Recovery of Calcium Phosphate from Ultrafiltration Permeates

Recovery of Calcium Phosphate from Ultrafiltration Permeates

C. F. BROTHERSEN, N. F. OLSON, and T. RICHARDSON Department of Food Science

and Cheese Research Institute University of Wisconsin

Madison 53706

ABSTRACT

Phosphate was precipitated preferen- tially from ultrafiltration permeate of whole milk without affecting the lactose concentration in the permeate. Precipi- tates formed by addition of up to .090 molal calcium in the form of calcium chloride, calcium hydroxide, and calcium oxide resulted in removal of approximately 80% of the phosphate in the permeate. Removal of phosphate with all calcium salts increased as pH was raised to 8 with little increase above pH 8. There was no significant difference among weights of precipitates produced by calcium chloride, calcium hydroxide, or calcium oxide at equivalent pH's.

INTRODUCTION

Under utilization of whey continuously has plagued the cheese industry because increased cheese production with concomitant generation of whey has nullified the expanded usage of whey and whey components. Separation of whey components by uhrafihration, gel per- meation, ion exchange chromatography, electrodialysis, or selective precipitation is being and will be used to a greater extent to produce high protein products for food ingredients (7, 8, 12, 13, 14, 17, 18). These processes yield a by-product that is usually a dilute solution of lactose and milk salts. This by-product stream has been used commercially to produce lactose and as a substrate for alcohol production (8, 22). Laboratory and pilot-plant studies have suggested a variety of outlets such as hydrolyzed-lactose syrup, fermentation medium for various bacteria and yeasts, and as an ingredient

Received April 16, 1981.

in foods and feeds (8, 10, 15). A limited number of uses as an industrial chemical have been proposed also (4, 21).

It has been suggested that fractionation of whey and milk by ultrafiltration is likely to increase in the future which will generate large volumes of uhrafiltration (UF) permeate. Con- centrating this dilute solution by traditional vacuum evaporation will become less attractive as thermal energy costs increase. Alternative low-energy methods to recover the solids from UF permeate are being considered. One poten- tial technique for lactose removal is the Steffen process which has been tested for effectiveness in precipitating lactose from aqueous solutions (3, 16, 20). The Steffen process is used in the beet-sugar industry to remove sucrose from molasses by precipitating the sugar with a calcium salt, usually calcium oxide (11). How- ever, applying this process to milk products is more difficult since calcium will also precipitate lipids (2), proteins (3, 9), and phosphates, in addition to lactose. To use this process effec- tively for removing lactose from UF permeates or deproteinized whey, the calcium-lactose complex must be obtained in a relatively pure form free from other calcium complexes.

Several successful techniques have been developed for removal of proteins and lipids from milk or whey systems (2, 12, 13, 14, 17, 18). Phosphates can be removed by electrodialysis or ion exchange before lactose precipitation or following precipitation when lactose is resolubilized (6, 10, 19). However, electrodialysis is becoming tess favorable with increasing energy costs, and ion exchange has economic limitations because of cost of regen- eration. Also, neither of these methods solves the problem of final disposal of phosphates.

Precipitation of phosphates with calcium may circumvent the above problems and be economically advantageous because calcium phosphate could be used as a feed supplement.

1982 J Dairy 8ci 65:17-23 17

18 BROTHERSENETAL.

Calcium phosphate prepared from milk products should be relatively free of fluoride which may be important in some sections of the United States. Residual calcium in UF permeates or whey after removing calcium phosphate can be utilized subsequently in the Steffen process to remove lactose. This study was undertaken to determine conditions neces- saD/ to remove phosphorus from UF permeates with calcium as a precipitating agent without causing precipitation of lactose.

MATERIALS AND METHODS

All chemicals were reagent grade. Permeate was obtained from ultrafihration of whole milk at 43°C with a Romicon pilot-plant unit equipped with PM 50 membranes and operating at a back pressure of 100 kPa. Lactose was measured by polarimetry (24), calcium was determined by titration (1), and phosphorus was assayed by the molybdate/ant imony colori- metric method with the Technicon Autoanaly- zer II (23). All analyses were in triplicate, with averages reported.

Three calcium salts, CaC12, Ca(OH)2, and CaO, were tested for efficiency of precipitating phosphate. Each salt was added to 3,000 g of permeate to increase the calcium concentration

14- ~z7 ,2-

0.09

o~,~, ~ 0 " ~ \ / ~

(/ ~0 o

Figure 1. Effect of pH and calcium addition on the mass of precipitate produced by Ca(OH) 2 added to 3,000 g permeate at 20°C.

of the permeate by .015, .040, .065, or .090 molal. The pH of permeates was adjusted to 6, 7, 8, 9, or 10 with HCI or KOH. Mixtures were allowed to stand for 30 min before precipitates were removed by continuous centrifugation at 20,200 × g and a flow rate of 50 ml/min through a Sorvall SS 3 centrifuge equipped with a Sorvall KS3 continuous flow system. The supernatant liquids were analyzed for lactose, calcium, and phosphorus. Precipitates were weighed after drying overnight in a forced air oven at 110°C. Percentage recoveries of lactose, phosphorus, and calcium were calculated from amounts in precipitates found by difference

TABLE 1. Grams of precipitate recovered from 3,000 g of UF permeate at 20°C.

Ca added (molal) Salt pH .015 .040 .065 .090

Ca(OH) 2

CaCla

CaO

(g) 6 .43 4.18 5.66 7.31 7 5.45 8.22 10.43 11.19 8 6.40 10.17 12.52 13.10 9 7.28 10.15 12.78 13.82

10 7.00 10.57 12.73 13.39

6 0 3.26 4.48 6.62 7 6.17 9.10 10.89 9.45 8 8.00 10.82 12.10 11,12 9 8.08 11.50 12.45 11.74

10 8.48 11.32 12.63 12.18

6 .26 5.35 7.00 7.60 7 5.82 10.83 10.76 11.93 8 6.60 10.46 12.10 13.00 9 7.48 10.71 12.75 14.16

10 7.37 11.00 13.00 14.60

Journal of Dairy Science Vol. 65, No. 1, 1982

PHOSPHATE RECOVERY FROM ULTRAFILTRATION PERMEATES 19

and by amounts in the original permeate . Dif- ferences be tween amounts in supernatant fluids and the original permeate were assumed to be in precipitates.

The ef fec t o f t empera ture on precipi ta t ion of phosphate f rom UF permeates, w i thou t added calcium, was de termined by adjusting 3,000 g of permeate to pH's of 6, 7, 8, 9, or 10 with HCI or KOH. Solut ions were held for 30 rain at 20 or 70°C before precipitates were separated f rom solutions, dried, and analyzed for lactose, calcium, and phosphorus in super- na tant l iquids as described.

R ESU LTS

Precipitate Mass The mass of precipi tate formed, when cal-

c ium salts were added to UF permeate , increased f rom a min imum at the lowest pH and calcium concent ra t ion to a m a x i m u m at highest pH and calcium concentrat ions. This is illus- t ra ted in Figure 1 for Ca(OH)2 ; con tour plots were similar for data with the o ther two salts. Weights of precipi tates with each exper imenta l condi t ion are in Table 1. Precipi tate fo rmat ion reached a broad m a x i m u m that generally en- compassed the two highest concent ra t ions of added calcium, .065 and .09 M, and pH be-

Q

3 &70"C .,~ • 2ff'C

¢Y

0 6 7 8 9 10

pH

Figure 2. Effect of pH of permeate on mass of precipitate recovered from 3,000 g of permeate at 20 and 70°C with no added calcium.

tween 8 and 10 (Table 1). Some except ions can be noted in the data for each salt, but these do not negate the general trends. Greatest amounts of precipi tate were obta ined with CaO.

When no calcium was added, measurable precipi ta t ion did no t occur below pH 8 at 20°C. The amoun t of precipi ta te recovered increased over the entire pH range when the tempera ture was raised to 70°C as in Figure 2, but recoveries were only 12 to 15% of those with calcium salts. Tests were not made above

TABLE 2. Percentage of calcium recovered from UF permeate at 20°C.

Ca added (molal) Salt pH ,015 .040 .065 .090

Ca(OH) 2

CaCI 2

CaO

(%)

6 .8 15.7 12.6 12,3 7 48.2 36.6 26.7 19.2 8 60.4 41.2 28.6 21.0 9 69.0 42.1 28.8 22.0

10 70.7 43.9 30.4 21.8

6 .2 12.5 12.2 9.5 7 47.9 35.5 25.0 17.0 8 63.8 42.5 29.0 18.0 9 70.6 45.2 30.2 18.9

10 73.6 45.3 31.2 19.9

6 6.8 22.8 20.5 16.7 7 54.9 45.9 30.2 23.0 8 63.3 45.5 32.6 26.6 9 68.0 44.3 32.7 24.6

10 71.5 46.5 34.9 28.8


20 BROTHERSEN ET A L

70-

Figure 3. Effect of pH and amount of Ca(OH) 2 added on removal of calcium from UF permeate by precipitation at 20°C.

pH 10 since degradation of lactose can occur at this pH at higher temperatures (5).

Calcium Recovery

The amount of calcium removed was affected by pH and amount of calcium salts

added. Data in Table 2 and Figure 3 must be interpreted carefully since varying amounts of calcium salts were added which would influence apparent extensiveness of removal of calcium from the UF permeate. Trends illustrated in Figure 3 for Ca(OH)2 were similar to those when CaCI2 or CaO was used as in Table 2. Increasing the pH of permeate caused a greater increase in percentage removal of calcium in samples to which .015 M calcium salts were added as compared to samples which were fortified with greater amounts of these salts. This was expected because increasing the pH caused precipitat ion of calcium phosphate, and there was less excess of calcium when calcium salts were added at .015 M as compared to higher concentrations. The low recovery when calcium salts were added at .015 M at pH 6 reflected the lack of precipitate formation. When no calcium was added (Figure 4), the trend of increasing calcium removal with increasing pH was also evident. More calcium was removed at 70°C than at 20°C, but the differ-

8 0 • 70 C ~ ence decreased as pH approached 10.

Phosphorus Recovery

70 Recovery of phosphate in the precipitate followed the same general trend as changes in weight of the precipitate; more phosphorus

60 precipitated with increasing pH and calcium w concentration. The percentage of phosphorus

removed from permeates by adding Ca(OH)2 is in Figure 5. Phosphorus recovery was similar when CaC12 and CaO were added as in Table 3. The percentage of recovery of phosphorus was

tw5 40 not affected significant!y by concentration of ~_~ calcium added at pH s above 8. However,

3 0

<o 2 0 ~ - ~ - -

lO ;o ° 20

V - - - - - 0 ' ' , , L I t o, ,_ ~ 0 " ~ r ~ •

p H . ~ eo " 6

Figure 4. Effect of pH of permeate on percentage of calcium removed from UF permeate by precipitation at 20 and 70°C.

Figure 5. Effect of pH and calcium addition on removal of phosphorus from UF permeate at 20°C following addition of Ca(OH) 2 .


21

Ca added (molal)

(%)

Salt p H .015 .040 .065 .090

Ca(OH) 2

CaCI 2

CaO

70

6o

5o

tY

o 3o

2o

10

6 12.0 44.2 48.5 55.9 7 53.7 73.1 76.1 76.1 8 64.2 77.2 77.1 77.2 9 70.3 77.5 76.5 77.6

I0 71.6 77.7 77,4 77.3

6 .1 47.9 53,9 61.3 7 60.9 74.6 79,4 73.6 8 70.7 82.1 81.0 77.0 9 74.7 83.8 83,8 78.3

10 75.9 83.9 83,4 76.4

6 6.5 27.0 57.5 55.5 7 53.9 75.6 77.5 74.7 8 63.4 77.2 78,8 74.8 9 69.9 69.4 77,8 75.2

10 68.9 86.7 79.6 77.1

recoveries t e n d e d to be lower at t he lowes t cal- c ium, .O15 M (Table 3). The pe rcen tage of p h o s p h o r u s r emoved increased wi th increasing pH when ca lc ium was n o t added to the per- m e a t e (Figure 6). P h o s p h o r u s removal was m o r e e f f ic ien t at 70°C t h a n at 20°C, h u t the e x t e n t of removal was m u c h less t h a n samples wi th added calc ium.

Lactose Recovery

Percentages of original lac tose r ema in ing in the s u p e r n a t a n t l iquid a f t e r removal o f precipi- t a tes are in Table 4, Lactose r e t e n t i o n in super- n a t a n t l iquid was grea ter t h a n 93% for all t reat- m e n t s wi th no s igni f icant effects f r o m t y p e of salts, pH, or added calc ium.

When ca lc ium was no t added, lac tose was n o t p r ec ip i t a t ed be low pH 8. T he lactose re- ma in ing in s u p e r n a t a n t l iquids at 20°C ranged b e t w e e n 96 to 98% w h e n the pH was increased to 9 and 10. The pe rcen tage remain ing at 70°C was 97% at pH 9 and 88% at pH 10. The lower recovery wi th increas ing pH at 70°C t h a n at 20°C could have been caused by alkal ine de- g rada t ion o f lac tose (5).

• 70°C • 20°C

PHOSPHATE RECOVERY FROM ULTRAFILTRATION PERMEATES

TABLE 3. Percentage of phosphorus removed from UF permeate at 20°C.

0 I I I 6 7 8 - 9 10

pH Figure 6. Effect of pH on removal of phosphorus

from UF permeate by precipitation at 20 and 70°C.


22 BROTHERSEN ET AL.

TABLE 4. Percentage of original lactose remaining in the supernatant liquid after removal of the precipitates from UF permeate at 20°C.

Calcium added (molal) Salt pH .015 .040 .065 .090

Ca(OH) 2

CaCI 2

CaO

(%)

6 100.9 100.6 101.6 101.3 7 101.1 100.2 99.6 100.1 8 101.3 100.3 100.3 100.3 9 100.1 100.0 100.9 !00.4

10 99.9 99.7 98.5 99.2

6 100.0 101.3 101.3 101.3 7 98.5 98.7 98.7 100.6 8 98.6 98.4 98.4 100.4 9 98.8 100.9 100.9 100.3

10 97.9 97.9 97.9 99.3

6 100.2 99.8 98.7 96.5 7 99.9 99.0 95.8 89.9 8 99.8 99.6 100.1 95.9 9 98.7 99.6 93.3 99.5

10 93.0 98.8 94.8 96.3

DISCUSSION

Precipitation of calcium phosphate from UF permeates of whey or milk appears to be a feasible pretreatment before application of the Steffen process. The precipitate can be centri- fuged easily and may be removed by settling if small volumes of permeate are to be treated. The extensive removal of phosphate should yield a relatively pure lactose solution for sub- sequent precipitation. The final effluent stream would require minimum treatment to remove residual phosphorus and lower the biological oxygen demand (BOD).

Concentrations of added calcium required for precipitation of phosphate are lower than those used for precipitating sugars in the Steffen process. Consequently, calcium left in the supernatant liquid after precipitation of calcium phosphate can be used, with supple- mentation, for precipitating lactose. At the highest calcium concentration in this study, .09 M, approximately 4.0 g of calcium should remain in 1 liter of supernatant liquid. About 50 g of lactose would be in this liquid so the calcium:lactose molar ratio is .67. Preliminary experiments indicate that the minimum molar ratio of calcium to lactose to precipitate lactose is 3.0. Consequently, at least 13.5 g calcium

would have to be added to each liter of phosphate-depleted permeate to recover lactose.

Data were based upon precipitation of milk salts from UF permeate of milk. Similar trends would be anticipated with whey permeates except for some shifts in profiles of the data. The pH of the sweet whey permeate would be 5.8 to 5.9 and that of acid whey permeate would be 4.4 to 4.6. The lower pH's are advantageous since CaO or Ca(OH)2 added at .065 molal increased the pH of milk permeate from 6.5 to 12 which is higher than desirable. Lower pH's would be anticipated with the whey permeates as compared to milk permeate at the same molality of CaO or Ca(OH)2 unless pH was adjusted as in this study.

Commercial application of the process might involve adding a combination of CaO or Ca(OH)2 with CaCI2 equal to .065 molal which will produce a pH of 9 in the permeate. This should result in maximum precipitation and phosphorus removal without altering the lactose concentration.

ACKNOWLEDGMENTS

This research was supported by the College of Agricultural and Life Sciences and by grants from Dairy Research Incorporated, Rosemont,


PHOSPHATE RECOVERY FROM ULTRAFILTRATION PERMEATES 23

IL, USDA-SEA Hatch Formula Funds, and the Graduate School , Universi ty of Wisconsin. UF uni t use cour tesy of C. H. A m u n d s o n .

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Recovery of Calcium Phosphate from Ultrafiltration Permeates