The effect of extended low-temperature storage of raw milk on the quality of pasteurized and UHT milk

The effect of storage of raw milk at 2 and 6°C on the quality of pasteurized and UHT milks produced from them has been investigated. There was no difference in shelf-lives of pasteurized milks produced from raw milks which had no obvious physical defects, odours and taints after storage at 2 and 6°C. This was true for pasteurized milks in the presence and absence of post-heat-treatment contamination. However, pasteurized milks of good quality could be produced from more than 80% of raw milk samples which had been stored for up to 5 days at 2°C, but this was only possible with raw milks which had been stored at 6°C for 2 days.Failure rates of experimentally-produced UHT milks were much higher in products manufactured from raw milks stored at 6°C for 4 days than those produced from raw milks stored at 2°C for 4 days. The main cause of failure was due to thermostable bacterial protease associated with high levels of bacterial growth in the raw milks. Other causes of failure included spore-forming bacteria, which may have survived UHT processing, and other organisms probably introduced as contaminants on filling.     

Additive milks*

Over the last 10 years the range of ‘milks’ which dairy companies market has increased enormously. In addition to true low fat milks (ie, skimmed milk and semi-skimmed milk), various additive ‘milks’ are now available. These include low fat ‘milks’ with the taste of whole milk, ‘milks’ with added vitamins, ‘milks’ enriched with calcium and ‘milks’ low in lactose. There are even high fat ‘milks’, and of course flavoured milks. Many of these products are available fresh (ie, pasteurized) rather than long-life (ie, UHT). Improved technology and chilled distribution give the fresh products additional shelf-life. The equipment for making these products is outlined, as well as the quality control requirement. The relevant legislation is also considered.     

PROGRESS AND DEVELOPMENT OF UHT MILK AND MILK PRODUCTS IN THE UK DAIRY INDUSTRY

The paper covers the history of UHT sales in the UK and explains reasons for the small sale of UHT milk. Sales are expected to rise significantly - more sterilized milk is being replaced by UHT, and skimmed milk and flavoured milks being promoted. UHT product sales are encouraging with cream, custards, desserts and ice-cream established. With continual price increases, caterers and shops would want to reduce waste and require non-perishable goods. UHT processing could be beneficial to dairies for doorstep delivery as well as shop sales. Particular mention is made of the success dairies have had with aseptically packed juice.     

Changes in phosphoglyceride composition during storage of ultrahigh-temperature milk, as assessed by 31P-nuclear magnetic resonance: possible involvement of thermoresistant microbial enzymes

Soluble phosphoglycerides were studied in ultrahigh-temperature (UHT) milk by 31P-nuclear magnetic resonance. It was shown that, during storage of UHT milk, manufactured from raw milk with poor microbial quality, glycerophosphocholine and glycerophosphoethanolamine disappeared in parallel with an increase in alpha-glycerophosphate (GP). Storage at 10, 20, and 30 degrees C showed a faster transformation as the temperature increased. UHT milk samples manufactured from raw milks with better microbial quality and submitted to severe heat processes did not display changes in phosphoglycerides during storage. Screening of commercial UHT milks showed variations regarding the presence of GP, while in pasteurized milk samples, the appearance of GP occurred when the commercial life had been exceeded. Inoculation of sterile milk with Pseudomonas fluorescens NCIB9046 and incubation at 10 degrees C supported that changes in phosphoglycerides could be the consequence of a phosphodiesterase activity of bacterial origin, able to survive UHT processing. A similar behavior was observed between this activity and proteolytic activity. The potential application of the detection of these compounds as spoilage predictor indices is discussed.     

Fluorometric determination of chemically available lysine: adaptation, validation and application to different milk products

A spectrophotometric method based on the reaction between available lysine and ortho-phthaldialdehyde (OPA) was adapted and validated for fluorometric determination of the chemically available lysine contents in milk matrices (UHT and conventional in-bottle sterilized cow milk, milk-based infant formulas and infant formula ingredients). The values of the analytical parameters show its usefulness as a routine method (linearity, r = 0.9992; detection limit, 0.0066 mg/mL assay; accuracy, 99-108%; precision, intra-day 2.1-5.9% and inter-day 3.5 10.2%). No statistically significant differences (p < 0.05) were found between the values obtained with the adapted method and those obtained applying the 1-fluoro-2,4-dinitrobenzene (FDNB) (Carpenter) technique. The OPA method was used to measure the chemically available lysine contents in UHT and sterilized milk marketed in Spain, to study the evolution of chemically available lysine during the shelf-life of UHT milks, and finally the quality of name- and store-brand UHT milks was also compared. No statistically significant differences (p < 0.05) were found between either the available lysine contents of the same type of UHT or sterilized milk or between store- and name-brand UHT milks. Statistically significant differences (p < 0.05) were found between the chemically available lysine contents in UHT and sterilized milk. Losses of chemically available lysine ranging from 2.7 to 29% were obtained during the shelf-life of UHT milk.     

Proteolysis in ultra‐heat‐treated skim milk after exposure to multispecies biofilms under conditions modelling a milk tanker

Bacteria from milk tankers can form multispecies biofilms and produce heat‐stable enzymes. In this study, milk was exposed to multispecies biofilms in stainless steel vessels and was then used to produce ultra‐heat‐treated (UHT) milk, which was stored for 5 months. The UHT milks were assessed for microbial counts, free peptide concentration and pH. The free peptide concentration, which indicated proteolysis, was higher in UHT milk that had been exposed to multispecies biofilms than in untreated UHT milk. Biofilm formation may be promoted in milk tankers that are not properly cleaned, which may compromise the quality of the final dairy product.     

Marketing premium milk and products for specialist groups

The problems faced by the producers of Channel Islands premium-quality milk in maintaining their market share are considerable. To counteract a fall in liquid milk sales on the doorstep. there have been increased sales in cartons through supermarkets where a separate product identity can be established. New product types with the Gold Top image have been developed and successfully marketed: frozen double cream, UHT semi-skimmed milk and dairy ice creams containing butter fat from Channel Islands milk. An unsalted butter gave keeping quality problems, but it is hoped that improved raw milk quality control will allow the product to be re-introduced.     

Distinguishing pasteurized, UHT and sterilized milks by their lactulose content

Commercial pasteurized, UHT and sterilized milk samples, and laboratory autoclaved milks, were analysed for lactulose by an enzymatic method. No lactulose was detected in the pasteurized milk. On the basis of the measured lactulose concentrations, the UHT and sterilized milk samples fell into five groups according to the type of process that they had received.
The lactulose concentrations in the laboratory autoclaved milks rose on storage, but this was not observed in the other milks.
It appears that pasteurized, UHT and sterilized milks can be differentiated by their lactulose content.     

Electrochemical sensor detecting free sulfhydryl groups: evaluation of milk heat treatment

We describe a new and rapid method for the evaluation of reactive sulfhydryl groups in whey proteins obtained after precipitation of casein by acetic acid at pH 4.6. The procedure is based on the use of a wire tungsten electrode operating at -0.2 V versus saturated calomel electrode in flow injection analysis. The method was applied to raw milks and to commercial pasteurized and UHT milks. Results showed that the tungsten electrode constituted a robust amperometric sensor that could be used to differentiate milks that underwent different heat treatments. The decrease of thiol content in the whey proteins from samples was in agreement with the whey protein content found by HPLC. The procedure is suitable for on-line quality control of heat-treated milks.     

Determination of whey protein to total protein ratio in UHT milk using fourth derivative spectroscopy

A fourth derivative spectroscopy method was applied for the quantification of whey protein to total protein ratio in UHT milks. Some analytical features such as model compounds, selection of wavelength, linearity, repeatability and interference of milk fat were studied. The effect of refrigerated storage of raw milk, UHT treatment, and storage of UHT milk at room temperature on whey protein to total protein ratio was evaluated. No significant (p<0.05) differences among samples were found in any case. The ratio of whey protein to total protein was also determined in batches of whole (n=28) and skimmed (n=27) commercial UHT milks from different Spanish geographic areas processed by direct or indirect UHT systems in different periods of the year. The mean value was 18.1% for both whole and UHT skimmed milks. The analysis of laboratory-made mixtures of UHT milk with acid and rennet whey (2.5–15% of whey in milk expressed in protein) indicated that adulterations of UHT milk with whey up from 5% could be detected by the proposed method.     

Photoacoustic analysis of some milk products in ultraviolet and visible light

The research reported here demonstrates the possibility of using photoacoustic spectroscopy for milk product analysis. Milk products including yogurt, cheese, and market milk were analyzed in the ultraviolet visible range. A strong absorption peak was present at 280 nm for all the products. Relationship was linear between relative protein concentration of skim milk and the photoacoustic signal at 280 nm (r2 greater than .99). Powdered milks, prepared from skim milk that had been subjected to different heat treatments before drying, were analyzed, and a second absorption peak at 335 nm was noted for milks subjected to high heat treatment prior to the drying process. This second absorption peak appears associated with Maillard reaction products. Analysis of stored UHT heat-treated milk and infant formulas showed a similar peak at 335 nm. The results suggest that the Maillard reaction is initiated during UHT treatment of milk, and associated pigments develop only during storage. The presence of the 335-nm band in the photoacoustic spectra of infant formulas is considered as the result of heat sterilization. It is anticipated that as photoacoustic spectroscopy becomes more common, its usefulness in the milk industry, in particular, and in food science, in general, will increase.     

影响UHT稀奶油脂肪球大小的因素

大豆磷脂是UHT稀奶油适合的乳化剂。适合的复合乳化剂HLB值约为7.4。稀奶油的均质宜采用低压均质,为4×106-5×106Pa。与常温贮存相比,低温贮存时,稀奶油的脂肪球聚集、结晶、固化,稀奶油脂肪球体积平均粒径增大。在6个月的保质期内,随着保质期的延长,稀奶油脂肪球体积平均粒径由2.833μm升高到11.770μm,搅打率从115%降低到92%。文中所讨论的稀奶油产品适合蛋挞制作,蛋挞切面整齐、组织致密,无析水现象。     

Improvement of storage stability and foaming properties of cream by addition of carrageenan and milk constituents

The increasing automation in the dairy industry and the longer guaranteed shelf life of the packed products of cream lead to a loss in the quality of liquid and whipped cream associated with the formation of irreversible plugs, too long whipping times and high degrees of liquid separation of the whipped product. In test series 0.015% carrageenan, 0.25% protein—fat as the dry matter (whey proteins with high-melting milk fat fractions) or a combination of both were added to homogenized and unhomogenized, pasteurized or ultra-high temperature treated (UHT) cream from the winter and summer feeding period (fat content: 30%). The creaming behaviour after storage at 7 or 20°C was characterized by determining the fat content in different layers of a cream package. In cream samples without additives ünstirrable layers had been formed after 2–7 weeks, in particular during the long storage times of UHT cream without cooling. A desired (i.e. low) degree of creaming of the liquid cream as well as little separation in the whipped product could be achieved for all samples only by means of a combination of carrageenan and protein—fat powder. Of the rather varying carrageenan fractions examined, a combination of similar proportions of kappa- and iota-carrageenan has proved to be particularly effective without excessively increasing viscosity.     

Influence of proteolysis of milk on the whey protein to total protein ratio as determined by capillary electrophoresis

Capillary electrophoresis (CE) was used to determine the whey protein to total protein ratio in raw and UHT milk samples with different degrees of proteolysis caused by storage. In raw milks, the analysis of samples taken at regular times demonstrated the influence of proteolysis in the whey protein to total protein determination, which was overestimated after 4 d of storage. In UHT milks, the overestimation of the whey protein to total protein ratio took place after 30 or 60 d of storage. However, the ratios alphaS1-CN/beta-CN and alphas1-CN/kappa-CN permitted detection of the samples of raw or UHT milk with degraded proteins. The distorted capillary electrophoretic pattern obtained for UHT milks made necessary an integration of the electropherograms in a "valley-to-valley" way. Results for raw milk samples were identical when "valley-to-valley" was compared to standard integration techniques. This CE method could be considered an alternative method to derivative spectroscopy for the determination of the whey protein to total protein of milk and could be used to detect samples with proteolysis.     

Heat-induced and other chemical changes in commercial UHT milks

The properties of commercial directly and indirectly heated UHT milks, both after heating and during storage at room temperature for 24 weeks, were studied. Thermally induced changes were examined by changes in lactulose, furosine and acid-soluble whey proteins. The results confirmed previous reports that directly heated UHT milks suffer less heat damage than indirectly heated milk. During storage, furosine increased and bovine serum albumin in directly heat-treated milks decreased significantly. The changes in lactulose, alpha-lactalbumin and beta-lactoglobulin were not statistically significant. The data suggest that heat treatment indicators should be measured as soon as possible after processing to avoid any misinterpretations of the intensity of the heat treatment.     

The use of the chromogenic Limulus amoebocyte lysate assay to distinguish between ultra high temperature and in-container sterilized milks

The chromogenic Limulus amoebocyte lysate (LAL) assay may be used to distinguish between ultra high temperature (UHT) and in-container sterilized (ICS) milks, when applied to samples which have received an additional heat treatment of 100°C for 10 minutes. This heat treatment resulted in a significant increase (p < 0.001) in the availability of lipopolysaccharides in ICS milks which did not occur in UHT milk samples. This method for distinguishing between these milk types is, however, limited to samples whose sera require extensive dilution prior to testing. This possibly reflects the calcium levels present.     

Effect of addition of CO₂ to raw milk on quality of UHT-treated milk

The objective of this study was to evaluate the effect of addition of CO(2) to raw milk on UHT milk quality during storage. Control milk (without CO(2) addition) and treated milk (with CO(2) addition up to pH 6.2) were stored in bulk tanks at 4°C for 6d. After storage, both samples were UHT processed using indirect heating (140°C for 5s). Samples were aseptically packed in low-density polyethylene pouches and stored in the dark at room temperature. Raw milk was evaluated upon receipt for physicochemical composition, proteolysis, lipolysis, standard plate count, psychrotrophic bacteria, and Pseudomonas spp. counts, and after 6d of storage for proteolysis, lipolysis, and microbial counts. After processing, UHT milk samples were evaluated for physicochemical composition, proteolysis, and lipolysis. Samples were evaluated for proteolysis and lipolysis twice a month until 120d. Peptides from pH 4.6-soluble N filtrates were performed by reversed-phase HPLC after 1 and 120d of storage. A split-plot design was used and the complete experiment was carried out in triplicate. The results were evaluated by ANOVA and Tukey's test. After 6d of storage, CO(2)-treated raw milk kept its physicochemical and microbiological quality, whereas the untreated milk showed significant quality losses. A significant increase in proteolysis occurred during 120d of storage in both treatments, but the increase occurred 1.4 times faster in untreated UHT milk than in CO(2)-treated UHT milk. In both UHT milks, the proteolysis was a consequence of the action of plasmin and microbial proteases. However, the untreated UHT milk showed higher microbial protease activity than the treated UHT milk. The addition of CO(2) to the raw milk maintained the quality during storage, resulting in UHT milk with less proteolysis and possibly longer shelf life, which is usually limited by age gelation of UHT milk.     

Deterioration of protein fraction by Maillard reaction in dietetic milks.

The development of the Maillard reaction in pasteurized, UHT and in-bottle sterilized dietetic milks was studied. In these products damage caused by heat treatments could increase as a result either of the addition of various ingredients or of manufacturing processes that alter their content of reducing carbohydrates. Protein damage was evaluated by measuring furosine by reversed-phase ion-paired HPLC. The levels of furosine detected made it possible to assess the amounts of biologically unavailable lysine. In all milks analysed blocked lysine values were < 340-350 mg/g total lysine, the level at which lysine becomes the limiting amino acid in milk. Pasteurized dietetic milks had levels of blocked lysine similar to that in ordinary pasteurized cows' milk. In some UHT and in-bottle sterilized dietetic milks their different composition resulted in an increase in the blocked lysine content. In some in-bottle sterilized milks, protein damage greatly reduces the beneficial effects of milk as a dietary supplement. Lactose-free milks, which are more susceptible to protein deterioration because of their higher content of reducing carbohydrates, were also analysed after storage at 20 degrees C and at < or = 4 degrees C. At the end of their recommended storage times, they contained limited amounts of blocked lysine only if they had been stored at < or = 4 degrees C.     

Bioavailability of Calcium in Heat-Processed Milk

Bioavailability to rats of calcium in milk subjected to the DASI Free Falling Film system (UHT) of pasteurization was compared with the bioavailability of the mineral in raw, high temperature-short time pasteurized, and stored UHT milks. In two other studies, the calcium in the milks was equilibrated with ⁴⁷Ca. Intestinal absorption (both uncorrected and corrected for endogenous fecal excretion) and femoral deposition of calcium were measured. In another study, femur calcium was determined in rats deriving calcium and phosphorus exclusively from the milks. No significant effects were attributable to heat treatment.     

Determination of the casein content in bovine milk by 31P-NMR

The relative proportion of caseins to total protein is a parameter that can be used to control the protein quality in standardised milk, an increasing tendency in dairy industries. 31P-NMR was used to analyse the casein content of milk, by the quantitation of the area under the resonances belonging to SerP, and using methylenediphosphonic acid as internal standard. This procedure yielded good results, as similar values of caseins were obtained from N Kjeldahl and NMR analysis for slightly heated milk samples. Heating at 95 degrees C for 15 min did not alter the casein content results. Casein content of raw, pasteurised and UHT milks (25.6 +/- 1.4, 26.4 +/- 1.8, 25.5 +/- 1.6 g casein/l milk, respectively), obtained by NMR, were not significantly different, giving an average of 25.8 +/- 1.6 g casein/l for bulk liquid milk. This work concluded that 31P-NMR could be used as an alternative method to determine casein in raw, pasteurised, dry and UHT milks.