Heated Skim Milk Research Articles

QUALITY OF PROTEIN IN MILK REPLACERS FOR YOUNG CALVES. II. EFFECTS OF HEAT TREATMENT OF SKIM MILK POWDER AND FAT LEVELS ON CALF GROWTH, FEED INTAKE AND NITROGEN BALANCE

Three spray-dried powders were prepared after heating skim milk for 30 min at 60 C (LT), 73.9 C (MT) or 85 C (HT). Each was reconstituted to two concentrations of dietary fat and fed to five or six calves for 26 days after purchase at approximately 1 wk of age. Calves receiving the HT skim milk had significantly (P < 0.05) higher incidence of diarrhea and slower rate of gain over 19 days on test as compared with those fed the MT and LT skim milks. Subsequently growth rate was unaffected by the skim milk source and there were interactions of fat level with skim milk source. The high fat level (22.2% of dietary dry matter) resulted in a higher incidence of alopecia than in calves receiving the low fat level (16.2% of dietary dry matter). Nutrient digestibilities and nitrogen balance, measured from day 20 to day 26, were unaffected by skim milk treatments or fat concentrations. This experiment indicated that young calves, less than 3 wk of age, require a diet based on skim milk that has not been severely heat-treated and the curd formation is a suitable index of quality for such milk replacer diets. Where control diets for milk-replacer studies are designated "all milk protein," it is important for correct interpretation to define their curd-forming characteristics. For older calves, coagulation appears not to be a requirement.

Milk Ultracentrifugal Opalescent Layer.2. Physico-Chemical Properties

Casein micelles in skim milk crude opalescent layer were not fractionated by rate-zone ultracentrifugation because they failed to sediment sufficiently into the gradient. Gel filtration indicated that soluble casein in the ultracentrifugal supernatant from crude opalescent layer consisted of submicellar casein complexes larger than those in skim milk soluble casein. Inelastic light scattering indicated that the hydrodynamic radii of casein micelles in opalescent layer ranged from 100 to 200nm. Electron microscopy revealed that opalescent layer casein micelles are polydisperse and predominantly smaller than those in skim milk casein pellet (≤100nm diameter). Irregularly shaped particles, that may be lipoprotein fragments of fat globule membranes, were also observed. Casein micelles, pelleted from heated skim milk, exhibited intermicellar interaction and were not readily redispersed by grinding and stirring at 0 to 5C. The temperature dependent association-dissociation of soluble casein from opalescent layer casein micelles and the high concentration of heat-induced β-lactoglobulin-κ-casein complex in opalescent layer were further confirmed by zonal electrophoresis.

Milk Ultracentrifugal Opalescent Layer. I. Composition as Influenced by Heat, Temperature, and pH

The effect of heating skimmilk to 90C and adjusting it to temperatures of 0 to 5 and 35 to 40C and to pH 5.0 and 7.5 upon amount and composition of the three major skimmilk ultracentrifugal fractions, i.e., pellet, supernatant, and opalescent liquid layer located above tne pellet, was determined. Recentrifugation of the crude opalescent layer produced an opalescent layer supernatant and pellet.The composition of opalescent layer supernatant and pellet fractions was generally intermediate between that of skimmilk supernatant and pellet fractions but contained higher concentrations of certain constituents than either of these two skimmilk fractions. Opalescent layer pellet was more highly solvated (3.65 to 4.03g H2O/g dry pellet) than skimmilk pellet (2.85g H2O/g dry pellet) at 0 to 5C. Heating skimmilk caused a marked increase in the calcium/phosphorus ratio and in the disulfide group and sialic acid content of opalescent layer pellet and altered the opalescent layer supernatant composition but, otherwise, had little effect on composition of other ultracentrifugal fractions. Sialic acid and disulfide data indicated that heat induced β-lactoglobulin-κ-casein complex is concentrated in the opalescent layer fraction. The translucent portion of skimmilk pellet was more highly solvated and contained lower mineral and protein content than the opaque portion of the pellet. Temperature adjustments of milk to 0 to 5 and 35 to 40C altered skimmilk pellet solvation and casein and mineral dissociation from casein micelles. Adjusting skimmilk pH to 5.0 dissociated substantial amounts of casein from the micelles, thus increasing the casein content of opalescent layer and skimmilk supernatant fractions. Skimmilk pellet fractions 0 to 5C from pH 6.6 skimmilk had the lowest solvation, i.e., 2.85g H2O/g dry pellet compared to 3.5 and 3.8g H2O/g dry pellet for pH 7.5 and 5.0. The effect of pH adjustment to 5.0 upon calcium and inorganic phosphorus dissociation from casein micelles at 0 to 5C also was determined.

Heat-Induced Milk Gels. IV. Nutritional Evaluation

Gels (50%) obtained by heating skim-milk at 100C for 10min retained the nutritional value of milk as tested on rats, though the total and available lysine contents decreased by 9 and 17%. Heating at 100C for 90min decreased the protein efficiency ratio of the gelled product by 21%. Autoclaving 50m milk suspensions at 120C for 10min decreased protein efficiency ratio by 15% and destroyed 23% of total and 34% of available lysine. Autoclaving at 120C for 90min decreased protein efficiency ratio by 77% and destroyed 49% of total and 60% of available lysine. A loss of cystine was also noted in the gelled product.

Chemical nature of malty flavor and aroma produced by Streptococcus lactis var. maltigenes.

Mature skim milk cultures of Streptococcus lactis var. maltigenes were steam distilled at low temperature under reduced pressure. Ethyl ether extracts were prepared from the distillates and analyzed by gas-liquid chromatography and mass spectrometry. Twenty of 31 components detected in the culture distillates were identified positively and 11 tentatively, whereas 10 of 19 components detected in the heated skim milk control were identified positively and 9 tentatively. Among components detected in the culture distillate, but not detected in the heated skim milk distillate, and which have not been previously identified in milk cultures of the organism were phenylacetaldehyde and phenethanol. Quantitative analyses of the volatiles entrained from milk cultures of several strains of S. lactis var. maltigenes revealed a probable relationship between variation in the character of the aroma of the cultures and the alcohol/aldehyde ratio.

Heat-Induced Milk Gels. II. Preparation of Gels and Measurement of Firmness

A penetrometric procedure was devised for determining gel firmness to evaluate effects of various factors on heat-induced gelation of concentrated skimmilk. Milk gels were prepared by heating skimmilk powder suspensions (50% total solids) in sausage casings 19mm in diameter at 100C for 10 minutes. Gel firmness was measured by the force required for penetration of a disk-shaped probe, 6.2mm in diameter, through encased milk gels 4.5cm high and 1.9cm in diameter. The firmness thereby measured was reproducible with a coefficient of variation of±3.3%.

Chemical Nature of Malty Flavor and Aroma Produced by Streptococcus lactis var. maltigenes1

Mature skim milk cultures of Streptococcus lactis var. maltigenes were steam distilled at low temperature under reduced pressure. Ethyl ether extracts were prepared from the distillates and analyzed by gas-liquid chromatography and mass spectrometry. Twenty of 31 components detected in the culture distillates were identified positively and 11 tentatively, whereas 10 of 19 components detected in the heated skim milk control were identified positively and 9 tentatively. Among components detected in the culture distillate, but not detected in the heated skim milk distillate, and which have not been previously identified in milk cultures of the organism were phenylacetaldehyde and phenethanol. Quantitative analyses of the volatiles entrained from milk cultures of several strains of S. lactis var. maltigenes revealed a probable relationship between variation in the character of the aroma of the cultures and the alcohol/aldehyde ratio.

Protein Aggregation in Conventional and Ultra High-Temparature Heated Skimmilk

Ultracentrifugate supernatant fractions from raw and heated skimmilk were examined by Sephadex G-100 gel-filtration and zonal electorphoresis to investigate the protein aggregation produced by heating skimmilk at 90C for 10 or 30min, and at temperatures to 149C for as long as 16sec. Heating at 90C caused preferential formation of intermediate size ≥75 to 100 S ≤425 to 500 S protein particles; whereas, the ultra high-temperature heat treatments favored formation of larger size ≥425 to 500 S particles. Ultra high-temperature heat treatments produced only slight alteration of Sephadex gel-filtration patterns for the smallest size ≤75 to 100 S protein components; whereas, all of the ≤75 to 100 S protein components in 90C heated skimmilk were sufficiently aggregated as to be totally excluded from Sephadex G-100. Polyacrylamide gel electrophoresis data indicated that the rate and extent of heat-induced protein aggregation to particle sizes in excess of about 75 to 100 S closely parallel those for whey protein denaturation. The findings support the concept that ultra high-temperature heat treatments of 146 to 150C may produce small amounts of nonsedimenting casein components by disaggregation of casein micelles.

Effect of Heat upon Electrophoresis and Ultracentrifugal Sedimentation Properties of Skimmilk Protein Fractions

Skimmilk was centrifuged at 11,700×g, 46,900×g, and 92,000×g and the supernatant and sediment fractions were incorporated into phosphate buffer or treated with potassium oxalate to remove calcium and disperse the proteins. They were examined by free-boundary electrophoresis and ultracentrifugal sedimentation.Heating skimmilk (88 C-10min) had little effect upon electrophoretic properties of the three protein sediment fractions, but reduced the magnitude of the differences in electrophoretic composition of the supernatant fractions. These results indicate that interaction of α-casein with β-lactoglobulin is favored among the smaller skimmilk protein particles.Skimmilk and its sediment and supernatant fractions all exhibited two ultra-centrifugal sedimentation components at pH 6.98 and 25C. Oxalate treatment of unheated skimmilk increased Component I from 28 to 57% of the pattern area and reduced its sedimentation constant from 12.7 to 6.9 S. Heating skimmilk increased Component I from 28 to 83% of the pattern area and reduced its sedimentation constant to 6.7 S. The sedimentation patterns of heated skimmilk protein fractions were relatively unaffected by the oxalate treatment. No evidence of 25-48 S casein-serum protein complex particles was obtained in any of the heated skimmilk fractions.

A Method for Standardizing the Biacetyl Content of Creamed Cottage Cheese

Summary The addition of 0.15% citric acid to creamed cottage cheese held at 45° F. did not increase the biacetyl content; whereas, addition of Streptococcus citrovorus gave a slight increase and addition of both citric acid and S. citrovorus gave still more of an increase. No biacetyl was formed in a 12% fat creaming mixture containing S. citrovorus or S. citrovorus plus 0.15% citric acid. Considerable biacetyl was produced in a creaming mixture containing large numbers of S. citrovorus after it was adjusted to pH 4.2 with citric acid and held at 70° F. Cottage cheese creamed with such a preparation had a high initial biacetyl content. The most satisfactory method of preparing a creaming mixture which contains sufficient biacetyl and volatile acid to influence the flavor of creamed cottage cheese is the following: Heated skimmilk is inoculated with a pure culture of S. citrovorus and, after sufficient growth, adjusted to pH 4.3 with sterile citric acid. Further incubation at 70° F. results in the formation of considerable biacetyl and volatile acid. The skimmilk culture thus prepared is used to standardize the fat content of cream to about 12%. Standardization of biacetyl is accomplished by varying the fat content of the cream in the creaming mixture and, therefore, the amount of skimmilk culture to standardize the fat content. Cheese creamed with such a preparation has an appreciable amount of flavor immediately after creaming.

Volatile Carbonyl Compounds Produced in Skimmilk by High-Heat Treatment

Summary Furfural and acetaldehyde are the principal volatile carbonyl compounds generated in skimmilk by high-heat treatment. None of the aldehydes which could result from a heat-induced reaction between the free leucine, isoleucine, or valine and the dicarbonyl compounds present in raw and heat skimmilk was detected in skimmilk heated as long as 90 minutes at 121° C. When leucine or isoleucine was added to skimmilks prior to heat treatment, 3-methylbutanal or 2-methylbutanal was isolated from the distillates of the heated skimmilks. The apparent low yield of these compounds suggests that the Strecker degradation may be somewhat inhibited in a medium such as skimmilk.

The Ion-Exchangeable Calcium in Raw and Heated Skimmilk as Determined by the “Resin Contact Time Method”

The Ion-Exchangeable Calcium in Raw and Heated Skimmilk as Determined by the “Resin Contact Time Method”

Human type caries distribution in Osborne-Mendel rats kept on heated skim milk powder diet.

SummaryDental caries with occlusal and surface lesions approximating human caries distribution has been produced in the Osborne-Mendel strain of the white rat on heated skim milk powder diet of McClure and Folk. The lesions were developed fully in 97% of the animals at the age of 81 days. The data presented suggested independent etiologies for the surface and occlusal types of caries lesions.

An Electrophoretic Study of the Protein in Skimmilk

Summary Electrophoretic studies of unfractionated and unheated skimmilk proteins at ionic strengths of 0.02, 0.1 and 0.5 at pH's 8.4, 6.6, 5.4, 4.1 and 3.2 have shown that: (a)The electrophoretie mobilities of the proteins in unfractionated skimmilk are appreciably lower than the mobilities of the corresponding isolated proteins. (b)The isoelectric points of the proteins in unfractionated skimmilk are approximately the same as the isoelectric points of the corresponding isolated proteins reported by other workers. (c)The best electrophoretic resolution of the proteins in skimmilk occurs at pH 6.6 and ionic strengths of 0.02 and 0.1. (d)α-Casein migrates as two distinct components, α 1 - and α 2 -casein, over a range of pH's above 4.2 at ionic strengths of 0.02 and 0.10. (e)An interaction occurs between the α- and β-casein components of skimmilk which increases in degrees as pH is lowered from 8.4 to 5.4 at ionic strength 0.02. Heat treatments of skimmilk for 30 min. at 65, 75 and 85 C. were found to cause the following changes in the electrophoretic properties of the protein components as revealed by electrophoresis measurements at pH 6.6, r/2 = 0.1: (a) The mobilities of α -casein, β -casein and β -lactoglobulin are lowered by heating, the effect being greater at the higher temperatures; (b) The concentrations of β -lactoglobulin and β -casein are decreased and the concentration of protein migrating with a-casein is increased by heating skimmilk under the conditions given.

Studies of Heated Milk. I. Formation of 5-Hydroxymethyl-2-Furfural

Summary Hydroxymethylfurfural has been identified as one of the compounds formed during prolonged heat treatment of concentrated skim milk. From studies of simplified systems, it has been shown that this compound is formed from lactose. It further was demonstrated that either the glucose or galactose portion of the lactose molecule may serve as the origin of hydroxymethylfurfural. Glycine, casein or the heat degradation products of casein were found essential in the conversion of lactose to hydroxymethylfurfural. No measurable quantities of hydroxymethylfurfural were formed by heating lactose solutions alone under the experimental conditions employed. The formation of hydroxymethylfurfural appeared to be directly associated with heat-induced browning in these experiments. In addition to hydroxymethylfurfural, furfuryl alcohol and maltol were formed during the heating of lactose-casein systems. Previous findings (4, 5) with regard to furfuryl alcohol and maltol formation in heated skim milk were confirmed.

The Isolation of Maltol from Heated Skim Milk

The Isolation of Maltol from Heated Skim Milk

Observations on the Application of the Nitroprusside Test to Heated Milk

Summary The procedure of Josephson and Doan (7) for the nitroprusside test was found to be satisfactory in its application to heated milk when the factor of temperature was controlled. For optimum results, samples should be cooled to 20° C. or lower before subjecting them to the test. Experiments utilizing the nitroprusside test as an indicator of sulfhydryl behavior demonstrated that these groups disappear in skim milk heated for prolonged periods at high temperatures. This disappearance depends upon a reaction involving lactose and casein and is correlated with the development of caramelized flavor and browning. The distribution of sulfhydryl substances in heated skim milk was studied, using the nitroprusside test. These substances associate themselves physically and/or chemically with the casein in heated skimmilk. This phenomenon may be an important factor in explaining the soft curd characteristics of highly heated milk.