D.M. Barbano

High temperature, short time pasteurization temperatures inversely affect bacterial numbers during refrigerated storage of pasteurized fluid milk

The grade A Pasteurized Milk Ordinance specifies minimum processing conditions of 72 degrees C for at least 15 s for high temperature, short time (HTST) pasteurized milk products. Currently, many US milk-processing plants exceed these minimum requirements for fluid milk products. To test the effect of pasteurization temperatures on bacterial numbers in HTST pasteurized milk, 2% fat raw milk was heated to 60 degrees C, homogenized, and treated for 25 s at 1 of 4 different temperatures (72.9, 77.2, 79.9, or 85.2 degrees C) and then held at 6 degrees C for 21 d. Aerobic plate counts were monitored in pasteurized milk samples at d 1, 7, 14, and 21 postprocessing. Bacterial numbers in milk processed at 72.9 degrees C were lower than in milk processed at 85.2 degrees C on each sampling day, indicating that HTST fluid milk-processing temperatures significantly affected bacterial numbers in fluid milk. To assess the microbial ecology of the different milk samples during refrigerated storage, a total of 490 psychrotolerant endospore-forming bacteria were identified using DNA sequence-based subtyping methods. Regardless of processing temperature, >85% of the isolates characterized at d 0, 1, and 7 postprocessing were of the genus Bacillus, whereas more than 92% of isolates characterized at d 14 and 21 postprocessing were of the genus Paenibacillus, indicating that the predominant genera present in HTST-processed milk shifted from Bacillus spp. to Paenibacillus spp. during refrigerated storage. In summary, 1) HTST processing temperatures affected bacterial numbers in refrigerated milk, with higher bacterial numbers in milk processed at higher temperatures; 2) no significant association was observed between genus isolated and pasteurization temperature, suggesting that the genera were not differentially affected by the different processing temperatures; and 3) although typically present at low numbers in raw milk, Paenibacillus spp. are capable of growing to numbers that can exceed Pasteurized Milk Ordinance limits in pasteurized, refrigerated milk.

Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C

Raw milk (2,710 kg) was separated at 4 degrees C, the skim milk was pasteurized (72 degrees C, 16 s), split into 3 batches, and microfiltered using pilot-scale ceramic uniform transmembrane pressure (UTP; Membralox model EP1940GL0.1microA, 0.1 microm alumina, Pall Corp., East Hills, NY), ceramic graded permeability (GP; Membralox model EP1940GL0.1microAGP1020, 0.1 microm alumina, Pall Corp.), and polymeric spiral-wound (SW; model FG7838-OS0x-S, 0.3 microm polyvinylidene fluoride, Parker-Hannifin, Process Advanced Filtration Division, Tell City, IN) membranes. There were differences in flux among ceramic UTP, ceramic GP, and polymeric SW microfiltration membranes (54.08, 71.79, and 16.21 kg/m2 per hour, respectively) when processing skim milk at 50 degrees C in a continuous bleed-and-feed 3x process. These differences in flux among the membranes would influence the amount of membrane surface area required to process a given volume of milk in a given time. Further work is needed to determine if these differences in flux are maintained over longer processing times. The true protein contents of the microfiltration permeates from UTP and GP membranes were higher than from SW membranes (0.57, 0.56, and 0.38%, respectively). Sodium-dodecyl-sulfate-PAGE gels for permeates revealed a higher casein proportion in GP and SW permeate than in UTP permeate, with the highest passage of casein through the GP membrane under the operational conditions used in this study. The slight cloudiness of the permeates produced using the GP and SW systems may have been due to the presence of a small amount of casein, which may present an obstacle in their use in applications when clarity is an important functional characteristic. More beta-lactoglobulin passed through the ceramic membranes than through the polymeric membrane. The efficiency of removal of serum proteins in a continuous bleed-and-feed 3x process at 50 degrees C was 64.40% for UTP, 61.04% for GP, and 38.62% for SW microfiltration membranes. The SW polymeric membranes had a much higher rejection of serum proteins than did the ceramic membranes, consistent with the sodium-dodecyl-sulfate PAGE data. Multiple stages and diafiltration would be required to produce a 60 to 65% serum protein reduced micellar casein concentrate with SW membranes, whereas only one stage would be needed for the ceramic membranes used in this study.

Comparison of composition and sensory properties of 80% whey protein and milk serum protein concentrates

Milk serum protein concentrates (SPC) are proteins found in cheese whey that are removed directly from milk. Because SPC are not exposed to the cheese-making process, enzymatic or chemical reactions that can lead to off-flavors are reduced. The objectives of this study were to identify and compare the composition, flavor, and volatile components of 80% protein SPC and whey protein concentrates (WPC). Each pair of 80% SPC and WPC was manufactured from the same lot of milk and this was replicated 3 times. At each replication, spray-dried product from each protein source was collected. Commercial 80% WPC were also collected from several manufacturers for sensory and volatile analyses. A trained sensory panel documented the sensory profiles of the rehydrated powders. Volatile components were extracted by solid-phase microextraction and solvent extraction followed by solvent-assisted flavor evaporation with gas chromatography-mass spectrometry and gas chromatography-olfactometry. Consumer acceptance testing of acidified 6% protein beverages made with 80% SPC and WPC produced in the pilot plant and with WPC from commercial sources was conducted. The SPC was lower in fat and had a higher pH than the WPC produced in the pilot plant or commercial WPC. Few sensory differences were found between the rehydrated SPC and WPC manufactured in this study, but their flavor profiles were distinct from the flavor of rehydrated commercial WPC. The pilot-plant WPC had higher concentrations of lipid oxidation products compared with SPC, which may be related to the higher fat content of WPC. There was a large difference in appearance between 80% SPC and WPC: solutions of SPC were clear and those of WPC were opaque. Concentrations of lipid oxidation products in commercial WPC were generally higher than those in pilot-plant SPC or WPC. Sensory profiles of the peach-flavored protein beverage included cereal, free fatty acid, and soapy flavors and bitter taste in beverages made from pilot-plant products, whereas cardboard flavors were detected in those made with commercial WPC. Consumer liking scores for the beverages made with SPC were ranked highest or equally high with beverages made with WPC for aroma, appearance, and mouthfeel, but the beverages made with SPC had lower flavor and overall liking scores compared with beverages made with 3 of the 4 WPC.

Comparison of composition, sensory, and volatile components of thirty-four percent whey protein and milk serum protein concentrates

The objectives of this study were to identify and compare the composition, flavor, and volatile components of serum protein concentrate (SPC) and whey protein concentrate (WPC) containing about 34% protein made from the same milk to each other and to commercial 34% WPC from 6 different factories. The SPC and WPC were manufactured in triplicate with each pair of serum and traditional whey protein manufactured from the same lot of milk. At each replication, SPC and WPC were spray dried (SD) and freeze dried (FD) to determine the effect of the heat used in spray drying on sensory properties. A trained sensory panel documented the sensory profiles of rehydrated SD or FD powders. Volatile components were extracted by solid-phase microextraction (SPME) and solvent extraction followed by solvent-assisted flavor evaporation (SAFE) with gas chromatography-mass spectrometry and gas chromatography-olfactometry. Whey protein concentrates had higher fat content, calcium, and glycomacropeptide content than SPC. Color differences (Hunter L, a, b) were not evident between SPC and WPC powders, but when rehydrated, SPC solutions were clear, whereas WPC solutions were cloudy. No consistent differences were documented in sensory profiles of SD and FD SPC and WPC. The SD WPC had low but distinct buttery (diacetyl) and cardboard flavors, whereas the SD SPC did not. Sensory profiles of both rehydrated SD products were bland and lower in overall aroma and cardboard flavor compared with the commercial WPC. Twenty-nine aroma impact compounds were identified in the SPC and WPC. Lipid and protein oxidation products were present in both products. The SPC and WPC manufactured in this study had lower total volatiles and lower concentrations of many lipid oxidation compounds when compared with commercial WPC. Our results suggest that when SPC and WPC are manufactured under controlled conditions in a similar manner from the same milk using the same ultrafiltration equipment, there are few sensory differences but distinct compositional and physical property differences that may influence functionality. Furthermore, flavor (sensory and instrumental) properties of both pilot-scale manufactured protein powders were different from commercial powders suggesting the role of other influencing factors (e.g., milk supply, processing equipment, sanitation).

Effect of mastitis on plasminogen activator activity of milk somatic cells

This study was conducted to examine the effects of mastitis and stage of lactation on plasminogen activator (PA) activity in milk somatic cells. An assay system, which measures the plasmin-mediated hydrolysis of the chromogenic substrate D-valyl-leucyl-lysine p-nitroanilide, was used to assess PA activity present within milk somatic cells. Milk cell associated PA activity was increased (P < 0.05) by 50% in the presence of fibrin fragments. This suggests that milk somatic cells contain tissue PA which, unlike urokinase PA, is preferentially activated in the presence of fibrin fragments. An increase of the milk somatic cell count from < 5 x 10(4) to > 10(6) cells/ml resulted in an 8-fold increase in PA activity per cell. Elevated levels of PA activity were associated with milk somatic cells isolated from mastitic quarters obtained from cows in early (< 4 months in lactation) or late lactation (> 8 months in lactation). We conclude that PA activity is increased during severe mastitic inflammation. Although the physiological function of this enzyme is as yet unclear, we propose that it may be involved in the conversion of plasminogen to plasmin, contributing to the higher levels of plasmin occurring in milk isolated from mastitic quarters.

Micellar casein concentrate production with a 3X, 3-stage, uniform transmembrane pressure ceramic membrane process at 50°C.

The production of serum protein (SP) and micellar casein from skim milk can be accomplished using microfiltration (MF). Potential commercial applications exist for both SP and micellar casein. Our research objective was to determine the total SP removal and SP removal for each stage, and the composition of retentates and permeates, for a 3×, continuous bleed-and-feed, 3-stage, uniform transmembrane pressure (UTP) system with 0.1-μm ceramic membranes, when processing pasteurized skim milk at 50°C with 2 stages of water diafiltration. For each of 4 replicates, about 1,100 kg of skim milk was pasteurized (72°C, 16s) and processed at 3× through the UTP MF system. Retentate from stage 1 was cooled to <4°C and stored until the next processing day, when it was diluted with reverse osmosis water back to a 1× concentration and again processed through the MF system (stage 2) to a 3× concentration. The retentate from stage 2 was stored at <4°C, and, on the next processing day, was diluted with reverse osmosis water back to a 1× concentration, before running through the MF system at 3× for a total of 3 stages. The retentate and permeate from each stage were analyzed for total nitrogen, noncasein nitrogen, and nonprotein nitrogen using Kjeldahl methods; sodium dodecyl sulfate-PAGE analysis was also performed on the retentates from each stage. Theoretically, a 3-stage, 3× MF process could remove 97% of the SP from skim milk, with a cumulative SP removal of 68 and 90% after the first and second stages, respectively. The cumulative SP removal using a 3-stage, 3× MF process with a UTP system with 0.01-μm ceramic membranes in this experiment was 64.8 ± 0.8, 87.8 ± 1.6, and 98.3 ± 2.3% for the first, second, and third stages, respectively, when calculated using the mass of SP removed in the permeate of each stage. Various methods of calculation of SP removal were evaluated. Given the analytical limitations in the various methods for measuring SP removal, calculation of SP removal based on the mass of SP in the skim milk (determined by Kjeldahl) and the mass SP present in all of the permeate produced by the process (determined by Kjeldahl) provided the best estimate of SP removal for an MF process.

Effect of bleaching whey on sensory and functional properties of 80% whey protein concentrate

Whey is a highly functional food that has found widespread use in a variety of food and beverage applications. A large amount of the whey proteins produced in the United States is derived from annatto-colored Cheddar cheese. Color from annatto is undesirable in whey and must be bleached. The objective of this study was to compare 2 commercially approved bleaching agents, benzoyl peroxide (BP) and hydrogen peroxide (HP), and their effects on the flavor and functionality of 80% whey protein concentrate (WPC80). Colored and uncolored liquid wheys were bleached with BP or HP, and then ultrafiltered, diafiltered, and spray-dried; WPC80 from unbleached colored and uncolored Cheddar whey were manufactured as controls. All treatments were manufactured in triplicate. The WPC80 were then assessed by sensory, instrumental, functionality, color, and proximate analysis techniques. The HP-bleached WPC80 were higher in lipid oxidation compounds (specifically hexanal, heptanal, octanal, nonanal, decanal, dimethyl disulfide, and 1-octen-3-one) and had higher fatty and cardboard flavors compared with the other unbleached and BP-bleached WPC80. The WPC80 bleached with BP had lower norbixin concentrations compared with WPC80 bleached with HP. The WPC powders differed in Hunter color values (L, a, b), with bleached powders being more white, less red, and less yellow than unbleached powders. Bleaching with BP under the conditions used in this study resulted in larger reductions in yellowness of the powders made from whey with annatto color than did bleaching with HP. Functionality testing demonstrated that whey bleached with HP treatments had more soluble protein after 10 min of heating at 90°C at pH 4.6 and pH 7 than the no-bleach and BP treatments, regardless of additional color. Overall, HP bleaching caused more lipid oxidation products and subsequent off-flavors compared with BP bleaching. However, heat stability of WPC80 was enhanced by HP bleaching compared with control or BP-bleached WPC80.

Survey of the fatty acid composition of retail milk in the United States including regional and seasonal variations.

Consumers are increasingly aware that food components have the potential to influence human health maintenance and disease prevention, and dietary fatty acids (FA) have been of special interest. It has been 25 years since the last survey of US milk FA composition, and during this interval substantial changes in dairy rations have occurred, including increased use of total mixed rations and byproduct feeds as well as the routine use of lipid and FA supplements. Furthermore, analytical procedures have improved allowing greater detail in the routine analysis of FA, especially trans FA. Our objective was to survey US milk fat and determine its FA composition. We obtained samples of fluid milk from 56 milk processing plants across the US every 3 mo for one year to capture seasonal and geographical variations. Processing plants were selected based on the criteria that they represented 50% or more of the fluid milk produced in that area. An overall summary of the milk fat analysis indicated that saturated fatty acids comprised 63.7% of total milk FA with palmitic and stearic acids representing the majority (44.1 and 18.3% of total saturated fatty acids, respectively). Unsaturated fatty acids were 33.2% of total milk FA with oleic acid predominating (71.0% of total unsaturated fatty acids). These values are comparable to those of the previous survey in 1984, considering differences in analytical techniques. Trans FA represented 3.2% of total FA, with vaccenic acid being the major trans isomer (46.5% of total trans FA). Cis-9, trans-11 18:2 conjugated linoleic acid represented 0.55% total milk FA, and the major n-3 FA (linolenic acid, 18:3) composed 0.38%. Analyses for seasonal and regional effects indicated statistical differences for some FA, but these were minor from an overall human nutrition perspective as the FA profile for all samples were numerically similar. Overall, the present study provides a valuable database for current FA composition of US fluid milk, and results demonstrate that the milk fatty acid profile is remarkably consistent across geographic regions and seasons from the perspective of human dietary intake of milk fat.

Gravity separation of fat, somatic cells, and bacteria in raw and pasteurized milks1

The objective of experiment 1 was to determine if the extent of gravity separation of milk fat, bacteria, and somatic cells is influenced by the time and temperature of gravity separation or the level of contaminating bacteria present in the raw milk. The objective of experiment 2 was to determine if different temperatures of milk heat treatment affected the gravity separation of milk fat, bacteria, and somatic cells. In raw milk, fat, bacteria, and somatic cells rose to the top of columns during gravity separation. About 50 to 80% of the fat and bacteria were present in the top 8% of the milk after gravity separation of raw milk. Gravity separation for 7h at 12°C or for 22h at 4°C produced equivalent separation of fat, bacteria, and somatic cells. The completeness of gravity separation of fat was influenced by the level of bacteria in the milk before separation. Milk with a high bacterial count had less (about 50 to 55%) gravity separation of fat than milk with low bacteria count (about 80%) in 22h at 4°C. Gravity separation caused fat, bacteria, and somatic cells to rise to the top of columns for raw whole milk and high temperature, short-time pasteurized (72.6°C, 25s) whole milk. Pasteurization at ≥76.9°C for 25s prevented all 3 components from rising, possibly due to denaturation of native bovine immunoglobulins that normally associate with fat, bacteria, and somatic cells during gravity separation. Gravity separation can be used to produce reduced-fat milk with decreased bacterial and somatic cell counts, and may be a critical factor in the history of safe and unique traditional Italian hard cheeses produced from gravity-separated raw milk. A better understanding of the mechanism of this natural process could lead to the development of new nonthermal thermal technology (that does not involve heating the milk to high temperatures) to remove bacteria and spores from milk or other liquids.

Processing factors that influence casein and serum protein separation by microfiltration.

Our objective was to demonstrate the effect of various processing factors on the performance of a microfiltration system designed to process skim milk and separate casein (CN) from serum proteins (SP). A mathematical model of a skim milk microfiltration process was developed with 3 stages plus an additional fourth finishing stage to standardize the retentate to 9% true protein (TP) and allow calculation of yield of a liquid 9% TP micellar CN concentrate (MCC) and milk SP isolate (MSPI; 90% SP on a dry basis). The model was used to predict the effect of 5 factors: 1) skim milk composition, 2) heat treatment of skim milk, 3) concentration factor (CF) and diafiltration factor (DF), 4) control of CF and DF, and 5) SP rejection by the membrane on retentate and permeate composition, SP removal, and MCC and MSPI yield. When skim milk TP concentration increased from 3.2 to 3.8%, the TP concentration in the third stage retentate increased from 7.92 to 9.40%, the yield of MCC from 1,000 kg of skim milk increased from 293 to 348 kg, and the yield of MSPI increased from 6.24 to 7.38 kg. Increased heat treatment (72.9 to 85.2°C) of skim milk caused the apparent CN as a percentage of TP content of skim milk as measured by Kjeldahl analysis to increase from 81.97 to 85.94% and the yield of MSPI decreased from 6.24 to 4.86 kg, whereas the third stage cumulative percentage SP removal decreased from 96.96 to 70.08%. A CF and DF of 2× gave a third stage retentate TP concentration of 5.38% compared with 13.13% for a CF and DF of 5×, with the third stage cumulative SP removal increasing from 88.66 to 99.47%. Variation in control of the balance between CF and DF (instead of an equal CF and DF) caused either a progressive increase or decrease in TP concentration in the retentate across stages depending on whether CF was greater than DF (increasing TP in retentate) or CF was less than DF (decreasing TP in retentate). An increased rejection of SP by the membrane from an SP removal factor of 1 to 0.6 caused a reduction in MSPI yield from 6.24 to 5.19 kg/1,000 kg of skim milk, and third stage cumulative SP removal decreased from 96.96 to 79.74%. Within the ranges of the 5 factors studied, the TP content of the third stage retentate was most strongly affected by the target CF and DF and variation in skim milk composition. Cumulative percentage SP removal was most strongly affected by the heat treatment of skim milk, the SP removal factor, and the target CF and DF. The MCC yield was most strongly affected by initial skim milk composition. Yield of MSPI was strongly affected by skim milk composition, whereas the heat treatment of milk and SP removal factor also had a large effect.