Vol48, No 1 February 1995 Journal of the Society of Dairy Technology Nanofiltration of whey: quality, environmental and economic aspects JAMES KELLY and PHILIP KELLY National Dairy Products Research Centre, Moorepark, Fermoy, Co Cork, Ireland Flux rate performance for both rennet and cheddar wheys were similar to those described for acid casein whey beginning at 37-41 1 mP2 h-' and declining to 10 I mP2 h-I at volume concentration ratio of 4. The chloride in dry matter reduction for these wheys was much greater at 71% compared to -41 % for acid casein whey. Losses of organic solids from acid casein whey in terms of chemical oxygen demand were similar to those published for cellulose acetate membranes. Lactose and true protein nitrogen (total protein nitrogen less non-protein nitrogen; NPN) losses amounted to 2.6% and 8.1 YO respectively. NPN constituted the main nitrogen loss (77%) through the HC-50 membrane. True protein loss increased as p H was lowered to 3.6. Solubility index values obtained for nanojiltered powders produced from acid casein whey were comparable with those obtained for conventional spray dried powders and whey protein concentrates. In a case study based on the performance of the HC-50 membrane the economic feasibility of nanofiltration along with other demineralization processes was assessed. INTRODUCTION Nanofiltration (NF) is widely used to describe the pressure driven separation of electrolytes. Although the principal application is for separation of mineral ions within the hD (100 x D) or dD (10 x D) molecular range, the term nanofiltration reflects the corresponding nanometer m) size exclusion range. Ultrafiltration, on the other hand, is based on the concept of molecular exclusion, in which the retained particles have molecular dimen- sions in the order of kD, ie, mainly proteins, compared to the solution diffusion mechanism favoured for description of the ideal reverse osmosis (RO) process. In a previous paper (Kelly and Kelly, 1995) the application of an NF membrane for desalination of acid casein whey was considered with particular attention being paid to chloride permeation. It is now proposed to explore the application of NF in the wider context of whey processing by examining organic losses in acid whey per- meates, examining performance on other types of whey, monitoring quality aspects of spray dried NF whey concentrates from acid casein whey and carrying out an economic appraisal of its usefulness alongside other demineralization processes such as electrodialysis. MATERIALS AND METHODS Plant configuration An APV Pasilac (APV Pasilac AS, DK 8600, Silkeborg, Denmark) NF plant was used in all experiments. The plate and frame unit featured polymeric membranes (type HC-50) with a specific porosity (-66% at 30 bar operating pressure) to a standard NaCl solu- tion. The membrane surface areas totalled 4.5 m2 and the plant was configured in a batch Original paper. recirculation mode. The hydraulic circuit comprised a centrifugal feed pump to give a circulation flow of 1500 1 h-' and a positive piston pump to deliver greater than 30 bar operating pressure. A prefilter (1000 pm mesh) and a tubular heat exchanger were also included. Processing temperature was main- tained by circulating water through the jacket of the balance tank via the tubular heat exchanger. Experimental trials were carried out in duplicate. Diafiltration Diafiltration was carried out by introducing deionized water to the retentate after concen- trating by NF to a volume concentration ratio (VCR) of either 1 or 2. The quantity of water added varied occasionally but in most cases amounted to 50%, ie, an amount equal to 50% of the whey volume processed. Non-protein nitrogen (NPN) After precipitation of protein using 12% trichloroacetic acid (TCA) (ie, 20 g of sample + 80 g of 12% TCA) Kjeldahl nitrogen (IDF standard 20A: 1986) was determined on the TCA filtrates from various wheys. Lactose Samples for lactose determination were first deproteinized as described for NPN. A Boehringer enzymatic food test kit (Cat. No. 176393) was used for the determination of lactose in the TCA filtrates. Chemical oxygen demand (COD) Following dilution the COD of whey and permeates was determined by the rapid 15 min method developed by Palmer (1981). 20
Transcript
Vol48, No 1 February 1995 Journal of the Society of Dairy Technology
Nanofiltration of whey: quality, environmental and economic aspects
JAMES KELLY and PHILIP KELLY National Dairy Products Research Centre, Moorepark, Fermoy, Co Cork, Ireland
Flux rate performance fo r both rennet and cheddar wheys were similar to those described fo r acid casein whey beginning at 37-41 1 mP2 h-' and declining to 10 I mP2 h-I at volume concentration ratio of 4. The chloride in dry matter reduction f o r these wheys was much greater at 71% compared to -41 % f o r acid casein whey. Losses of organic solids f r o m acid casein whey in terms of chemical oxygen demand were similar to those published fo r cellulose acetate membranes. Lactose and true protein nitrogen (total protein nitrogen less non-protein nitrogen; NPN) losses amounted to 2.6% and 8.1 YO respectively. NPN constituted the main nitrogen loss (77%) through the HC-50 membrane. True protein loss increased as p H was lowered to 3.6. Solubility index values obtained fo r nanojiltered powders produced f rom acid casein whey were comparable with those obtained fo r conventional spray dried powders and whey protein concentrates. In a case study based on the performance of the HC-50 membrane the economic feasibility of nanofiltration along with other demineralization processes was assessed.
INTRODUCTION Nanofiltration (NF) is widely used to describe the pressure driven separation of electrolytes. Although the principal application is for separation of mineral ions within the hD (100 x D) or dD (10 x D) molecular range, the term nanofiltration reflects the corresponding nanometer m) size exclusion range. Ultrafiltration, on the other hand, is based on the concept of molecular exclusion, in which the retained particles have molecular dimen- sions in the order of kD, ie, mainly proteins, compared to the solution diffusion mechanism favoured for description of the ideal reverse osmosis (RO) process. In a previous paper (Kelly and Kelly, 1995) the application of an NF membrane for desalination of acid casein whey was considered with particular attention being paid to chloride permeation. It is now proposed to explore the application of NF in the wider context of whey processing by examining organic losses in acid whey per- meates, examining performance on other types of whey, monitoring quality aspects of spray dried NF whey concentrates from acid casein whey and carrying out an economic appraisal of its usefulness alongside other demineralization processes such as electrodialysis.
MATERIALS AND METHODS
Plant configuration An APV Pasilac (APV Pasilac AS, DK 8600, Silkeborg, Denmark) NF plant was used in all experiments. The plate and frame unit featured polymeric membranes (type HC-50) with a specific porosity (-66% at 30 bar operating pressure) to a standard NaCl solu- tion. The membrane surface areas totalled 4.5 m2 and the plant was configured in a batch Original paper.
recirculation mode. The hydraulic circuit comprised a centrifugal feed pump to give a circulation flow of 1500 1 h-' and a positive piston pump to deliver greater than 30 bar operating pressure. A prefilter (1000 pm mesh) and a tubular heat exchanger were also included. Processing temperature was main- tained by circulating water through the jacket of the balance tank via the tubular heat exchanger. Experimental trials were carried out in duplicate.
Diafiltration Diafiltration was carried out by introducing deionized water to the retentate after concen- trating by NF to a volume concentration ratio (VCR) of either 1 or 2. The quantity of water added varied occasionally but in most cases amounted to 50%, ie, an amount equal to 50% of the whey volume processed.
Non-protein nitrogen (NPN) After precipitation of protein using 12% trichloroacetic acid (TCA) (ie, 20 g of sample + 80 g of 12% TCA) Kjeldahl nitrogen (IDF standard 20A: 1986) was determined on the TCA filtrates from various wheys.
Lactose Samples for lactose determination were first deproteinized as described for NPN. A Boehringer enzymatic food test kit (Cat. No. 176393) was used for the determination of lactose in the TCA filtrates.
Chemical oxygen demand (COD) Following dilution the COD of whey and permeates was determined by the rapid 15 min method developed by Palmer (1981).
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Vol48, No I February 1995 Journal of the Society of Dairy Technology
Solubility index Nanofiltered retentates were spray dried and the solubility of the powder after reconstitu- tion with water was determined as an indica- tion of the degree of protein denaturation that may have taken place during the drying process. Solubility index was determined according to the American Dried Milk Insti- tute method (1971).
Powder production After nanofiltering acid whey to VCR4 reten- tates were further concentrated (to ca 40% total solids) before spray drying in a single effect falling film evaporator (Anhydro F1 Lab). Retentates were dehydrated in a pilot scale Anhydro spray drier (Model Lab 3) at an inlet temperature range of 20CL220"C and outlet of 90-100°C. Control powders were produced by evaporating raw acid whey to ca 40% total solids and spray drying under similar conditions.
Gel permeation chromatography (GPC) Whey proteins permeating the NF membrane were identified by GPC using a fast protein liquid chromatograph and a superose 12 gel permeation column (Pharmacia HW30) according to the methods of Andrews et a1 (1985).
Permeate samples (100 p1) were applied undiluted to the column while the feed and retentate samples were diluted using an elu- tion buffer in order to maintain the chromato- grams on scale; the elution buffer was 0.1 m Tris-HC1, pH 7.0. Sodium chloride (0.1 M) was added to prevent ionic interactions occur- ring on the column and sodium azide (10 mM) was added as an antimicrobial agent. The absorbance (280 nm) of the eluate was mea- sured using a single path monitor (UV-1 Pharmacia) and the absorbance profiles were automatically recorded. A calibration curve (loglo molecular weight versus retention volume) was prepared by applying standard proteins and peptides to the column. The calibration curve was used to estimate the molecular weights of unknown peaks in the chromatograms.
RESULTS AND DISCUSSION
Processing cheddar cheese and rennet casein wheys The flux rate pattern for both rennet casein and cheddar cheese wheys was similar to that already described for acid casein, ie starting from an initial flux rate of 3 7 4 1 m-* h-' and declining radually to a final rate of just under 10-12 1 m .h at VCR4. A 33-35% reduction in ash in dry matter was recorded during 4 times concentration of the acid casein and cheddar wheys (Table 1). In the case of rennet casein whey the initial reduction after 4 times NF concentration was only 25%, but there was a greater response during the diafiltration step
8
to give an overall reduction in ash of 40.5%, which was comparable to that obtained with the other wheys following diafiltration.
For example, by introducing diafiltration after 2 times concentration it is possible to achieve a reduction in ash content corres- ponding to 86% of that obtained after uninter- rupted Concentration to higher values (VCR4) without diafiltration of acid casein whey. The loss of protein in dry matter (8.3%) after concentration was of the same order of magnitude as that for acid casein whey.
Chloride: permeation Chloride permeation from rennet casein and cheddar wheys was much greater than for acid casein whey where it was only possible to achieve a reduction of 41.1% after concen- tration (Table 1). Over 70% of the chloride content of the other wheys could be removed under the same conditions.
NF of preconcentrated whey The effect of preconcentrating acid casein whey b y a factor of 2 : l using thermal evaporalion on the performance of NF was examined. Plant performance data were limited as the maximum NF concentration achievable was only VCR2. The higher total solids (1'2.7%) of the whey at start-up greatly influenced flux rates. The initial flux was 21.1 1 mP2 h- l , and this declined sharply to 4.4 1 m-2 h- ' at VCR2 concentration. The total protein loss after VCR2 was 2.25% (a reduc- tion from 10.29 to 10.06% in dry matter), which compares with 6.0% for unconcen- trated wlhey after a similar volume reduction. A reduction of 25.8% ash in dry matter at VCR2 compared favourably with a value of 32.8% on processing unconcentrated whey to higher volume reduction levels (VCR4). Given the lower flux rates, this level of ash removal was significant and could possibly be improved upon by diafiltration.
Permeation of organic solids Lactose ,and NPN losses in the permeates are illustrated in Fig. 1. The dominant feature is the dramatic increase in the lactose permea- tion from 0.068 to 0.15% at the higher
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Vol48, N o 1 February 1995 Journal of the Society of Dairy Technology
O.* 1 I
- 0
m m
C 0
0 0.1 P
0
0
- 2
c m
E n
1 2 3 4
Volume concentration ratio (VCR)
Fig. 1. Percentage of lactose and non-protein nitrogen (NPN) in permeates from the nanofiltration of acid whey.
GPC. Peak No. 1, representing large aggre- gates such as fat and residual casein fines, is only evident in the whey feed sample (Fig. 2). As expected, all permeates were clear of suspended matter, which is retained by the membrane. Similarly, immunoglobulin G and bovine serum albumin are retained (R, =
There is evidence of a lactalbumin and 8 lactoglobulin permeating the membrane in trace amounts (Fig. 2). It may be concluded, therefore, from these data that the nitrogen- ous constituents permeating the membrane are mainly low molecular weight compounds (MW < 1200 daltons). The GPC technique was unable to identify the individual compo- nents which contributed to increased nitrogen losses in the permeate (as measured by Kjeldahl) during processing of the whey at a lower pH value of 3.6. GPC chromatograms of permeates produced at pH 3.6 and 4.6 were distinguishable by higher peak areas and elution volumes at the lower pH value with no additional peaks being detected. Retention coefficients of constituents less than 7000 daltons are only slightly influenced by pH. Mean retention coefficient values for nitrogenous material of 6000-7000 daltons are 0.46 at pH 4.6 and 0.62 at pH 3.6. A similar trend is true for compounds of less than 1200 daltons.
1.0).
concentration factor (VCR4). The total lac- tose loss amounted to 2.67% of lactose in the raw whey. NPN permeated readily through the membrane, resulting in a 39.3% loss of total NPN with respect to the raw whey level. This accounted for 77% of total nitrogen in the permeate. Some true protein nitrogen (Total N less NPN) was also detected in the permeate, equivalent to 8.12% of the raw whey level.
Effect of pH on protein yield Processing acid casein whey at pH values other than at the normal occurring level (typically pH 4.6) had an affect on the yield of protein in dry matter, particularly if the whey was adjusted to more acidic pH values. Losses of protein in dry matter after concentration tQ VCR4 were in the range 8.6-8.9% for whey processed at pH 4.6 and 6.6, but increased to 10.5% when concentrated at pH 3.6. The data provided in Table 2 were compiled from experiments conducted at different times, and the exceptionally high protein in dry matter of acid whey at pH 4.6 (14.8% protein in dry matter) was due to seasonal variation.
Identification of nitrogenous fractions in permeates As nitrogen losses through the membrane were higher at pH 3.6, the protein compo- nents of the permeate were identified using
Environmental considerations The biological loading of permeates produced during NF of acid whey was established in order to predict waste treatment require- ments. The organic losses through the mem- brane were quantified in terms of permeate COD at each volume concentration ratio. COD values increased from 1000 mg/l at VCRl to 2592 mg.1-l at VCR4, resulting in a mean value of 1592 mg.1-l (Fig. 3). These results were similar to those found by Smith and MacBean (1978) using the CA T2/40 membrane on a similar type of whey.
Solubility index of spray dried NF whey concentrates Proximate analysis and solubility index (SI) values of spray dried acid whey powders (control and NF) are shown in Table 3 for normal and pH adjusted concentrates respec- tively. Neutralization of acid whey concen- trates to pH 6.6 before drying increased marginally the SI values of the spray dried control powders from 0.25 to 0.30, while the SI values for the nanofiltered whey powders at pH 4.6 and 6.6 were similar at 0.50 and 0.65 respectively; however, the diafiltered powders differed considerably with SI values of 0.7 and 0.3 for the same pH values. It should be noted that the moisture level in the diafiltered powder at pH 4.6 was 1.83%, which was considerably lower than in the other samples, which had typical values of - 3 4 % moisture..
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Vol48, No I February 1995 Journal of the Society of Dairy Technology
void vol (vo) Aggregates Immmoglobulln BSA B l g a h
Molecular w l w t o w
350,000 150,000 67,000 36,000 14,700
6,000-7,OOo ilJ00
on startding, protein precipitation occurred more readily as a result of decreasing ionic strength. It should be emphasized that the process of neutralizing NF concentrates con- tributes additional ash to the final product, which is an undesirable feature in the produc- tion of acid whey powders adjusted to neutral pH values; however, these SIs compare favourably with results in the range 0.10 to 0.30 f'or commercially produced whey powdens.
NF as an adjunct to other demineralization processes Higher levels of demineralization than those achievable using NF require the use of addi- tional processes such as electrodialysis (ED) andor ion exchange (IE). The selection from among IVF, ED, IE and various combinations thereof depends very much upon the type of whey, irelative cost of local services and especially upon the level of demineralization and plant capacity. With the assistance of an internationally recognized supplier of ED equipment an attempt was made to examine a number of options based on preconcentrating whey by NF before final demineralization to different degrees (62% and 90% demin) by ED.
The case study was based on exploring the additional ED facilities required to perform more extensive demineralization following NF of whey produced from hydrochloric acid precipitation of casein from skim milk. Using
,: The lower SI value (0.30) for the pH 6.6 powder may be due to an increase in ionic strength (which influences solubility) resulting
0 .
from the addition of sodium hydroxide in the observation Of wheys
concentrated by NF at pH 4.6 confirmed that,
Volume concentration ratio (VCR)
Chemical oxygen demand of permeates from the nanofiltration of acid whey.
step- Fig. 3.
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Vo148, N o I February 1995 Journal of the Society of Dairy Technology
Evaporation + drying at pH 6.6 12.75 4.2 13.85 0.30
NFlDF + evap + dry~ng 11.43 3.07 8.82 0.30
a selected capacity of 275 000 kglday acid casein whey, details of membrane area and estimated capital (1991 data), inflated by 12% for E D plant with and without NF as in preconcentration step, are presented in Table 4. The selected demineralization levels of 62% and 90% correspond to demineralized whey powders containing 4.0% and 1.0% ash in dry matter respectively-two of the most common forms in which demineralized whey powder is marketed.
Some guidelines are provided for the econ- omic application of NF in whey processing where further demineralization is required using existing and/or additional E D equipment.
With the present state of knowledge and equipment availability the following obser- vations (Table 4) for NF applicability in whey processing can be made:
0-32% demineralization: NF is the lowest cost operation for a plant that does not intend to process beyond ca 32% deminer- alization limit. The most practical situation would be to upgrade the quality of acid whey to that of sweet whey, while at the same time preconcentrating to approx 20% total solids. Combined processes: In the case of manu- facturing whey powder to a 62% demin- eralization level, the capital costs of a combined NF and E D approach would
exceed that of an E D plant only by almost 20% (Table 4). However, as the deminera- lization specification is increased to 90% the gap between the capital costs of an ED only or NF plus E D is reduced considera- bly to less than a 3% difference. It may then be a matter of deciding on the preferred option on the basis of operating cost savings or other site specific factors. Preconcentration: Since preconcentration of whey to ca 20% total solids is required for ED, NF can compete favourably against evaporation and RO if such an investment decision arises. In those cir- cumstances NF can provide ‘free’ deminer- alization. The transport of whey from an outlying site to a central processing plant can provide a suitable opportunity in which to preconcentrate the whey by NF instead of RO in order to reduce transport costs. Expansion of existing E D plant: Depend- ing on several factors including the suita- bility of the existing E D plant for expan- sion, there is scope for considering NF as an option to increase throughput. In any event, for an E D plant producing 90% demineralized product, the amount of expansion possible by an NF installation is limited to about 20%.
CONCLUSIONS Flux rates for ‘neutral’ (sweet) wheys were similar to those of acid casein wheys. Even though flux rates were significantly lower for preconcentrated wheys ash removal was signi- ficant and could be improved upon by diafil- tration. GPC succeeded in detecting trace quantities of a lactalbumin and fi lactoglobu- lin in the permeates from the HC-50 mem- brane. Nitrogen compounds permeating the membrane were generally of low molecular weight.
Losses of organic solids in terms of COD (-1600 mg.1-l) were similar to those pub- lished for cellulose acetate membranes. Bio- logical treatment of the permeate stream from NF plants is, therefore, necessary. True pro- tein loss increased as pH was lowered to 3.6. Diafiltration had an almost identical effect on protein loss. SI values obtained for spray dried NF powders demonstrated that this physico- chemical property was comparable with that
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Vol48, No 1 February 1995 Journal of the Society of Dairy Technology
Original paper.
obtained for conventional spray dried powders and whey protein concentrations. The authors wish to acknowledge the assis- tance of Mr Bruce Batcheldor, Ionics Tnc, USA, and Mr Niels Ottesen, APV Pasilac AS, Denmark, in the compilation of the data contained in Table 4.
REFERENCES American Dried Milk Institute (1971) Solubility Index
Reference Method. Chicago IL: AMDI.
Andrews A T, Taylor M D and Owen A J (1985) Rapid analysis of bovine milk proteins by fast protein liquid chromatography. Journal of Chromatography 348 177-185.
International Dairy Federation (1986) IDF Standard No. 20A. Brussels: IDF.
Kelly J and Kelly P M (1995) Desalination of acid casein whey by nanofiltration. International Dairy Journal (in press).
Palmer J (1981) Rapid method for the estimation of chemical oxygen demand of dairy wastes. Irish Journal of Food Science 5 149-155.
Smith B R and MacBean R (1978) Fouling in reverse osmosis of whey. Australian Journal of Dairy Technology 5 5742 .
The incidence of Salmonella and Listeria in raw milk from farm bulk tanks in England and Wales
E T O’DONNELL Northern Foods Dairy Group, Holme on Spalding Moor, Near York, YO4 4AN
Raw milks from farm bulk tanks in England and Wales were sampled over a 15 month period in 1992-93 and analysed for Salmonella spp, Listeria spp and Listeria monocytogenes. Of 1673 samples tested for Salmonella spp, six (0.36%) yielded a positive result. A total of 2009 samples were analysed for Listeria spp. Of these, 310 (15.43%) were found positive for Listeria spp, I02 (5.08%) of the positive samples yielded L monocytogenes, which represented 33% of the Listeria isolations. There was a signifcant rise in the isolation rate for listerias between October and March broadly in line with the period during which the cows were housed indoors. The highest isolation rate was in January, 25.89% in 1992 and 28.4% in 1993, and the lowest isolation rate (3.1 %) in August. Although the incidence of L monocytogenes broadly paralleled the trend for total Listeria, the highestpercentage of L monocytogenes to total Listeria was found in the months of April to September when the total Listeria isolations were at a minimum. Thirty two (1.590/,) samples were found to be positive for Listeria spp prior to enrichment procedure, with 17 (0.85%) positive for L monocytogenes. The highest counts reported for L monocytogenes were 62 and 30 colony forming units (cfu)fml with over 60% at a level of less than 10 cfufml. Of the direct isolations, some 78% were obtained between the months of March and July.
INTRODUCTION Ex-farm milk can be a vehicle for pathogens such as Salmonella spp and Listeria monocyto- genes. Despite considerable attention to hygiene in milking operations raw milk will contain micro-organisms of concern, albeit generally at low incidence and in low numbers. To date, surveys of pathogens in raw milk have in the main used small numbers of samples over a limited time scale and geographical area. In addition, little attempt has been made to enumerate pathogens in raw milk.
The objectives of this survey sponsored by the Joint Committee for Milk Quality were to estimate the incidence of Salmonella spp, Listeria spp and L monocytogenes in farm bulk tank milks in England and Wales over a 15 month period. An estimate of numbers of Listeria spp was also made.
MATERIALS AND METHODS
Sampling was carried out monthly on a random basis across England and Wales from farm bulk tanks and the samples transported in insulatedl boxes to the central laboratories of each of the participating companies. The samples were held at 4°C and tested within 48 hours of milking. The sampling programme was carried out over a 15 month period and included seasons when the cows were grazing and when they were confined inside and fed on silage.
The method used for the detection of Salmonella spp is given in Fig. 1 and for L monocytogenes in Fig. 2. The analysis for both micro-organisms was on the basis of presence/absence in 25 ml of raw milk. A numerical estimation was also carried out for Listeria spp and L monocytogenes by direct plating of 0.5 ml aliquots from the enrichment
