Food process engineering1

Microfiltration and Thermal Treatment for Extending the Shelf Life of Milk:

Factors affecting product quality and stability

Veronika Kaufmann1, Verena Schmidt2, Siegfried Scherer2 & Ulrich Kulozik1

1Chair for food process engineering and dairy technology; 2 Chair for microbial ecology

Food process engineering2

Factors affecting quality and shelf life of milk

ESL technology

= use of synergistic factorsfor efficient debacterization by

minimal processing impact on product properties

Thermal processing

High heattreatment

- 125-127°C → 2-4 s- 135°C → 0,5 s

directindirect

MicrofiltrationBactofugation

Non-thermal processing

Cross-flow

UTP-Concept

Gradientmembrane

Depthfiltration+

- Cream (and Retentate): 115-125°C → 3-4 s- Low bacteria milk: 72-75°C → 15-30 s

Thermal treatment

- Raw milk quality

- Process design and hygiene

- Filling and packaging

- Storage and distribution conditions

Food process engineering3

Conflicting targets of ESL-MF

Particle size [µm]0,01 0,1 1 10

Vol

ume

[%]

0

10

20

30

40

50

Casein(micelles)

Bacteria

Fat globules

Classicsterile

filtration

CurrentESL-MF

→ Objectives: efficient and selective debacterization of milk

Food process engineering4

Permeation of whey protein depending on pore diameter

Average pore diameter [µm]

Per

mea

tion

[%]

0

20

40

60

80

100a-lactalbumin xxxß-lactoglobulin

0,1 0,16 0,2 0,8 1,4

α-lactalbuminβ-lactoglobulin

MF:Skim milk ϑ = 50°C

100%)(CC

Permeationretentate

permeate ⋅=

Food process engineering5

Factors affecting filtration characteristics

Microorganisms/Particles

- Quantity- Size- Shape

Process- Temperature ϑ- Transmembrane pressure ΔpTM- Wall shear stress τw- Time t

Membrane- Material and surface structure- Effective filter area- Pore size and -distribution

Product- PH/IEP/ionic strength- Viscosity- Surface activity - Chem./physic. characteristics

Food process engineering6

Schematic diagramm of (UTP-)MF pilot plant

Plate heatexchanger

Tube heatexchangerMF (1,4 µm)

Feed tank

Plate heatexchanger

Tube heatexchangerMF (1,4 µm)

Feed tank

→ Operating conditions for ESL-MF: ΔpTM=0,2 barτW=150 Paϑ=55°C

Food process engineering7

Time [hours]

0 2 4 6 8 10 12

Per

mea

te fl

ux [l

m-2

h-1]

0

200

400

600

800

ϑ=55°CΔpTM=0,2 barτw=150 Pa

ϑ=10°CΔpTM=0,2 barτw=150 Pa

ϑ=55°CΔpTM=0,2 barτw=40 Pa

ϑ=55°CΔpTM=0,1 barτw=150 Pa

MF (1,4 µm):Skim milk

Influence of operating conditions on permeate fluxduring long-term filtrations

Food process engineering8

Permeation of protein and bacterial reductionduring long-term filtrations

Time [hours]

0 2 4 6 8 10 12

Pro

tein

per

mea

tion

[%]

98,0

98,5

99,0

99,5

100,0

log (N/N

0 )

0

1

2

3

4

0milkraw

permeate

NN log

NN

logreductiondecimalBacterial ==

MF (1,4 µm):Skim milk ϑ=55°CΔ pTM=0,2 barτw=150 Pa

100%)(CC

Permeationretentate

permeate ⋅=

Food process engineering9

Reduction of several microorganisms by MF* (I)

log (N/N0) [-]

- coccus:Ø = 1 - 3,5 µm

- gram-positive4,06 ± 0,26Micrococcus luteus

MorphologyGenus

- rods:1,5 - 3,0 x 0,5 - 1 µm

- gram-negative4,73 ± 0,18Citrobacter freundii

- rods:1,2 - 10 x 0,5 - 2 µm

- gram-positive4,37 ± 0,09Bacillus cereus

3,66 ± 0,23

- rods:1 - 2,2 x 0,6 - 0,8 µm

- gram-negativePseudoxanthomonas mex.

*MF (1,4 µm): Skim milk; ϑ=55°C; ΔpTM=0,2 bar; τw=150 Pa

Cell size

Food process engineering10

0,85 1,85 2,85 3,85

log

(N/N

0) [-

]

0

1

2

3

4

5

Reduction of several microorganisms by MF (II)

Bacilluscer.

Citro-bacter fr.

Pseudo-xanthomo-nas mex.

Micro-coccus lut.

MF (1,4 µm):Skim milkϑ=55°CΔpTM=0,2 barτW=150 Pa

5

4

3

2

1

log (N/N

heatedto 55°C ) [-]

0

gram-negative gram-positive

Cell size

→ Debacterization by MF = mechanical removal and thermal inactivation

Food process engineering11

...

References

Reference-databaseSpectrum

Unknown isolate

Comparison

Identification of microorganisms by FTIR-Spectroscopy

Identification

Food process engineering12

Comparison of flora compostion in skim, heated and MF-milk

3

3

20

21

10

43

Skim milkTBC=7,3*104 cfu/ml

other gram-positives

Bacillus ssp.

Coryneformes

Lactic acid bacteria

other gram-negatives

Pseudomonas ssp.

Genus

815

210

410

113

524

MF-milk*TBC=43 cfu/ml

Milk heated to 55°CTBC=3,1*104 cfu/ml

28

Quantity [%]

80

*MF (1,4 µm): ϑ=55°C; ΔpTM=0,2 bar; τw =150 Pa

Food process engineering13

Days of storage

0 5 10 15 20 25

Tota

l bac

teria

l cou

nt [c

fu/m

l]

1e+0

1e+1

1e+2

1e+3

1e+4

1e+5

1e+6

1e+7

Shelf life limit

Microbial growth during storage of MF- vs. MF+PAST-milk

→ MF decisive process step for debacterization of milk

104

105

106

107

103

102

101

0

MF

MF+PAST

stored at 10°CMF (1,4 µm)+ PAST (7373°°C/25 s)

→ Combination of MF+PAST essential for product safety and shelf life extension

rawskimmilk

off-flavour

Food process engineering14

Quantity [%]

Genus

10

30

58

2

-

-

TBC=18 cfu/ml

Day 0

5

12

83

-

-

-

TBC=80 cfu/ml

Day 7

other gram-positives

Bacillus ssp.

Coryneformes

Lactic acid bacteria

other gram-negatives

Pseudomonas ssp.

167

--

Day 21Day 14

7840

TBC=7·103 cfu/mlTBC=1,7·10² cfu/ml

53

-

-

6

-

-

Flora composition of MF+PAST*-milk during storage at 10°C

*MF (1,4 µm) + PAST (7373°°C/25 s)

Food process engineering15

Heat resistance of bacteria, spores and enzymes

12110,411Mesophilic spores (milk)

at ϑ [°C]z [°C]D [s]Species

1217,3 - 10,72,3Bacillus cereus (skim milk)

12110,425Thermophilic spores (milk)

79,36,90,50Enterobacter liquefaciens

Pseudomonas pseudomallei 0,52 9,3 73,3

Kessler, 1996

Lipase from15025100Pseudomonas

Pseudomonas flourescens

EnzymeProteinase from

660 34,5 130

Food process engineering16

Lipolysis and proteolysis during storage of MF+PAST-milk at 10°C

Days of storage

0 5 10 15 20 25 30

Free

fatty

aci

ds [µ

equi

v/m

l]

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

1,5

1,0

0,5

equiv. Tyr-Leu [µmol/m

l]

0,0

2,0

2,5

3,0

3,5→

Met

hod

byD

eeth

→Flourescam

in-Method

MF (1,4 µm)+ PAST (7373°°C/25 s)stored at 10°C

Food process engineering17

Days of storage

0 5 10 15 20 25 30

equi

v. T

yr-L

eu [µ

mol

/ml]

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

Influence of raw milk quality on proteolysis of MF+PAST-milk

off-flavour

MF (1,4 µm)+ PAST (7373°°C/25 s)stored at 10°C

c0=2,0 µmol/ml

c0 = 1,6 µmol/ml

Food process engineering18

Perspectives: Variation of process steps

Current processing: Alternative processing:

Raw milk

Cream

Retentate

Skim milk

Permeate

ESL milk

PasteurisationHigh HeatTreatment

Separation

Microfiltration

Raw milk

Cream

Retentate

Skim milk

Permeate

ESL milk

Pasteurisation

Separation

MicrofiltrationMicrofiltration

High HeatTreatment

Food process engineering19

Conclusions

- ESL-MF:

- Bacterial (decimal) reduction depends on morphology of microorganisms

- Reduction of bacteria = mechanical removal and thermal inactivation

- Decisive process step for debacterization of milk

- Combination of MF and thermal treatment:

- Ensures product safety and shelf life extension of milk

- Shelf life of MF+PAST-milk can be limited by enzymatical reactions

and depends on (microbial/enzymatical) raw milk quality

Acknowledgement:AiF/FEI → research project FV 15047 N