Bile Salt-stimulated Lipase Activity Research Articles

The change of bile salt stimulated lipase during 6 months and the correlation with macronutrients in Chinese human milk

Bile salt stimulated lipase (BSSL) is the most abundant lipase in human milk which plays a pivotal role in new-born fat digestion, especially in the first six months of life. Lipid as the main energy supply for infant was very depending on BSSL activity for obtaining optimal lipid absorption under immature digestion track and pancreatic system condition. Due to the important role of BSSL, this research was addressed to study the effect of gender and lactation stage on BSSL activity and the correlation of BSSL with macronutrients content in Chinese human milk. Cross sectional and longitudinal study were used to investigate the BSSL activity and concentration pattern during 6 months of lactation. A declining pattern of both BSSL activity and concentration was observed as lactation stage progressed. The BSSL concentration significantly declined from 138.56 to 97.07 μg/ml during 6 months. Significant differences of BSSL activity and concentration were also observed in human milk for different gender babies. Male babies had breastmilk with less BSSL rather than female babies. Protein had strong correlation with BSSL activity and concentration, while carbohydrate had non-significant negative coefficient correlation. However, no correlation observed between fat content and BSSL.

High-Pressure Processing of Human Milk: A Balance between Microbial Inactivation and Bioactive Protein Preservation

BackgroundDonor human milk banks use Holder pasteurization (HoP; 62.5°C, 30 min) to reduce pathogens in donor human milk, but this process damages some bioactive milk proteins. ObjectivesWe aimed to determine minimal parameters for high-pressure processing (HPP) to achieve >5-log reductions of relevant bacteria in human milk and how these parameters affect an array of bioactive proteins. MethodsPooled raw human milk inoculated with relevant pathogens (Enterococcus faecium, Staphylococcus aureus, Listeria monocytogenes, Cronobacter sakazakii) or microbial quality indicators (Bacillus subtilis and Paenibacillus spp. spores) at 7 log CFU/mL was processed at 300–500 MPa at 16–19°C (due to adiabatic heating) for 1–9 min. Surviving microbes were enumerated using standard plate counting methods. For raw milk, and HPP-treated and HoP-treated milk, the immunoreactivity of an array of bioactive proteins was assessed via ELISA and the activity of bile salt-stimulated lipase (BSSL) was determined via a colorimetric substrate assay. ResultsTreatment at 500 MPa for 9 min resulted in >5-log reductions of all vegetative bacteria, but <1-log reduction in B. subtilis and Paenibacillus spores. HoP decreased immunoglobulin A (IgA), immunoglobulin M (IgM), immunoglobulin G, lactoferrin, elastase and polymeric immunoglobulin receptor (PIGR) concentrations, and BSSL activity. The treatment at 500 MPa for 9 min preserved more IgA, IgM, elastase, lactoferrin, PIGR, and BSSL than HoP. HoP and HPP treatments up to 500 MPa for 9 min caused no losses in osteopontin, lysozyme, α-lactalbumin and vascular endothelial growth factor. ConclusionCompared with HoP, HPP at 500 MPa for 9 min provides >5-log reduction of tested vegetative neonatal pathogens with improved retention of IgA, IgM, lactoferrin, elastase, PIGR, and BSSL in human milk.

Adaption of a commercial lipase kit to measure bile salt-stimulated lipase in human milk

A highly sensitive, fluorometric method for measuring activities of two major lipases in milk, lipoprotein lipase (LPL) and bile salt-stimulated lipase (BSSL) is described. Human milk LPL showed linear activity from 65 to 347 U/L (U = 1 μmol/min) and the quantification limit was theoretically about 65 U/L. For human milk BSSL, the linear range was from 10 to 267 U/L and the quantification limit was 18 U/L. The mean LPL and BSSL activities were 409 ± 13 U/L and 5263 ± 132 U/L, respectively. This corresponded well to the values reported in literature. The residual LPL and BSSL activities were reduced dramatically to 21%, and 2%, respectively, when the milk was heated to 55 °C; heating to 50 °C caused only nominal reductions of LPL and BSSL (activities remaining ∼82% and 87%, respectively). No BSSL activity was detected in cow, camel, goat milk or in infant formula.

Effects of High-Pressure Processing, UV-C Irradiation and Thermoultrasonication on Donor Human Milk Safety and Quality.

Holder pasteurization (HoP) is the current recommended treatment for donor human milk. Although this method inactivates microbial contaminants, it also negatively affects various milk components. High-pressure processing (HPP, 400, 500, and 600 MPa), ultraviolet-C irradiation (UV-C, 2,430, 3,645, and 4,863 J/L) and thermoultrasonication (TUS, 1,080 and 1,620 kJ/L) were investigated as alternatives to thermal pasteurization (HoP). We assessed the effects of these methods on microbiological safety, and on concentration and functionality of immunoglobulin A, lactoferrin, lysozyme and bile salt-stimulated lipase, with LC-MS/MS-based proteomics and activity assays. HoP, HPP, TUS, and UV-C at 4863 J/L, achieved >5-log10 microbial reduction. Native protein levels and functionality showed the highest reduction following HoP, while no significant reduction was found after less intense HPP and all UV-C treatments. Immunoglobulin A, lactoferrin, and lysozyme contents were also preserved after low intensity TUS, but bile salt-stimulated lipase activity was significantly reduced. This study demonstrated that HPP and UV-C may be considered as suitable alternatives to HoP, since they were able to ensure sufficient microbial inactivation while at the same time better preserving the bioactive components of donor human milk. In summary, our results provide valuable insights regarding the evaluation and selection of suitable processing methods for donor human milk treatment, which may replace HoP in the future.

High-Temperature Short-Time Preserves Human Milk's Bioactive Proteins and Their Function Better Than Pasteurization Techniques With Long Processing Times.

Donor human milk is generally processed by holder pasteurization (HoP) at 62. 5°C for 30 min. This temperature-time combination is sufficient for eliminating pathogens in donor milk, but also negatively affects several bioactive milk components. Long heating up times may further affect the bioactive properties of pasteurized milk. High-Temperature-Short-Time (HTST), a treatment with shorter processing times (72°C for 15 sec), was investigated as a suitable alternative to HoP. In addition, pasteurization methods that follow the same temperature regime but with varying heating up times were compared. Human milk samples from four different donors were combined into one pool, which was then used to perform all analyses. The effects of these methods on the levels and functionality of immunoglobulin A, lactoferrin, lysozyme and bile salt-stimulated lipase, were evaluated with LC-MS/MS-based proteomics and activity assays, while the pasteurization efficacy was evaluated with an alkaline phosphatase test. HoP, a treatment with long processing times, times, caused the highest reduction in all proteins studied (reduced by 50–98%). Compounds such as lactoferrin and bile salt-stimulated lipase that are more sensitive to heat treatments were better retained with HTST, but their levels and functionality were still significantly lower than those of untreated donor milk (52 and 81% reduction of lactoferrin and bile salt-stimulated lipase activity, respectively). Our findings showed that a treatment with considerably shorter processing times, such as HTST, may reduce the thermal damage caused to the bioactive proteins compared to HoP, without affecting pasteurization efficacy. Since the vast majority of the donor human milk banks that are currently operating on a global level apply HoP to donor milk, our findings may provide relevant information for the optimization of donor milk processing.

High-Temperature Short-Time and Holder Pasteurization of Donor Milk: Impact on Milk Composition.

Holder pasteurization (HoP; 62.5 °C, 30 min) is commonly used to ensure the microbiological safety of donor human milk (DHM) but diminishes its nutritional properties. A high-temperature short-time (HTST) system was designed as an alternative for human milk banks. The objective of this study was to evaluate the effect of this HTST system on different nutrients and the bile salt stimulated lipase (BSSL) activity of DHM. DHM was processed in the HTST system and by standard HoP. Macronutrients were measured with a mid-infrared analyzer. Lactose, glucose, myo-inositol, vitamins and lipids were assayed using chromatographic techniques. BSSL activity was determined using a kit. The duration of HTST treatment had a greater influence on the nutrient composition of DHM than did the tested temperature. The lactose concentration and the percentage of phospholipids and PUFAs were higher in HTST-treated than in raw DHM, while the fat concentration and the percentage of monoacylglycerides and SFAs were lower. Other nutrients did not change after HTST processing. The retained BSSL activity was higher after short HTST treatment than that following HoP. Overall, HTST treatment resulted in better preservation of the nutritional quality of DHM than HoP because relevant thermosensitive components (phospholipids, PUFAs, and BSSL) were less affected.

Bile Salt-Stimulated Lipase Activity in Donor Breast Milk Influenced by Pasteurization Techniques.

Breast milk contains bile salt-stimulated lipase (BSSL), which significantly increases the fat digestion capacity of newborns who have limited pancreatic lipase secretion in the first few months after birth. Problematically, Holder pasteurization used in non-profit milk banks to ensure the microbiological safety of donor milk for infants, particularly preterm infants (<37 weeks gestation age), destroys milk BSSL, thus limiting infant fat absorption capacity. Alternative strategies are needed to ensure the safety of donor milk while preserving BSSL activity. Three alternative pasteurization techniques—high-pressure processing (HPP, 550 MPa, 5 min), gamma cell irradiation (IR, 2.5 Mrads) and UV-C (254 nm, 0–33,000 J/L)—were compared with Holder pasteurization (low-temperature long-time, LTLT, 62.5°C, 30 min) for retention of BSSL activity in donor breast milk. As the time required for donor milk pasteurization by UV-C in published methods was not clear, donor breast milk was spiked with seven common bacterial strains and treated by UV-C for variable time periods and the minimum UV-C dosage required to achieve a 5-log10 reduction of CFU/mL was determined. Eight thousand two hundred fifty J/L of UV-C exposure was sufficient to achieve 5-log10 reduction of each of bacterial targets, including Bacillus and Paenibacillus spores. The retention of BSSL activity was highest after HPP (retaining 62% of the untreated milk BSSL activity), followed by UV-C (16,500 J/L), IR and LTLT (35, 29, and 0.3% retention, respectively). HPP was an effective alternative to pasteurize milk with improved retention of BSSL activity compared to Holder pasteurization. Future work should investigate the effect of alternative pasteurization techniques on the entire array of bioactive components in donor breast milk and how these changes affect preterm infant health outcomes. Implementation of HPP technique at milk banks could improve donor milk-fed infant fat absorption and growth.

Pasteurization of human milk by a benchtop High-Temperature Short-Time device

A new small-scale continuous-flow High-Temperature Short-Time (HTST) pasteurizer has been designed for treating human milk. The efficacy of the new HTST device was assessed on inoculated Listeria monocytogenes, Staphylococcus aureus and Chronobacter sakazakii, as well as on raw human milk bacteria. The milk biochemical quality after HTST pasteurization was assessed in comparison to a standard Holder pasteurization, by determining the secretory IgAs (sIgAs) content, the protein profile, lysozyme and the Bile Salt Stimulated Lipase (BSSL) activities. No pathogen or bacterial growth was detected after HTST pasteurization with the new instrument. Changes in the protein profile were observed in the milk pasteurized according to both processes. The sIgAs content and BSSL activity were significantly higher in the milk pasteurized with the new device than in the same milk treated by the standard Holder pasteurization. In conclusion, the new HTST apparatus: (i) can effectively pasteurize human milk with a better retention of sIgAs content and BSSL activity; (ii) comply to human milk banking safety requirements. Currently, 210 active human milk banks are located in Europe (and 17 more are planned). The majority of the European banks still use Holder-based pasteurizers, which, despite efficacy in ensuring microbiological safety, are known to reduce/disrupt important nutritional and non-nutritional biological factors. Although already widely established in food industry, the advantages of HTST technology were tested only at small laboratory scale for human milk. The device tested in the present research was specifically designed to provide human milk banks with the technology they need to ensure a safe and lower-impact pasteurization process, that is suitable for processing different volumes of donations. The device can pasteurize up to 10 L of milk per hour, with a minimum volume of 100 mL. The system is designed to be cleaned-in-place (CIP) after each pasteurization run and sanitized immediately prior to the next use, being thus more suitable for treating pools of milk from different donors than milk from single donations. Italian and EU patents have been filed for the device, within a partnership between public research institutions, stakeholders (Italian association of donor milk banks), and a private company in the sector of dairy processing equipment. The device has achieved a Technology Readiness Level (TRL) 6 (Prototype demonstration in a relevant environment). The cost of the new device will be comparable to that of a typical human milk Holder pasteurizer.

Ultraviolet-C Irradiation: A Novel Pasteurization Method for Donor Human Milk.

BackgroundHolder pasteurization (milk held at 62.5°C for 30 minutes) is the standard treatment method for donor human milk. Although this method of pasteurization is able to inactivate most bacteria, it also inactivates important bioactive components. Therefore, the objective of this study was to investigate ultraviolet irradiation as an alternative treatment method for donor human milk.MethodsHuman milk samples were inoculated with five species of bacteria and then UV-C irradiated. Untreated and treated samples were analysed for bacterial content, bile salt stimulated lipase (BSSL) activity, alkaline phosphatase (ALP) activity, and fatty acid profile.ResultsAll five species of bacteria reacted similarly to UV-C irradiation, with higher dosages being required with increasing concentrations of total solids in the human milk sample. The decimal reduction dosage was 289±17 and 945±164 J/l for total solids of 107 and 146 g/l, respectively. No significant changes in the fatty acid profile, BSSL activity or ALP activity were observed up to the dosage required for a 5-log10 reduction of the five species of bacteria.ConclusionUV-C irradiation is capable of reducing vegetative bacteria in human milk to the requirements of milk bank guidelines with no loss of BSSL and ALP activity and no change of FA.

Ultrasonication and the quality of human milk: variation of power and time of exposure

Donor human milk is pasteurized to prevent the potential risk of the transmission of pathogens to preterm infants. Currently, Holder pasteurization (human milk held at 62·5°C for 30 min) is used in most human milk banks, but has the disadvantage that it results in excessive inactivation of important bioactive components. Power-ultrasound (20-100 kHz) is an emerging technology for the preservation of foods and could be an alternative method for the treatment of human milk. The aim of this study was to investigate the effect of different ultrasound settings on the elimination of Escherichia coli and the retention of bile salt stimulated lipase (BSSL) activity. Ultrasonication with a constant power decreased Esch. coli viability exponentially over time until the processing temperature increased to sub-pasteurization level to between 51·4 and 58·5°C, then a log10 1·3 decrease was observed (P<0·05). BSSL activity decreased to 91% until a temperature of 51·4°C and then it decreased to 8% between 51·4 and 64·9°C. Ultrasonication with a constant energy and various power and exposure times showed the highest temperature (53·7°C) when treated with the longest exposure time and lowest ultrasound-power (276 s at 3·62 W) compared with 37·6°C for 39 s at 25·64 W. The findings predict that the viability of Esch. coli could be reduced by log10 5 with a minimal loss of activity of BSSL by applying 13·8 kJ of energy in 12 ml of human milk using high ultrasound power over a short exposure time to ensure that the temperature remains below the critical level for protein denaturation. Alternatively, the use of lower power settings such as the 26 W used in the present studies would require a cooling system to ensure the human milk BSSL was protected against temperature denaturation.

Incidence of molecular forms of bile salt-stimulated lipase in preterm and term human milk.

Preterm and term human milk samples obtained at various times after delivery were analyzed for the presence of molecular forms of the human milk enzyme, bile salt-stimulated lipase (BSSL). Thirty-five percent of both the preterm and term milk samples contained two molecular forms of BSSL, of variable molecular mass. The remainder contained only one molecular species of either 115 kD (50%) or 120 kD (15%). The number of molecular forms present was not related to length of lactation, maternal age, gestation, or maternal blood group. The specific activity of BSSL purified from term milk was similar to that purified from preterm milk, and there was no difference in specific activity whether one or two molecular forms were present. This study demonstrates heterogeneity of both molecular mass and molecular forms. We conclude that preterm babies fed their own mother's milk are unlikely to be disadvantaged with respect to fat digestion as BSSL secreted in preterm milk appears to be very similar to that produced in term milk, although we cannot exclude other functional differences.

A Study of the Relationship between Bile Salts, Bile Salt-Stimulated Lipase, and Free Fatty Acids in Breast Milk

Breast milk jaundice has been reported to be associated with increased lipase activity and elevated free fatty acid (FFA) concentrations within breast milk. We have previously shown that bile salts are present in small concentrations in breast milk and the aim of this study was to examine the relationship of bile salt-stimulated lipase (BSSL) activity, FFA concentration, and bile salt concentration in milks of normal infants and the milk of infants with breast milk jaundice. Mothers of healthy newborn infants were recruited in the early newborn period and 42 provided breast milk samples at 2 weeks, 30 at 6 weeks, 16 at 10 weeks, and 13 at 14 weeks postnatally. We initially studied the effect of lactation on bile salts and found there was a significant decline in both cholate and chenodeoxycholate levels with duration of lactation (p less than 0.05). There was also a significant fall in BSSL activity with duration of lactation (p less than 0.05), but no correlation was found between BSSL activity and bile salt concentration. FFA concentrations were similar throughout lactation and were not related to either BSSL activity or bile salt concentration. There was a significant increase in the concentration of cholate and the cholate-to-chenodeoxycholate ratio in the milks of 12 infants with breast milk jaundice compared with normal milks, the BSSL activity was similar and contrary to previous reports, the FFA concentration was not increased in the milks of infants with breast milk jaundice.

A Study of the Relationship between Bile Salts, Bile Salt‐Stimulated Lipase, and Free Fatty Acids in Breast Milk

Summary:Breast milk jaundice has been reported to be associated with increased lipase activity and elevated free fatty acid (FFA) concentrations within breast milk. We have previously shown that bile salts are present in small concentrations in breast milk and the aim of this study was to examine the relationship of bile salt‐stimulated lipase (BSSL) activity, FFA concentration, and bile salt concentration in milks of normal infants and the milk of infants with breast milk jaundice. Mothers of healthy newborn infants were recruited in the early newborn period and 42 provided breast milk samples at 2 weeks, 30 at 6 weeks, 16 at 10 weeks, and 13 at 14 weeks postnatally. We initially studied the effect of lactation on bile salts and found there was a significant decline in both cholate and chenodeoxycholate levels with duration of lactation (p &lt; 0.05). There was also a significant fall in BSSL activity with duration of lactation (p &lt; 0.05), but no correlation was found between BSSL activity and bile salt concentration. FFA concentrations were similar throughout lactation and were not related to either BSSL activity or bile salt concentration. There was a significant increase in the concentration of cholate and the cholate‐to‐chenodeoxy‐cholate ratio in the milks of 12 infants with breast milk jaundice compared with normal milks, the BSSL activity was similar and contrary to previous reports, the FFA concentration was not increased in the milks of infants with breast milk jaundice.

Lipases in human milk: effect of gestational age and length of lactation on enzyme activity.

Human milk contains two lipases, bile salt-stimulated lipase (BSSL) and lipoprotein lipase (LPL). In the mammary gland, LPL provides long-chain fatty acid for milk fat synthesis. LPL has no known function in milk, but has been implicated in milk fat hydrolysis during cold storage. BSSL may have an important role in infant fat digestion. The aims of the present studies were to assess (1) the methodological validity of using whole milk to analyze BSSL activity, (2) the longitudinal variation of BSSL and LPL activity in the milk of mothers delivering premature and full-term infants, and (3) the stability of BSSL and LPL activity during cold storage. Diluted whole milk and purified BSSL were shown to have similar characteristics. LPL activity was equally stable at -20 and -70 degrees C, whereas BSSL activity was higher in milks stored at -70 than at -20 degrees C (38.8 +/- 0.88 vs 33.3 +/- 0.87 U/ml milk, respectively; 1U = 1 mumol free fatty acid release/min). Levels of BSSL activity in preterm and term milk were similar. LPL activity tended to be higher in term milk. Overall, BSSL activity showed significant longitudinal variation, being highest at 1 and 3 weeks of lactation (43.2 +/- 0.04 and 42.6 +/- 1.03 U/ml milk, respectively). For LPL, the longitudinal pattern of activity depended upon the length of pregnancy. Implications for infant nutrition and mammary gland biology are discussed.

Heat Stability of the Bile Salt-Stimulated Lipase in Human Milk Fortified with Sodium Taurocholate

Time-temperature relationships for heat-inactivation of the bile salt-stimulated lipase activity were compared in whole human milk and in the same product fortified to 9 mM/ml with sodium taurocholate. Heat treatments were varied from 45 to 70°C for times ranging from 15s to 40 min. Enzyme activity was more heat stable in human milk fortified with taurocholate than in control samples. The temperature required for the onset of heat inactivation at 30-min holding time was increased from 45°C for control samples to 60°C following addition of taurocholate. A temperature differential of approximately 12°C was required in the fortified milks to produce inactivation equivalent to that observed in the control milks over the heating range studied.

Bile Salt-Stimulated Lipase of Human Milk

Human milk contains a lipase (bile salt-stimulated lipase) that is considered to have an important role in infant fat digestion. In this study we compared the characteristics of bile salt-stimulated lipase activity in milk samples from mothers delivering prematurely (26-30 and 31-37 weeks of gestation) and in milk from mothers delivering at term (38-42 weeks of gestation). Preterm milks were collected at day 1-5 and during week 6 of lactation. Term milks were collected during week 6 of lactation. The characteristics of the enzyme (kinetics, enzyme concentration, pH optimum, and pH stability, effects of bile salt structure and concentration, eserine inhibition) were identical regardless of length of pregnancy or duration of lactation. Bile salt-stimulated lipase had a neutral to alkaline pH optimum (pH 7.3-8.6), was stable for 1 h at a wide pH range (pH 3.1-8.6), was active only in the presence of primary bile salts, and was inhibited by eserine. The data indicate that, following parturition at as early as 26 weeks of gestation, the mammary glands synthesizes bile salt-stimulated lipase with identical characteristics as does the mammary gland after a full-term pregnancy.

BREAST MILK JAUNDICE REVISITED: NO ROLE FOR B-GLOCORONIDASE OR “UNSTIMULATED LIPASE”

To date, the milk factor(s) repsonsible for breast milk jaundice (BMJ) has not been found. The lipases of human milk, lipoprotein lipase (LPL) and bile salt-stimulated lipase (BSSL), require specific activators, apoprotein CII for LPL and primary bile salts for BSSL. It has been suggested that, in BMJ milks, one or both lipases are active in the absence of activator [unstimulated lipase activity (USL) (Pediatr Res 14:1328, 1980)]. More recently, It has been reported that milk β-glucuronldase (β-glu) may be the causative agent (Lancet 1:644, 1986). We have reexamined these questions by analyzing β-glu activity and lipase activity [using highly sensitive techniques (Biochim Biophys Acta 878:209, 1986) for quantitation of stimulated and unstimulated LPL and BSSL activity] in milk samples from 13 mothers of infants with BMJ and 4 mothers of healthy infants. The following results (means and ranges, expressed as umol free fatty acids/min/ml milk for lipases and modified Sigma units/ml for β-glu) were obtained: Within-feed variation in milk collection does not affect the level of β-glu, the activity being 100 (48-189), 115 (48-233), and 132 (54-230) units in fore, mid and hind milk, respectively. These data show that the levels of β-glu, BSSL, LPL and USL activity are not higher in BMJ than in normal milks; therefore, a role for these enzymes in the etiology of BMJ is unlikely. (Supported by NIH grant HO 20833.)

Bile salt-stimulated lipase activity in human colostrum from mothers of infants of different gestational age and birthweight.

The bile salt-stimulated lipase (BSSL) activity was measured in milk collected at 3-4 days postpartum (colostrum) from 36 mothers divided into three groups according to gestational age and birthweight of their infants. BSSL activity changed with the length of gestation. Preterm colostrum presented a mean activity significantly higher than the term groups (small-for-gestational age and appropriate-for-gestational age) which had similar values. The ratio of BSSL activity to the estimated fat content was 6.33 in colostrum of mothers who delivered preterm and 4.20 in colostrum of both groups of term mothers. These data suggest that preterm colostrum has a higher fat digesting potential than term colostrum and that it is the gestational age rather than the adequacy or non-adequacy of birthweight to gestational age that may influence the BSSL activity.

Breast milk composition and bile salt-stimulated lipase in well-nourished and under-nourished Nigerian mothers.

Breast milk was analysed in 9 under-nourished Nigerian mothers and 23 well-nourished mothers who served as controls. Milk from the under-nourished mothers contained adequate amounts of lactose and total triglycerides, but had significantly lower bile salt-stimulated lipase activity (BSSL); their mean BSSL activity was only about 50% of the activity in milk from the control group. Total milk protein was also significantly lower than for the controls (1.45 vs. 1.09 g/dl, respectively; P less than 0.01). Our findings may have nutritional implications for breast-fed infants of under-nourished nursing mothers.

Diurnal and within-feed variations in lipase activity and triglyceride content of human milk.

This study examined the diurnal and within-feed variations in the two lipases of human milk, bile-salt-stimulated lipase (BSSL) and lipoprotein lipase (LPL). In addition, all milks were also analyzed for triglyceride content. As compared to BSSL, LPL activity was more variable both among mothers and among different samples from the same mother (largest variation, 0-2.8 U/ml milk). The only significant variation in lipase activity was in LPL activity during a 24-h period (0.96 +/- 0.113, 0.52 +/- 0.21, and 0.57 +/- 0.092 U/ml milk at 0801-1600, 0001-0800, and 1601-2400 h, respectively) (p less than 0.05). Triglyceride content showed no significant diurnal variation but did increase significantly during a single feeding (31 +/- 9.4, 52 +/- 11.7, and 72 +/- 14.8, mumoles triglyceride/ml milk for fore-, mid-, and hind-milks, respectively) (p less than 0.05). There was no correlation between either LPL or BSSL activity and triglyceride content of the milks. Our data indicate that milk obtained for banking or research purposes will contain similar levels of BSSL activity irrespective of diurnal or within-feed timing of milk collections.