechanical treatment. Butter being an emulsion, ratio of solid to liquid fat, crystal dimensions and polymorphic transformations play a major role.

26.2.1.1 Hardness and spreadability of butter

Butter hardness and spreadability are inversely related parameters and are two of the most important aspects of texture. They have been the two most commonly measured sensory properties as they greatly influence consumer acceptability. Butter possesses poor spreadability at refrigeration temperatures, and poor structural stability at room temperatures. At room temperature, butter also demonstrates oiling off and moisture migration. The change in butter’s solid fat content between 10 and 20°C is very pronounced, resulting in a very limited temperature range within which there is desirable spreadability. At lower temperatures, more triacylglycerols crystallize into the solid fat network. The degree of milk fat crystallinity is the largest determinant of the rheological properties of butter. For easy spreading, butter should contain between 20 and 40% fat in solid form and, in general, butters with “yield stresses” below roughly 125 kPa have satisfactory spreadability.

Butter texture depends on many interrelated parameters, and no single factor appears capable of explaining its consistency. Milk fat’s fatty acid composition and positional distribution influence the melting point, and therefore are of major importance. The iodine value and average fatty acid chain length account for much of the variation in butter firmness. A change in iodine value of 3% can effect a 50% change in firmness. Winter butter is harder than summer butter because of seasonal variations in cow feed which result in a more saturated fat overall during the winter.

26.2.1.2 Setting

Fat crystals, which are suspended in liquid fat, aggregate into 3-dimensional networks because of Brownian motion and Van der Walls forces. This aggregation continues in freshly made butter for months, resulting in increased firmness. The extent of this setting depends on a number of variables, including composition, storage temperature, storage time, the butter’s initial hardness, and the conditions during manufacturing.

26.2.1.3 Work softening

Butter may get softened by working. When the butter is subjected to extensive working, its firmness is considerably reduced. The effect of working varies according to the type and origin of butter. The greatest amount of breakdown occurs when the temperature is around 10-15°C. If the butter is allowed to stand after working, firmness gradually returns as discussed in setting of butter.