Principles of Dairy Science

1. Principles of Dairy Science Jordan University of Science and Technology Faculty of Agriculture Nutrition and Food Technology Department Lecture 5 Dr. Yousef Tawalbeh

2. Milk Components CARBOHYDRATES • Lactose is the major carbohydrate in milk. • The sugar has been found in milks of most mammals and is unique to milk. • Milk contains traces of other carbohydrates but no polysaccharides. • Lactose can be separated from milk by letting it crystallize. • In the industrial manufacture of lactose, crystallization is used on a large scale. • Lactose is, for instance, used in several foods. • The pharmaceutical industry uses large amounts; almost all pills contain lactose as a filling material.

3. Chemical Properties of Lactose • Lactose is a disaccharide composed of d-glucose and d-galactose • The aldehydegroup of galactose is linked to the C-4 group of glucose through a β-1, 4-glycosidic linkage

4. Chemical Properties of Lactose • Hydrolysis of lactose by acid does not occur easily. • If it occurs (high temperature, low pH), many other reactions take place as well. • Lactose can, however, simply be hydrolyzed using the enzyme lactase (β-d-galactosidase). • This enzyme is highly specific for the β-1,4 linkage of a galactopyranose residue. • Acting on lactose, the enzyme produces, besides glucose and galactose, some di- and oligosaccharides foreign to milk, up to a few percent of the hydrolyzed lactose.

5. Chemical Properties of Lactose • Several reactions of lactose occur when milk is heated. • Lactose may isomerize into lactulose. • That means that the glucose moiety converts to a fructose moiety as mentioned in the above figure . • The quantity of lactulose in heated milk products can be used as an indicator for the intensity of the heat treatment.

6. Milk Components CARBOHYDRATES • On heating, caramelization also can occur. A mixture of reaction products is formed, including: Hydroxymethylfurfural Furfuryl alcohol • Acetol CH3⋅CO⋅CH2OH • Methylglyoxal CH3⋅CO⋅CHO • Formaldehyde HCOH • Acetic acid CH3⋅COOH • Formic acid HCOOH • Pyruvic acid CH3⋅CO⋅COOH • Levulinic acid CH3⋅CO⋅CH2⋅CH2⋅COOH

7. Milk Components CARBOHYDRATES • The proportions of the products formed depend on concentration of sugar, pH, heating time, and temperature. • The very important Maillard reaction occurs in the presence of amino compounds (in milk it mainly concerns the ε-amino group of the lysine residue in proteins). • The initial reaction product undergoes a series of rearrangements, yielding nitrogenous reaction products in addition to such products as mentioned above for caramelization. • Further reactions lead to brown color, loss of nutritive value, and off-flavors. • All these changes occur on prolonged storage, and especially during heating

8. Milk Components CARBOHYDRATES • Lactose is approximately 0.3 times as sweet as sucrose. • The sweet taste in milk is somewhat masked by the protein, primarily the casein. • Because of this, whey has a sweeter taste than milk. • The mixture of glucose and galactose formed by hydrolysis tastes much sweeter than lactose.

9. Physicochemical Aspects of Lactose Solubility • As seen in the following Figure, α- and β-lactose differ considerably in solubility and in the temperature dependence of solubility. • If α-lactose is brought in water, much less dissolves at the outset than later. • This is because of mutarotation: α-lactose is converted to β, hence the α-concentration diminishes and more α can dissolve. • If β-lactose is brought in water, more dissolves at the outset than later (at least below 70°C): • on mutarotation more α-lactose forms than can stay dissolved, and α-lactose starts to crystallize. • The solubility thus depends partly on the mutarotation equilibrium, the rate of dissolution on the mutarotation rate

10. Structure of α- and β- Lactose • α- and β- Lactose are isomers, which are differentiated by the orientation of the hydroxyl group at the position 21 • α- Lactose β- Lactose

11. Physicochemical Aspects of Lactose Crystal Forms • Usually, α-lactose crystallizes as a hydrate containing one molecule water of crystallization. • The crystals are very hard, slightly hygroscopic, often fairly large, and dissolve slowly. • The water of crystallization is very strongly bound. • Above 93.5°C, anhydrous β-lactose crystallizes from an aqueous solution. • β-Lactose is not very hygroscopic, and it dissolves quickly; its solubility is good.

12. Physicochemical Aspects of Lactose • Amorphous lactose is formed during rapid drying, as in a spray drier. • It is present in the glassy state, which means that many properties, including hardness, density, and specific heat, are similar to those of the crystalline sugar but that the packing of the molecules does not show perfect order • Amorphous lactose contains at least a few percent of water and can quickly dissolve on addition of water. • But then, α-lactose hydrate may start to crystallize. • If the water content of the amorphous lactose is low, say 5%, crystallization is postponed.

13. Physicochemical Aspects of Lactose • However, the product attracts water from moist air, and when moisture content rises to about 8%, α-lactose hydrate starts to crystallize (at room temperature). • The postponed crystallization is an important factor in relation to spray-dried powders made from skim milk or whey because it leads to hard lumps in the powder; eventually, the whole mass of powder turns into one solid cake.

14. Crystallization of α-Lactose Hydrate • This crystallization is of great practical importance. Because α-hydrate is poorly soluble, it may crystallize in some milk products, especially ice cream and sweetened condensed milk. • Large crystals can easily be formed because both nucleation • and crystal growth are slow. • We usually have to add numerous tiny seed crystals to ensure the rapid formation of sufficient, hence small-sized, crystals. • To prevent segregation and development of ‘‘sandiness’’ in milk products, the largest crystals formed should be no more than 10 μm in size. • This implies that at least 1010 crystals per gram of crystalline lactose should be present.

15. Thanks