Unheated Milk Research Articles

Acid-Induced Dissociation of Casein Micelles in Milk: Effects of Heat Treatment

The effects have been studied of heat treatment of skim milk at temperatures from 70 to 90°C on the dissociation of caseins and minerals from the micelles during acidification at 5 and 22°C. The amounts and types of caseins that were dissociated depended on the pH and temperatures of acidification and heating. When acidification was carried out at 5°C, more caseins were dissociated in the heated milks than in the unheated milks; the reverse occurred after acidification at 22°C. Heating casein micelles dispersions that were free of serum proteins had no effect on casein dissociation during acidification at 22°C, but an effect similar to that observed for heated milk was apparent at 5°C. Solubilization of Ca, inorganic phosphate, and Mg from the micelles during acidification remained largely unaffected by heat treatment of milk. Heat treatment of milk appeared to weaken the interaction forces between casein components within the micelles, and, consequently, micelles dissociated more readily when pH was lowered at low temperatures. At higher temperatures, this dissociation was compensated, to a large extent, by the introduction of new crosslinks within the micelles through hydrophobic interactions between whey proteins and casein components.

Solvation of Rennet Curd from High-Heat-Treated Milk Influenced by Acid Type and pH

Mean solvation of caseins in unheated milk was 2.29 g of H2O/g of dry matter. Mean solvation of casein−whey protein complex in high-heat-treated (85 °C for 10 min) milk was 3.44 g of H2O/g of dry matter. Rennet treatment of heated or unheated milk reduced curd solvation, respectively, to 2.64 and 0.78 g of H2O/g of dry matter. Curd solvation decreased with milk pH and was dependent on the acid type used in acidification of the milk. Mean curd solvation, in decreasing order, resulting from acid type used was acetic = hydrochloric > phosphoric > lactic acids. Keywords: Solvation; acids; curd

Effect of heat and homogenization on riboflavin photolysis in milk

The influence of milk heat treatment and homogenization on photolytic degradation of riboflavin in milk was examined. Milk samples heated at 80, 100 and 120 °C were used for light exposure experiments and determinations of whey protein denaturation, casein micelle size and browning. Heat treatment of skim milk increased riboflavin photostability. The large decrease in rate constants in heated as compared to unheated milks could be due mainly to denaturation of whey proteins. Further decrease in rate constants with prolonged heat treatments could be due to an increase in casein micelle size and browning. Homogenization of whole milk slightly increased riboflavin photostability, but differences in pressure had no significant effect on photodegradation.

Enhancement of proteolysis in bovine skim milk by heat and chemical treatments

SummaryEffect of heating and of some chemical treatments on the proteolysis of skim milk caused by its major proteinase system, plasmin, was investigated. Degree of proteolysis was expressed as the increase in γ-casein content of skim milk (mg/ml) after incubation with 0·02% (w/v) NaN3 at 37 °C for 20 h. Apart from reduced proteolysis in skim milk heated at > 70 °C for 10 min or at 75 °C for ∼ 5 min, some enhancement (10–80%) was observed on heating at 55–63 °C for 10 min or at 60–65 °C for 15 s. Heating at pasteurization conditions (63 °C/30 min or 72 °C/15 s) had no apparent effect on the amount of proteolysis. Addition of ascorbic acid (0·2 mg/100 ml) or H2O2 (1/100 ml) to unheated skim milk also increased the amount of proteolysis by 38 and 28%, respectively. However, the extent of this increase diminished with increasing amounts of both chemicals. Furthermore, there was a steady increase (∼ 33%) in proteolysis with time of exposure to light for up to 3 h. These results suggest that oxidative conditions lead skim milk to enhanced proteolysis by inactivating an inhibitor(s) operating in the plasmin system in milk.

Rennet Effects and Partitioning of Bacterial Cultures During Quarg Cheese Manufacture

ABSTRACTQuarg was prepared with or without milk heating or rennet addition using yogurt, acidophilus, and buttermilk cultures. Total solids and protein contents in the quarg cheeses were 17.4‐34.7% and 7.4‐15.0%, respectively, depending on process conditions. Rennet addition increased the total solids content, especially when unheated milk was used; however, the corrected (18% T.S.) yield was unaffected. Yogurt culture resulted in the shortest incubation time (3.8‐4.6 hr), while the acidophilus culture alone was unsuitable. After whey separation, the concentration of bacteria from all three cultures were higher in the quarg (8.48‐9.05 log CFU/g) than in the whey (5.92‐7.90 log CFU/ g) using either centrifuge or gravity filtration methods.

Changes in the carbohydrate fraction of milk during heating processes

Carbohydrate composition of commercial pasteurized, UHT, sterilized and dried milk samples, and laboratory heat-treated milks was determined by gas-liquid chromatography. Glucose, galactose and lactulose were present in all types of commercial milk samples. The concentrations of galactose and lactulose increase with the severity of the heating process. The amount of glucose present in commercial milks was similar to that found in unheated milk. Epilactose was only detected in sterilized milk samples. UHT and sterilized milks can be differentiated by their lactulose, galactose or epilactose content.

Yield, Sensory Properties, and Nutritive Qualities of Quarg Produced from Lactose-Hydrolyzed and High Heated Milk

Quarg was produced from homogenized milk with or without lactose hydrolysis or prior heating to 95°C for 15min. Increased yields of 9.2 and 13.3% were obtained for unhydrolyzed and hydrolyzed high heated milk, respectively, in comparison with the corresponding unheated milk. The recovery of whey proteins in quarg from high heated milk was 40.5% in unhydrolyzed milk and 42.7% in hydrolyzed milk. No effect of lactose hydrolysis on yield was observed. The time required to attain the desired pH 4.5 in the quarg fermentation was reduced in lactose-hydrolyzed milk by 7.1 and 4.6% for the unheated and high heat-treated milk, respectively. The results of sensory evaluation indicated that heating resulted in graininess and hydrolysis plus high heating induced an unclean flavor in the quargs. In nutritional evaluation by bioassay methods, no differences were found in protein digestibility among any of the quargs studied; however, protein efficiency ratio, biological value, and net protein utilization values indicated that lactose hydrolysis plus high heating had a negative effect on the nutritional value of milk. Amino acid analysis of quarg samples indicated possible lowering of lysine content for the hydrolyzed quargs.

Influence of processing temperature and seasonal change in diet on lipase activity and lipolysis during the mechanical separation of bovine milk

SUMMARYThe effect of separating conditions on lipase activity and free fatty acid levels in preheated milk, cream and skim milk was measured on nine occasions during a 12-week period covering the seasonal change from winter feeding to summertime grazing. This change consisted of four periods each representing a different type of forage intake, namely: silage, kale, daytime grazing and 24 h grazing. Milk was separated at 30, 40, 50 and 60°C with preheating times of 10, 25 and 55 s. Results were expressed both as absolute values and in terms of changes relative to the original unheated milk. Lipase activity and free fatty acid concentration were significantly reduced as separation temperature increased but were not influenced by holding time. The loss of activity in cream was progressive so that at 60°C only 40% of the original activity remained. Up to 50°C little change occurred in preheated milk or skim milk activity, while at 60°C 83 and 76% respectively of the original activity remained. The amount of activity calculated to be associated with the fat fraction of the cream also decreased with temperature. Activity varied significantly with date; maximum values were observed during the first 3 weeks of summertime grazing. Relative activity values indicated that the susceptibility of milk lipase to heat inactivation also varied with date. Lipolysis was also significantly affected by date. Cream free fatty acid levels were lower during the period of daytime grazing and were significantly higher than those in preheated milk. The correlation between lipase activity and free fatty acid levels was generally poor, accounting for between 0 and 34% of the variance. Possible reasons for the effect of separating temperature on lipolysis in cream are discussed.

Reduction in the antigenicity of whey proteins by heat treatment: a possible strategy for producing a hypoallergenic infant milk formula.

Residual antigenic protein in heat-denatured cow's milk whey and in two commercial infant milk formulas was determined using enzyme-linked immunosorbent assays specific for beta-lactoglobulin, alpha-lactalbumin, bovine serum albumin, bovine IgG1 and alpha-casein. This immunochemical assessment of antigenicity was related to the capacity of the preparations to sensitize immunologically when fed to guinea-pigs for 2 weeks. Antibody production was measured and the susceptibility of the animals to systemic anaphylaxis was assessed by injecting them intravenously with heated or unheated milk proteins. Whey protein that had been heated at 100 degrees or 115 degrees for 30 min was extensively denatured and, in contrast to pasteurized whey, failed to sensitize guinea-pigs for anaphylaxis. Antibody production was undetected or very low. The proteins in SMA powder and SMA Gold Cap liquid concentrate were less denatured and animals given these formulas prepared according to the maker's instructions produced relatively high levels of antibodies to beta-lactoglobulin and alpha-casein and a majority developed anaphylaxis when injected intravenously with these products. As well as failing to sensitize, whey that had received severe heat treatment did not, in most cases, elicit anaphylaxis when injected into animals that had been sensitized with unheated milk. Discrimination between antibodies of the IgG1 and IgG2 subclasses specific for beta-lactoglobulin showed that IgG1, the principal anaphylactic antibody in guinea-pigs, was preferentially depressed in animals drinking heat-denatured milk preparations. The results suggest that heat denaturation of whey protein may be a logical and simple strategy for producing a hypoallergenic baby milk. Nevertheless, the value of experiments in guinea-pigs for predicting results in man is uncertain and the proposal awaits assessment in clinical trials.

Effects of heat treatment of cow's milk and whey on the nutritional quality and antigenic properties.

Recent experiments in guinea-pigs suggest that heat treatment applied during the manufacture of baby milk formulae reduces the immunological sensitising capacity of the cow's milk proteins. This immunological benefit must be weighed against possible damage that heat treatment may cause to the nutritional quality of the products. Severe heat treatment of skimmed milk (121 degrees C for 20 min) destroyed all the vitamin B12, about 60% of the thiamin and vitamin B6, 70% of the ascorbic acid, and about 30% of the folate. Available lysine was reduced by 21% and lactulose was formed (166 mg/100 ml). Despite extensive denaturation of the whey proteins the milk retained its capacity to sensitize guinea-pigs for systemic anaphylaxis when administered orally. Animals drinking heated milk also produced circulating antibodies to beta-lactoglobulin and casein, although titres were lower than for unheated milk. Unlike skimmed milk, heat-treated diafiltered whey failed to sensitize guinea-pigs orally. It caused the production of trace levels of antibodies in some of the animals, but these were specific for residual casein. We suggest that it may be possible to produce a non-sensitising baby milk without casein based on heat-denatured whey. The nutritional quality could be preserved by removing low molecular weight nutrients before heat treatment and adding back appropriate quantities later.

Subpasteurization Heat Treatment to Inactivate Lipase and Control Bacterial Growth in Raw Milk

Raw milk was heat-treated under subpasteurization and suprapasteurization conditions, cooled and stored for up to 72 h at 4.4 and 6.7°C. Milk lipase activity and bacteria counts were monitored in both unheated and heated milks. Inhibition of milk lipase activity ranged from 42 to 98% for treatments of 57.2°C for 10 sec to 73.9°C for 10 sec, respectively. The logs of Standard Plate Count after 72 h of storage at 6.7°C were 6.56, 4.86, 4.31, 4.00 and 2.82 for unheated and 10-sec heat treatments at 57.2, 65.6, 73.9 and 82.2°C, respectively. Psychrotrophic Bacteria Counts were also lower in the heated milks than in the unheated milk. The logs of Psychrotrophic Bacteria Counts after 72 h of storage at 6.7°C were 6.21, 2.45, 2.27, 1.33 and 1.00 for unheated and 10-sec heat treatments at 57.2, 65.6, 73.9 and 82.2°C, respectively. Heat treatment of raw milk supplies would result in limiting action of the milk lipase system and growth of bacteria.

Effects of pH, calcium and unheated casein micelle on physical properties of high-temperature heated milk rennet curd.

Skim milk, separated from bulk cow's milk gotten from a dairy district in Miyazaki Prefecture, was heated at 85°C for 5min, immediately followed by heating at 135°C in a moment on an experimental scale. The effects of pH, Ca and intact casein micelle on the physical properties (hardness, breaking energy and elastic modulus) of the rennet curd prepared from this high-temperature heated milk were investigated using a rheometer.The results obtained were summarized as follows:(1) The physical properties of rennet curd of high-temperature heated and pH-adjusted (6.7-6.0) milk could not be improved up to those of unheated milk. (2) Using Ca-added (80mg/100ml) and pH-adjusted (6.7-6.0) milk, it could be seen that the physical properties of high-temperature heated milk were improved with pH lowering, whereas even the highest values of the physical properties obtained for pH 6.0-adjusted milk rennet curd could not come up to those of unheated one. (3) When unheated casein micelle (360mgN/100ml) of 0.35 or above as unheated casein micelle ratio were added to high-temperature heated milk at 6.6 or 6.2, the physical properties of milk rennet curd were greater than those of unheated one and the rennet curd of pH 6.2-adjusted milk was somewhat better than those of pH 6.6-adjusted one. (4) Rennet curd of high-temperature heated milk at pH 6.6 or 6.2 was remarkably improved by the combination of Ca addition (80mg/100ml) and unheated casein micelle fortification; pH 6.6 or 6.2-adjusted milk rennet curd was attained nearly same as unheated one by fortifing to 0.2 or 0.15 respectively, as unheated casein micelle ratio. (5) From those results it was confimed that an adequate combination of both Ca addition and native casein micelle fortification as well as sutitable acidification was essential for the improvement of high-temperature heated milk rennet curd.

Apparent Viscosity of Milk and Cultured Yogurt Thermally Treated by UHT and Vat Systems

Apparent viscosity of ultra-high-temperature (UHT) treated milk and of yogurt prepared from this milk was studied and compared to that of vat-treated milk and yogurt. UHT-treated milks (149 C, 3.3 sec) had an apparent viscosity of 2.3 to 2.7 cp, while the apparent viscosity of vat-treated milks (63 C and 82 C, 30 min) ranged from 1.9 to 2.0 cp. The apparent viscosity of unheated (raw) milk was 1.7 cp. The apparent viscosity of yogurt prepared from UHT-treated milk became nearly constant at 0.8 cp after 14 min of shearing, while the apparent viscosity of yogurts prepared from vat-treated milks ranged from 1.8 to 3.8 cp under the same conditions. All yogurts exhibited thixotrophic behavior.

Effect of proteose-peptone on the heat gelation of whey protein isolates

SUMMARYWhey proteins have been isolated from cheese whey by complex formation with carboxymethylcellulose. The protein preparations form stable gels and foams when reconstituted in water. A study has been made of the effect of concentration, pH and temperature on the gelling behaviour of the whey protein isolates, and the conditions necessary for a standard gelling test have been defined. Proteose-peptones (p-p) have been prepared from a whey protein isolate which showed a strong tendency to gel, and also from heated and unheated milk. The gelling behaviour of whey protein isolates, which showed either no tendency or a strong tendency to gel, has been studied in the presence and absence of added p-p. The quantities added varied from 0·05 mg isolated from unheated milk to 15 mg from heated milk. No change in gelling time was observed, and it was concluded that p-p were not responsible for the gelling behaviour exhibited by whey protein isolates.

HEAT STABILITY OF PEANUT/MILK PROTEIN BLENDS

Protein blends were prepared from peanut flour or peanut protein concentrate and whey protein concentrate, sodium caseinate, calcium caseinate or nonfat dry milk. One per cent protein dispersions of these blends containing 0.0, 25.0, 50.0, and 100.0% peanut protein were heated at 60, 80, and 90° C for 30 min. Soluble protein was lower in unheated peanut protein preparations than in unheated milk proteins preparations with lowest soluble protein observed in peanut flour dispersions. Solubility of milk proteins dispersed in distilled water was generally not affected by heat treatment, while heating of similarly prepared peanut protein dispersions above 80° C decreased soluble protein. Systems containing peanut protein with whey protein calcium caseinate or nonfat dry milk were intermediate, in soluble protein and heat stability to that of milk and peanut proteins alone. Soluble protein in peanut/sodium caseinate blends increased slightly with heat treatment. Calcium addition to 30mM decreased soluble protein in all protein systems. Peanut lipoprotein concentrate was not affected by added calcium. Heating whey protein, caseinate and blends of peanut with whey or caseinate in the presence of calcium induced aggregation and lowered soluble protein.

Chemical changes in ultra-heat-treated milk during storage: I. Hydrolysis of casein by incubation with pronase and a peptidase mixture

SummaryCasein samples from untreated milk and stored ultra-heat-treated (UHT) milk were hydrolysed with pronase (protease exStreptomyces griseus K-1) and subsequently with a mixture of peptidases prepared from the microsomal fraction of hog kidneys. Incubation of casein from unheated milk with pronase alone hydrolysed 70–80% of peptide bonds involving Ile, Leu, Tyr, Phe and His residues; other amino acids were released less well and proline hardly at all. The pronase/peptidase treatment resulted in 90–100% hydrolysis of peptide bonds involving all amino acids, including proline.Caseins from stored UHT milks were more resistant to proteolysis than casein from unheated milk. Reduced release of all amino acids was observed from samples taken after storage at 37 °C for 12 months or longer and for Lys, Arg and Asn residues from samples taken after storage at 30 °C for 14 months. Resistance to proteolysis was attributed to the Maillard reaction between milk proteins and lactose during storage of UHT milk.

Survival of Foot-and-Mouth Disease Virus in Cheese

Persistence of foot-and-mouth disease virus during the manufacture of Cheddar, Mozzarella, Camembert cheese prepared from milk of cows experimentally infected with the virus was studied. Cheese samples were made on a laboratory scale with commercial lactic acid starter cultures and the microbial protease MARZYME as a coagulant. Milk was heated at different temperatures for different intervals before it was made into cheese. Food-and-mouth disease virus survived the acidic conditions of Cheddar and Camembert cheese processing but not that of Mozzarella. Foot-and-mouth disease virus survived processing but not curing for 30 days in Cheddar cheese preparaed from heated milk. However, the virus survived curing for 60 days but not for 120 days in cheese (pH 5) prepared from unheated milk. Foot-and-mouth disease virus survived in Camembert cheese (pH 5) for 21 days at 2 C but not for 35 days.

Enzymic differentiation between curds of heated and raw milk. II. Milk peroxidase.

AbstractUnlike phosphatase, peroxidase, once destroyed by heating of milk, is not regenerated by the acidity or microbial growth of curd formation or by reasonable ageing. This reveals that curd‐forming micro‐organisms do not have or form peroxidase. The test was repeated in various heating conditions of milk. Peroxidase is destroyed even when the temperature of milk is gradually raised to 80 °C (in 3 min). Less‐heated milk, e.g. pasteurised milk and its corresponding curd, retain this enzyme. A positive test in the curd of raw milk is not affected by reasonable ageing or increased acidity but by excessive fermentation (secondary fermentation or putrefaction) which again can be halted by addition of formalin. The peroxidase test, therefore, appears to be a test for differentiating curds of heated and unheated milk.

Hydrolysis of fat and protein in small cheeses made under aseptic conditions

SummaryAseptically drawn milk was made into small cheeses (100 g) under aseptic conditions. The influence of the cow's feeding régime on the composition of the cheese fat, and the hydrolysis of the cheese fat and protein by bacterial and native milk enzymes, was studied.In the absence of a strongly lipolytic bacterial cheese flora, the free fatty acid (FFA) content of cheese appeared to depend on several variables:1. The initial FFA content of the milk varied according to the mode of milking (the lowest was obtained by cannulation of the udder) and the feeding régime of the cow. When one of a pair of monozygotic twin cows was maintained on pasture and the other on winter feed (hay and concentrates), the cheese made from the milk of the latter contained less FFA than did that from the former.2. The major changes in the composition of the milk fat of the cows on winter regime were an increase of myristic and palmitic acids from 8 to 11% and 21 to 28%, respectively, and a decrease in stearic and oleic acid from 11 to 7·5% and 38 to 28%; minor changes in the other fatty acids were also observed.3. The milk lipase in raw milk cheese liberated substantially more FFA than did the weakly lipolytic lactic acid streptococci. Milk lipase was active at the low pH value of cheese, but was inactivated when the milk was pasteurized at 63°C for 30 min.4. Although rennet did not produce any amino acids in the mature cheeses it hydrolysed some of the cheese protein to nitrogen soluble in water (WS), trichloroacetic acid (TCA-S) or phosphotungstic acid (PTA-S). Some of the WS-N appeared to be utilized by the lactic acid streptococci. The native milk protease resisted the heat treatment used and liberated amino acids at the low pH value of cheese, but its contribution to the proteolysis was relatively less important than the contribution of the milk lipase to the fat breakdown in cheese.5. The free amino acids of the milk, which appeared in the whey, were not recovered in the cheese. The cheese made from unheated milk with δ-gluconic acid lactone contained low amounts of free amino acids apparently due to the action of milk protease. The proteolytic activity of the lactic acid bacteria increased the amino acid content appreciably.

Formation of Methyl Ketones from Milk Fat during Steam Distillation or Saponification1,2

Summary An homologous series of methyl ketones was identified, by means of their 2,4-dinitrophenylhydrazones, in both the steam distillate and the unsaponifiable matter from milk fat. This series contains the n-alkyl members with odd numbers of carbons from C 3 (acetone) through C 15 (pentadecanone-2). Evidence is presented that fresh unheated milk fat is devoid of such ketones, with the exception of acetone, but that it does contain carbonyl functions as suggested by its reaction with 2,4-dinitrophenylhydrazine reagent. The capacity of milk fat to generate methyl ketones is discussed in terms of origin and flavor significance. A means of overcoming anomalous melting-point behavior of certain 2,4-dinitrophenylhydrazones is presented.