James A. O’Mahony

Rehydration and Solubility Characteristics of High-Protein Dairy Powders

Dairy powders derived from membrane filtration processes, such as milk protein concentrate (MPC) and phosphocaseinate (PC) powders, have considerable potential as functional ingredients due to their high protein content and quality. However, the use of these powders is sometimes limited or impaired by their poor rehydration characteristics in aqueous media, which has been linked with the formation of an inter-linked network of casein micelles at particle surfaces during processing and storage. Analytical tools are now available which can monitor the rehydration of dairy powders dynamically. This is a considerable development, as the rate-limiting stages of rehydration for individual powders (e.g., wetting, dispersion) can now be identified, quantified and targeted in attempts to improve rehydration properties. In addition, these technologies allow the negative effects of sub-optimal processing or storage conditions on powder rehydration and solubility characteristics to be measured, which allows preventative strategies against loss of solubility to be developed. Moreover, it is foreseeable that some of these technologies could be useful for in-line analysis and process control at an industrial scale. This review provides a detailed description of the underlying principles, data outputs and industrial relevance of current methods to monitor dairy powder rehydration. The technologies discussed in this review include viscometry and rheometry, turbidimetry, static light-scattering, focused beam reflectance measurement (FBRM), image analysis, nuclear magnetic resonance (NMR) relaxometry, thermochemistry, conductimetry and sound-based technologies. The contribution that these technologies have made to the current understanding of rehydration phenomena, with a particular emphasis on high-protein dairy powders (≥80 % protein), is discussed throughout. In addition, a comprehensive overview of rehydration and solubility characteristics, and the effects of process-, storage-, and additive-induced changes thereon, is given for high-protein dairy powders.

Physicochemical and acid gelation properties of commercial UHT-treated plant-based milk substitutes and lactose free bovine milk

Physicochemical and acid gelation properties of UHT-treated commercial soy, oat, quinoa, rice and lactose-free bovine milks were studied. The separation profiles were determined using a LUMiSizer dispersion analyser. Soy, rice and quinoa milks formed both cream and sediment layers, while oat milk sedimented but did not cream. Bovine milk was very stable to separation while all plant milks separated at varying rates; rice and oat milks being the most unstable products. Particle sizes in plant-based milk substitutes, expressed as volume mean diameters (d4.3), ranged from 0.55μm (soy) to 2.08μm (quinoa) while the average size in bovine milk was 0.52μm. Particles of plant-based milk substitutes were significantly more polydisperse compared to those of bovine milk. Upon acidification with glucono-δ-lactone (GDL), bovine, soy and quinoa milks formed structured gels with maximum storage moduli of 262, 187 and 105Pa, respectively while oat and rice milks did not gel. In addition to soy products currently on the market, quinoa may have potential in dairy-type food applications.

The physical characteristics and emulsification properties of partially dephosphorylated bovine β-casein.

Bovine β-casein was purified from phosphocasein by rennet coagulation and cold solubilisation from the resultant curd. β-Casein was then dephosphorylated using potato acid phosphatase. Urea-polyacrylamide gel electrophoresis (PAGE) of partially dephosphorylated β-casein showed a number of bands, depending on the final level of phosphorylation. Dephosphorylating β-casein increased its pH of minimum solubility from ∼pH 5 to 5.5 and reduced its net negative charge from -30.8 to -27.0 mV. During the acidification of β-casein solutions, partially dephosphorylated β-casein failed to form a gel, unlike the phosphorylated (i.e., control) β-casein. Use of partially dephosphorylated β-casein to stabilise oil-in-water emulsions resulted in larger fat globules compared to control β-casein, but such globules were less susceptible to aggregation in the presence of 15 or 30 mM CaCl(2). Overall, the dephosphorylation of β-casein resulted in a protein similar to human β-casein in terms of physicochemical functionality, with increased stability against calcium-induced aggregation.

Impact of glucose polymer chain length on heat and physical stability of milk protein-carbohydrate nutritional beverages

This study investigated the impact of glucose polymer chain length on heat and physical stability of milk protein isolate (MPI)-carbohydrate nutritional beverages containing 8.5% w/w total protein and 5% w/w carbohydrate. The maltodextrin and corn syrup solids glucose polymers used had dextrose equivalent (DE) values of 17 or 38, respectively. Increasing DE value of the glucose polymers resulted in a greater increase in brown colour development, ionic calcium, protein particle size, apparent viscosity and pseudoplastic rheological behaviour, and greater reduction in pH, hydration and heat stability on sterilisation at 120°C. Incorporation of glucose polymers with MPI retarded sedimentation of protein during accelerated physical stability testing, with maltodextrin DE17 causing a greater reduction in sedimentation velocity and compressibility of sediment formed than corn syrup solids DE38. The results demonstrate that chain length of the glucose polymer used strongly impacts heat and physical stability of MPI-carbohydrate nutritional beverages.

Salts of Milk

The salts of milk are mainly the phosphates, citrates, chlorides, sulphates, carbonates and bicarbonates of sodium, potassium, calcium and magnesium. Approximately 20 other elements are found in milk in trace quantities, including copper, iron, lead, boron, manganese, zinc, iodine, etc. Strictly speaking, the proteins of milk should be included as part of the salt system since these carry positively and negatively charged groups and can form salts with counter-ions; however, they are not normally treated as such. There is no lactate in freshly drawn milk but may be present in stored milk and in milk products. Many of the inorganic elements are of importance in nutrition, in the preparation, processing and storage of milk products due to their marked influence on the conformation and stability of milk proteins, especially caseins, in the activity of some indigenous enzymes and to a lesser extent in the stability of lipids.

Enzymology of Milk and Milk Products

Like all other foods of plant or animal origin, milk contains several indigenous enzymes. The principal constituents of milk (lactose, lipids and proteins) can be modified by exogenous enzymes, added to induce specific changes; being a liquid, milk is more amenable to enzyme action than solid foods Exogenous enzymes may also be used to analyse for certain constituents in milk. In addition, milk and most dairy products contain viable microorganisms which secrete extracellular enzymes or release intracellular enzymes after the cells have lysed. Some of these microbial enzymes may cause undesirable changes, e.g., hydrolytic rancidity in milk and dairy products, bitterness and/or age gelation of UHT milk, bittiness in cream, malty flavours or bitterness in fluid milk, or they may cause desirable flavours, e.g., in ripened cheese.

Chemistry and Biochemistry of Cheese

Cheese is a very varied group of dairy products, produced worldwide; cheesemaking originated in the Middle East during the Agricultural Revolution, about 8,000 years ago. Cheese production and consumption, which vary widely between countries and regions is increasing in traditional producing countries and is spreading to new areas. Milk for cheese is coagulated by acidification, or more commonly, by enzymatic action, and the resulting gel is processed to encourage moisture loss. Curds are salted and ripered βr up to two years during which time extensive biochemical reactions occur leading to the development of flavour and texture.

Production and Utilization of Milk

Milk is a fluid secreted by the female of all mammalian species, of which there are more than 4,000, for the primary function of meeting the complete nutritional requirements of the neonate of the species. In addition, milk serves several physiological functions for the neonate. Most of the non-nutritional functions of milk are served by proteins and peptides which include immunoglobulins, enzymes and enzyme inhibitors, binding or carrier proteins, growth factors and antibacterial agents. Because the nutritional and physiological requirements of each species are more or less unique, the composition of milk shows very marked inter-species differences. Of the more than 4,000 species of mammal, the milks of only ~180 have been analysed and of these, the data for only about 50 species are considered to be reliable (sufficient number of samples, representative sampling, adequate coverage of the lactation period). Not surprisingly, the milk of the principal dairying species, i.e., cow, goat, sheep and buffalo, and the human are among those that are well characterized. The gross composition of milks from selected species are summarized in Table 1.1; very extensive data on the composition of bovine and human milk are contained in Jensen (1995).

Physical Properties of Milk

Milk is a dilute emulsion consisting of an oil/fat dispersed phase and an aqueous colloidal continuous phase. The physical properties of milk are similar to those of water but are modified by the presence of various solutes (proteins, lactose and salts) in the continuous phase and by the degree of dispersion of the emulsified and colloidal components. The principal physical properties of milk include its density, redox properties, colligative properties, surface activity buffering capacity, rheological behaviour, conductivity, thermal properties and color.

Drying: Effect on Nutrients, Composition and Health

Drying is an effective method of extending the shelf life of nutrient-dense foods and facilitating their transport around the world. To ensure that the nutritional quality of foods is retained, appropriate drying technologies (e.g., air-, spray-, and freeze-drying) and drying conditions (e.g., heating intensity and feed composition) must be selected based on the properties of the product to be dried. The macronutrients and micronutrients that are present and the interactions that may occur between them during drying largely determine the sensitivity of the system to degradative dehydration-induced changes. Factors influencing the nutritional quality of dehydrated foods are discussed in this article, with a particular focus on process- and storage-induced degradation of micronutrients and advances in encapsulation technologies.