Influence of pH on surface properties of aqueous egg albumen solutions in relation to foaming behaviour

The surface properties of aqueous egg albumen protein solutions (0.1 g litre⁻¹) were studied at pH values of 4.8, 7.0, 9.2 and 10.7 and related to foaming behaviour such as bubble size distribution, overrun and drainage. By measurements far from equilibrium of dynamic steady state surface dilation using the overflowing cylinder technique, egg albumen showed ability to slow down surface expansion and to lower the dynamic surface tension. The pH‐effect was small, but at pH 4.8 the film length, at which a motionless surface was created, was longer than at higher pH indicating a somewhat more rigid surface at low pH. Near equilibrium sinusoidal surface area deformation resulted in relatively high moduli of egg albumen, with a significant effect of pH. The surface modulus E showed at pH 4.8 an increase in the course of time, but at higher pH it was constant. Large deformation of egg albumen surface was not destructive, and for all pH values the surface behaved viscoelastic, with highest loss modulus E″ and tan θ values at pH 4.8. Surface deformation frequency sweeps revealed the relaxation processes to be relatively slow at pH 4.8 and faster at pH 7.0–10.7. Foamability measured as overrun of foam as a result of shaking and stirring was highest at pH 4.8 and lowest at pH 10.7. Foam stability against drainage was best at pH 7.0 after 30 min, but at a long‐term scale foam at pH 4.8 was most resistant to drainage. Foam samples were subjected to microscopy and image analysis. The smallest bubbles were found at pH 4.8 (mean diameter 142 µm) and the largest at pH 7.0 (mean diameter 328 µm). In conclusion, the foaming behaviour of an aqueous egg albumen solution at pH 4.8 can be related to dynamic surface properties as follows: the more rigid behaviour of the surface at this pH favours a small bubble size and slow drainage of liquid from the foam. The high overrun at this pH can be explained by a lower dynamic surface tension, but also here film stability during foam making can be promoted by a more rigid liquid surface. © 1999 Society of Chemical Industry     

Influence of pH-Induced Unfolding and Refolding of Egg Albumen on Its Foaming Properties

ABSTRACT: The foaming properties of egg albumen, which had been subjected to low and high pH unfolding followed by refolding, were investigated. The foaming capacity of egg albumen, the stability of the foam, or both, could be improved by an unfolding and refolding regime by choosing proper unfolding and refolding pH values. The foaming capacities of egg albumen were greatly improved when the refolding was at pH 6.5, 7.5, or 8.5, whereas the foaming capacities could be either slightly increased or decreased when the refolding was at pH 4.5 or 5.5 compared with the controls. The foam stability was in almost all cases improved by the unfolding and refolding treatments except for a few cases of unfolding at pH 1.5 or 10.5. The foam stability and liquid drainage were improved most when the unfolding was at pH 12.5. Analysis of total and surface sulfhydryl groups, surface hydrophobicity, and protein analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) provided strong evidence that the partial unfolding of egg albumen proteins as well as the interactions among egg albumen proteins through disulfide and/or hydrophobic groups dictated the improvements in foaming properties. The increase in surface hydrophobicity showed better correlation with the improvement of foaming properties than the change of surface sulfhydryl content did.     

The significance of critical processing steps in the production of dried egg albumen powder on gel textural and foaming properties

The purpose of the present work was to study the variation in egg albumen functional properties as a function of seven selected processing steps from the initially raw liquid albumen to the final dried egg albumen powder. Albumen samples in six replicates were analysed for dry matter, pH, glucose, triglycerides, albumen gel properties (ie textural stress and Hencky strain, water‐ and protein‐binding capacity and gel colour L*, a*, b*), foaming properties (ie overrun, stability against drainage and bubble breakdown) and surface tension. The gel texture and water‐holding capacity significantly decreased during the processing steps from raw albumen through storage, centrifugation and ion exchange, whereas the final dry‐pasteurisation resulted in increased gel properties. Covalently linked protein polymers formed during the dry‐pasteurisation, as revealed by SDS‐PAGE, may explain this improvement. The foam overrun increased twofold during the three final steps of ultrafiltration, spray‐drying and dry‐pasteurisation compared with the raw albumen; however, the foam stability decreased, ie drainage and foam volume breakdown rates increased. The surface pressure increase was positively correlated with the foam overrun. This paper reveals at which processing steps the control of functional properties of egg albumen powder can take place. Copyright © 2004 Society of Chemical Industry     

Rheological properties of angel food cake made with pH unfolded and refolded egg albumen

Angel food cake was made using egg albumen subjected to the pH-induced unfolded and refolded treatment. The effect of treatment on the rheological properties of angel food cake was investigated. Egg albumen solutions were prepared and pH was adjusted to 1.5, 4.5, 8.5 or 12.5 and then held 60 min to unfold the proteins. After holding, the solutions were readjusted to pH 4.5 or 8.5, and held for 45 min to partially refold the proteins. Egg white foam with sucrose was whipped and flour and the rest of sucrose were folded into the foam. The foam batter was heated in a TA Instrument AR 2000 controlled stress rheometer equipped in a parallel geometry. Samples were heated from 21 °C to 150 °C at a rate of 8.5 °C per min and then cooled down to 21 °C to bake the angel food cake. At 21 °C, oscillatory stress sweep was performed. There was no relationship between the G′ value of angel cake batter and its G′ value at 150 °C. Changes in rheological properties of batters and angel food cakes using different combinations of ingredients were studied. The pH unfolding and refolding procedure led to more rigid final products compared to the controls with egg albumen samples not subjected to pH treatment. Adding sucrose to the flour increased the starch gelation temperature up to 82 °C. Higher protein concentrations resulted in better foams in the cake batter, but the batter made with an intermediate protein concentration produced the most rigid angel food cake. Adding egg albumin did not change gelation temperature of the starch. It appears that incorporation of flour with the egg white foam, leads to about a ten times decrease in the strength of the foam, and a decrease in the gelatinization process of starch after adding sugar, are crucial in forming of an angel food cake texture.     

Rheological properties of foams generated from egg albumin after pH treatment

Rheological properties of foams at pH 4.5 and 8.5 made from egg albumin proteins after pH-induced unfolding and refolding treatments were investigated. The foams behaved as highly elastic materials. Static and dynamic yield stress was investigated using small, steady shear experiments. Yield stress was measured also in oscillation mode and high correlation was found between dynamic yield stress and critical stress amplitude. All pH treatments led to firmer foams than untreated foams at pH 8.5. At pH 8.5 the control foam had a very weak structure (static yield stress at 3.4 Pa and dynamic yield stress at 5.5 Pa for the fresh made foams), while pH-treated foams at pH 8.5 had values (static yield stress up to 39.2 Pa and dynamic yield stress up to 47.1 Pa for the fresh made foams) that were even higher than the values for the control at pH 4.5 (static yield stress at 22.4 Pa and dynamic yield stress at 25.4 Pa for the fresh made foams, which is a good foaming pH for egg albumin). It was shown that an increase in yield stress of foams after drainage is related to foam stability and liquid drainage. This study demonstrated that unfolding egg albumin at low or high pH followed by refolding leads to a substantial increase in foam firmness and gives the foam different properties than foams from untreated egg albumin.     

Effect of pH and Food Ingredients on the Stability of Egg Yolk Phospholipids and the Metal-Chelator Antioxidant Activity of Phosvitin

Egg yolk phosvitin has been previously shown to inhibit metal-catalyzed phospholipid oxidation. In this study, a phospholipid emulsion system was used to study the effect of pH and food ingredients on the antioxidant activity of phosvitin and the oxidative stability of yolk phospholipid. Oxidation of phospholipids was carried out at five different pH levels (3.0, 5.0, 5.7, 6.1, and 7.8). NaCl and freeze-dried egg albumen were incorporated into the pH 6.1 emulsion. Lipid oxidation was measured by the thiobarbituric acid (TBA) assay. Phospholipids were stable at pH 6.1 and 7.8. However, phosvitin was unable to inhibit Cu²⁺ catalysis at pH 7.8. Neither NaCl nor albumen affected the stability of phospholipids or the activity of phosvitin in inhibiting Fe²⁺ catalysis at pH 6.1.     

Degradation of Iso‐α‐Acids During Wort Boiling

A detailed study on the degradation of iso‐α‐acids was conducted. Because of the complexity of the wort matrix and interfering interactions during real wort boiling, the investigation of degradation kinetics was performed in an aqueous solution. Degradation was investigated as a function of time (0–90 min), temperature (80–110°C), pH value (4–7), original gravity (10–18°P) and ion content of the water (0–500 ppm Ca²⁺ and Mg²⁺). After 90 min of boiling, over 20% of the dosed iso‐α‐acids could no longer be detected. A strong dependence of degradation could be shown due to high temperature, low pH, high original gravity and a high Mg²⁺ content. The cis:trans ratio and co‐iso‐α‐acid content did not change significantly. Losses of isohumulones could be lowered by reducing the temperature and original gravity, as well as by heightening the pH value. High amounts of Ca²⁺ and Mg²⁺ salts also led to an increase in degradation products. Solutions to decrease degradation and thereby possible improvements in sensory bitter quality are discussed.     

Determination and depletion of amoxicillin residues in eggs

Eggs were used to study the determination and depletion of amoxicillin (AMO) residues after oral dosing hens (25.0 mg kg^–1, 50.0 mg kg^–1 body weight), once daily for five days. A reverse-phase high-performance liquid chromatography with fluorescence detection (RP-HPLC-FLD) method was developed to determine AMO residues in albumen, yolk and whole egg. By using pre-column derivatisation, an improved liquid–liquid extraction procedure was developed for sample preparation. AMO were extracted from eggs with acetonitrile. The extract solution was extracted using saturated methylene chloride. The supernatant was reacted with salicylaldehyde under acidic and heating conditions. Limits of detection (LODs) were 1.2 ng g^–1 and limits of quantification (LOQs) were 3.9 ng g^–1 for AMO. Recoveries of AMO from samples fortified at levels of 5.0–125.0 ng g^–1 ranged from 79.1% to 88.5% in albumen, 78.6–83.6% in yolk and 78.3–85.1% in whole egg, with coefficients of variation of ≤7.3%. The maximum concentrations of AMO in albumen, yolk and whole egg were found to occur at 1.5, 2.5, 1.5 days after withdrawal of medication respectively. AMO was not detectable in albumen at 7.5 days after final administration of AMO, at 10.5–11.5 days in yolk and 10.5 days in whole egg after administration of two oral doses. The method was applied during the residue study of AMO in order to formulate a reasonable withdrawal period to ensure food safety.     

Prediction of Egg Freshness and Albumen Quality Using Visible/Near Infrared Spectroscopy

Important changes occur in egg during storage leading to loss of quality. Prediction of these changes is critical in order to monitor egg quality and freshness. The aim of this research was to evaluate application of visible (VIS) and near infrared (NIR) spectroscopy as a rapid and non-destructive technique for egg quality assessment. Three hundred and sixty intact white-shelled eggs freshly laid by the same flock of hens fed with a standard feed were obtained. They were put under controlled conditions of temperature and humidity (T = 18 °C and RH = 55%) for 16 days of storage. Forty eggs were analyzed at day 0, 2, 4, 6, 8, 10, 12, 14, and 16. Transmission spectral data was obtained in the range from 350 to 2,500 nm. The non-destructive spectral data was compared to egg sample’s Haugh unit (HU) and albumen pH in terms of quality and to the number of storage days in terms of freshness. A partial least squares predictive model was developed and used to link the destructive assessment methods and the number of storage days with the spectral data. The correlation coefficient between the measured and predicted values of HU, albumen pH, and number of storage days were up to 0.94, R ² was up to 0.90 and the root mean square error values for the validation were 5.05, 0.06, and 1.65, respectively. These results showed that VIS/NIR transmission spectroscopy is a good tool for assessment of egg freshness and albumen pH and can be used as a non-destructive method for the prediction of HU, albumen pH, and number of storage days. In addition, the relevant information about these parameters was in the VIS and NIR ranging from 411 to 1,729 nm.     

The effect of pH and calcium ion on rheological behaviour of β‐lactoglobulin‐basil seed gum mixed gels

Effect of pH (4.5–7.5) and Ca²⁺ (0.01–0.5 m ) on gelation of single and mixed systems of 10% β‐lactoglobulin (BLG) and 1% basil seed gum (BSG) was investigated. The gelling point of BLG and BSG gels was strongly pH‐dependent, and stiffer gels formed at higher pH. The BLG gels were formed upon heating to 90 °C and reinforced on cooling to 20 °C; however, the gelation of BSG occurred at temperatures below 70 °C. By increasing Ca²⁺ concentration, storage modulus of BLG and BSG gels were increased, although pH had a greater effect than Ca²⁺. In contrast, mixed systems showed two distinct types of behaviour: BLG gel formation and BSG network, suggesting that phase‐separated gels were formed. In addition, higher strength was obtained for BLG‐BSG mixture at higher Ca²⁺ concentration.     

Effect of phytate on the in-vitro solubility of Al+, Ca2+, Hg2+ and Pb2+ as a function of pH at 37°C

The effect of phytate on the solubility of Al³⁺ and Pb^{2 +} has been investigated in vitro at 37°C across the pH ranges 3–7 (Pb²⁺) and 2–5 (Al³⁺). For both ions minimum solubility was found when the initial metal ion: phytate ratio was in the range 5:1–3:1 across the whole pH range. For Al³⁺ the solubility rose sharply either side of these ratios whereas for Pb^{2 +} solubility rose only slowly as the metal ion: phytate ratio was reduced. Hg²⁺ was totally soluble under all conditions tested. At pH 7 Ca²⁺ solubility was found to be at a minimum when the initial Ca : phytate ratio was 5 :1 irrespective of the initial Ca²⁺ concentration.     

Quality of shell eggs pasteurized with heat or heat-ozone combination during extended storage

The physical quality and functionality of shell eggs, pasteurized with heat or a combination of heat and ozone, were assessed during eight weeks of storage at 4 or 25 °C. Shell eggs were treated as follows: (1) immersion heating that mimics commercial pasteurization processes (egg internal temperature of 56 ± 0.1 °C for 32 min), or (2) a newly developed combination process comprised of heating (56 ± 0.1 °C, internal, for 10 min) followed by gaseous ozone treatment. Eggs were tested for yolk index, Haugh units, albumen pH, albumen turbidity, and percent overrun. Additionally, albumen samples were assayed for lysozyme activity and free sulfhydryl group content, and were analyzed using differential scanning calorimetry and Fourier transform infrared (FTIR) spectroscopy. Both processed and unprocessed eggs maintained superior quality when stored at 4 °C, as opposed to 25 °C. Pasteurization, regardless of method, led to superior maintenance of Haugh units during storage but also increased albumen opacity and decreased albumen overrun. Detrimental effects on quality markers were more severe in heat-pasteurized eggs than those treated with the ozone-based process. Pasteurization of shell eggs by either process did not affect lysozyme activity or sulfhydryl group content. Changes in protein secondary structure, as indicated by FTIR analysis, suggest that the ozone-based process is less damaging to albumen proteins than is the heat-alone process. In conclusion, heat-ozone pasteurization, by virtue of its less severe heat treatment, yields a safe final product that more closely resembles untreated shell eggs.     

The effect of instant green tea on the foaming and rheological properties of egg albumen proteins

The effects of three varieties of instant green tea (from China, Japan and Kenya) on the foaming and thermal properties of 1% (w/v) egg albumen and the gelation properties of 5 and 15% (w/v) egg albumen were investigated. All varieties produced similar effects on the foaming and gelation properties of egg albumen, but to different extents depending on the tea constituents. Mixtures of 1% (w/v) egg albumen and 0.25–0.4% (w/v) instant green teas in distilled water showed the greatest foam expansion (800–1140%) and foam stability (97–100%) at 10 min after whipping compared with 1% (w/v) egg albumen alone (226% for foam expansion and 34% for foam stability). Addition of instant green teas at levels above 0.5% (w/v) decreased foam expansion and stability. Small‐deformation rheology of mixtures of 5% (w/v) egg albumen and 1 or 2% (w/v) instant green teas showed an initial increase in elastic modulus (G′) and viscous modulus (G″) followed by a small, broad peak, indicating that the binding of tea constituents (polyphenols) with proteins may be reversible between 20 and 54 °C; this peak was not seen for 5% (w/v) egg albumen on its own. Large‐deformation rheological tests also indicated increased strength of mixed egg albumen/green tea gels with increasing levels of instant green teas. Differential scanning calorimetry thermograms showed that for the same instant green tea a higher concentration decreased the onset and peak (T_m) temperatures and enthalpy change values of all egg albumen protein peaks. Copyright © 2007 Society of Chemical Industry     

A comparative study of heat and high pressure induced gels of whey and egg albumen proteins and their binary mixtures

Large deformation rheological studies of either egg albumen or whey protein isolate (15% protein w/w) gels induced by heating at 90 °C for 30 min were compared to those induced by a range high pressures (400–800 MPa for 20 min). Gels made by heating indicated higher gel strength and Young's modulus values for whey protein from pressures of 400–600 MPa for 20 min but similar values at 650–800 MPa. In contrast, egg albumen showed no gelation below 500 MPa for 20 min, but there was an increase in both gel strength and Young's modulus with increasing pressure, although values remained lower than those of the heat-induced gels. A mixture of 10:5 whey/egg albumen showed the highest gel strength and Young's modulus for both heated and high pressure-treated (400–600 MPa) gels, although, the heated mixture had the highest values. Electron micrographs indicated that high pressure-treated gels had a porous aggregated network for egg albumen while whey proteins showed a continuous fine stranded network. The heated mixtures of whey:egg albumen (7.5:7.5) showed large dense aggregates whereas high pressure-treated mixtures produced smaller aggregates. Raman spectroscopy of both heated and high pressure-treated whey and egg albumen (15% w/w in D₂O pD7) and their binary mixtures (7.5:7.5, protein w/w) indicated changes in β-sheet structures in the Amide 111′ region (980–990 cm⁻¹); however, peak intensity was reduced for high pressure-treated samples. β-Sheet structure (1238–1240 cm⁻¹) present in heated whey was absent in high pressure-treated whey and in egg albumen. Involvement of hydrophobic regions was reflected by changes in the CH (1350 cm⁻¹) and CH₂ (1450 cm⁻¹) bending vibrations. In addition to the Trp residues at 760 cm⁻¹, there were broad peaks at 874–880 cm⁻¹ and tyrosine 1207 cm⁻¹ in the high pressure-treated samples. Disulphide bands (500–540 cm⁻¹) in heated whey and egg albumen proteins showed higher peak intensities compared to high pressure-treated samples. Differences in the experimental and theoretical spectra indicated changes in the hydrophobic regions, tyrosine (1207 cm⁻¹) and tryptophan (880 cm⁻¹) and CH₂ bending in high pressure-treated samples, whereas heated samples indicated marked changes in β-sheet structures (987 and 1238 cm⁻¹) as well as hydrophobic regions CH (1350 cm⁻¹) and CH₂ (1450 cm⁻¹) bending vibrations.     

Gelling Temperatures of Gellan Solutions as Affected by Citrate Buffers

Effects of citrate buffers at pH 3.5 and 5.0 on gelling temperatures of gellan solutions with 0.4–1.8% gellan and 1.5–60 mM Ca²⁺were studied. Partial dissociation of the carboxyl groups in gellan polymer in pH 3.5 solutions resulted in weakened gels. The pH 3.5 buffer exhibited weak chelating ability for Ca²⁺. The gelling temperature of gellan solutions at pH 3.5 was quantitatively related to polymer and cation concentrations using a similar model to that for gellan water solutions. The pH 5.0 buffer exhibited strong chelating ability. Gelling temperatures at pH 5.0 were generally lower than those at pH 3.5, except at low calcium concentrations.     

Biochemical changes in phosphorus compounds and in the activity of phytase and α-amylase in the rice (Oryza sativa) grain during germination

Changes in the phytic acid, inorganic phosphorus and ATP contents, and in the activity of phytase and α-amylase in rice (Oryza sativa L) grains were determined during 18 days of germination in a dark room. The effect of phytic acid on α-amylase activity was studied in vitro. Rice grains immersed in sterilised deionised water at 14^°C germinated on the fifth day. Phytase activity, detected in the ripening rice grains, increased linearly until the eighth day and reached a maximum on the tenth day. There was a marked decrease in phytate and an increase in inorganic phosphorus accompanying germination. There was a good inverse correlation between the levels of both phytase activity and inorganic phosphorus, and phytate breakdown. α-Amylase activity was detected on the fourth day and increased markedly from the 12th to the 16th day of germination. ATP level increased from the second to the fourth day and slightly decreased from the fourth to the eighth day; it increased rapidly again from the eighth to the 18th day of germination. α-Amylase activity was influenced by both pH and phytic acid concentration in the assay system. At 75 mM phytic acid, α-amylase activity was lowered by 23%, 93% and 52% at pH 4–0, 5–0 and 6–0 respectively. When the enzyme, phytate and Ca²⁺ were incubated together at pH 5–0, the inhibition of α-amylase by phytic acid was markedly decreased by addition of Ca²⁺. The chemical affinity of Ca²⁺ for phytic acid was higher in the reaction at pH 5–0 than in those at pH 4–0 and pH 6–0, and over 98% of Ca²⁺ in the reaction system was precipitated as Ca-phytate.     

Ca2+ Affects Physicochemical and Conformational Changes of Threadfin Bream Myosin and Actin in a Setting Model

The effect of Ca²⁺ on physicochemical and conformational changes of threadfin bream (TB) myosin and actin during setting at 25 and 40°C was investigated. Ca²⁺ ion at 10 to 100 mM induced the unfolding of myosin and actin as evident by an increase of surface hydrophobicity (S_o ANS) at 40 °C. Total SH groups also decreased with an increased Ca²⁺ concentration, suggesting that Ca²⁺ promoted the formation of disulfide bonds during setting at 40 °C. Both hydrophobic interactions and disulfide linkages were involved in formation of myosin aggregates at 40 °C and were enhanced by addition of 10 to 100 mM Ca²⁺. Myosin Ca‐ATPase activity decreased when Ca²⁺ was greater than 50 mM, indicating conformational changes of myosin head. Circular dichroism spectra demonstrated that Ca²⁺ reduced the α‐helical content of myosin and actin incubated at either 25 or 40 °C. Ca²⁺ induced conformational changes of TB myosin and actin incubated at 40 °C to a greater extent than at 25 °C.     

PURIFICATION AND PROPERTIES OF TRANSGLUTAMINASE FROM STREPTOVERTICILLIUM MOBARAENSE

Transglutaminase (TGase) was separated from the culture broth of an isolated strain of Streptoverticillium mobaraense. The crude enzyme was prepared by centrifugation, ultrafiltration, precipitation by alcohol, centrifugation and freeze‐drying. The yield after these processes was 65–70%. Then the enzyme was purified to homogeneity by chromatography on CM‐cellulose and Sephadex G‐75 on which the yields were about 70% and 80%, respectively; the purified folds reached 2.5–4.7 and 1.08–2.06, respectively. The molecular weight of this TGase was 39,500–40,100 Da by gel filtration chromatography. Optimum enzyme activity was observed in the pH range of 5.0–7.0, and it was maintained stable at 20–40C. The optimal temperature and pH was 52C and 6.0, respectively. At 1 mM and 5 mM metal ion or inhibitors concentration, TGase activity was strongly inhibited by Zn²⁺ and NEM, and not affected obviously by Ba²⁺, Ca²⁺, Co²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mg²⁺, Mn²⁺, Na⁺ as well as PMSF and EDTA. The effects of these additions on this TGase were compared with those of other microbial TGases.     

Observations on the determination of protein quality by the Thomas–Mitchell method

In the Thomas–Mitchell method for the assessment of protein quality, metabolic faecal nitrogen (MFN) and endogenous urinary nitrogen (EUN) are used to adjust faecal and urinary N excretion on test diets so that true digestibility (TD) and biological value (BV) can be calculated. Using an experimental technique where the urine and faeces from young growing rats were collected every 2 days for an experimental period of 28 days, trends in the estimation of MFN and in the determination of TD and BV were examined. A diet containing 20 g egg albumen kg^?1 on an as fed basis (2.85 g N kg^?1 dry matter) has previously been found to be most applicable for the estimation of EUN. This diet was also found to be appropriate for the estimation of MFN. To examine the effect of a change from an ad libitum intake of a stock diet (containing 150 g protein kg^?1) to the restricted intake of a test diet (containing 80–100 g protein kg^?1) on the determination of TD and BV, three protein sources (egg albumen, casein+1% L -methionine and soya bean meal) were fed at two levels of intake: 9 and 15 g day^?1. The time taken for the faecal N excretion to stablilise was very similar with all diets at both levels of intake. The TD of all three protein sources at both levels of intake increased gradually with increasing lengths of preliminary period. However, this increase was not great, and had little effect on the determination of the BV. Stabilisation of urinary N excretion with the three diets depended on the quality of the protein and the amount fed. This led to variation in BV with lengthening preliminary periods, especially at the lower level of intake. Urinary N excretion, and consequently BV, stabilised most rapidly with the highest quality protein (egg albumen) at the highest level of intake. However, stabilisation of BV with the lowest quality protein (soya bean meal) did not occur until after a preliminary period of 12 days, even at 15 g day^?1. This preliminary period is considerably longer than that usually employed, and thus it appears that for many proteins the BV may have been assessed during a period before N retention has fully stabilised.     

Naturally occurring variations in milk pH and ionic calcium and their effects on some properties and processing characteristics of milk

Considerable variations were found for ionic calcium and pH in milk from individual cows. Milk with lower pH tended to have a higher Ca²⁺ concentration, although the relationship was weak. Milk samples with a higher Ca²⁺ concentration and lower pH produced less sediment during in‐container sterilisation, which was contrary to expectations. Rennet coagulation time was lower for milk with a higher Ca²⁺ concentration, but curd firmness was not related to Ca²⁺ concentration. There was a poor correlation between the pH reduction caused by acid addition and that resulting from increasing temperature. Sediment formation was related to pH change at high temperature.