PROCESSING OF PASTEURIZED MILK BY
PULSED ELECTRIC FIELD
RAJAN D JAYKAR 11 FET 1015 T Y B Tech FOODS
INSTITUTE OF CHEMICAL TECHNOLOGY , MUMBAI , MAHARASHTRA
WHY PROCESS FOOD ?
1. Extend shelf life
2. Maintain sensory properties
3. Maintain nutritive properties
4. Ensure safety
5. Make more convenient.
6. Economic value
food processing can be classified into 2
- high hydrostatic pressure
- ultra sonication
- PULSED ELECTRIC FIELD
WHY PULSED ELECTRIC FIELD IS NEEDED ?
LET’S FINDTHE ANSWER …… BUT FIRST, LET’S SEE :
Importance ofThermal Processing :
• HTST has been used for decades as method to extend shelf life of milk and to
eradicate the bacteria from it.
• It is able to extend shelf life of milk to around 3 weeks.
• Important in order to transport milk to distant places.
• Under nonflavourable conditions : high initial microbial load, poor processing and
storage conditions, post pasteurization contamination shorten the shelf-life of
milk. Alterations in sensory and nutritional attributes of milk.
TO GET A BETTER IDEA; LET’S SEE ONE EXAMPLE :
• As global trade extends and mass production of foods gets centralized ; shipping of milk to
very distant places where no milk production is available is common.
• But raw milk cannot be stored for long time , even under refrigeration due to bacteria .
Raw milk cannot be stored in single container for more than 72 hrs.
• Solution to this problem is thermally pasteurize the milk at the point of production
bulk shipping it then thermally pasteurized it again before packaging for retail.
• But this preservation strategy ; increases production costs and causes
reduction of nutritional and sensory quality of milk.
• An approach to avoid this strategy is to ship pasteurized retail packed milk.
but again, not possible due to space constraints and limited shelf life of pasteurized milk.
• Alternative method is to extend shelf life of milk by Ultra-HighTemperature Pasteurization
(UHT) or Ultra pasteurization (UP) which involves use of very high temperatures but
again alters sensory qualities of milk.
INTRODUCTIONOF PEF APPLICATIONTO SOLVETHIS PROBLEM :
• PEF could be successfully applied to extend shelf life of milk or as a second preservation
treatment to thermally pasteurized milk without altering its sensory and nutritional
• PEF alone is able to extend shelf life of milk in refrigeration by 2 weeks.
On other hand, PEF in combination with mild thermal treatment extends shelf life of milk v
. For up to 4 weeks without any significant changes.
What is PULSED ELECTRIC FIELD (PEF) processing :
• Pulsed electric field (PEF) processing is a non-thermal method of food preservation that
uses short bursts of electricity for microbial inactivation and causes minimal or no
detrimental effect on food quality attributes.
• PEF can be used for processing liquid and semi-liquid food products.
How DoesThisTechnology Benefit Consumers?
• PEF processing offers high quality fresh-like liquid foods with excellent flavour, nutritional
value, and shelf-life.
• Since it preserves foods without using heat, foods treated this way retain their fresh
aroma, taste, and appearance.
How Does PEFWork ?
• PEF processing involves treating foods placed between electrodes by high voltage pulses in
the order of 20–80 kV (usually for a couple of microseconds)
• The applied high voltage results in an electric field that causes microbial inactivation.
• After the treatment, the food is packaged aseptically and stored under refrigeration.
BUT HOW DOES IT INACTIVATE MICROBES :
• PEF treatment has lethal effects on various vegetative bacteria, mold, and yeast.
• A series of short, high-voltage pulses breaks the cell membranes of vegetative
microorganisms in liquid media by expanding existing pores (electroporation)
• The membranes of PEF-treated cells become permeable to small molecules; permeation
causes swelling and eventual rupture of the cell membrane.
Ruptured cellUnruptured cell
It is one of the most appealing technology due to-
-short treatment time(typically below 1 second).
-reduced heating effect.
-energy lost during heating food is minimized.
-for fresh-like characteristics of food, along with high sensorial quality and .
. nutrient content.
PEF technology has been suggested for the pasteurization of foods such as
juices, milk, yogurt, soups, and liquid eggs.
SHORTCOMINGSOF PEF :
High capital cost.
PEF treatment is effective for the inactivation of vegetative bacteria only.
Micro-organisms are destroyed by PEF but spores, with their tough protective coats, and
dehydrated cells are able to survive.
Refrigeration is required to extend shelf-life.
Treatment does not inactivate enzymes
PEF treatment has considerable added value for specific product ranges.
PEF is a continuous processing method, which is not suitable for solid food products that
are not pump able.
PEF processing is restricted to food products with no air bubbles and with low electrical
COMPARITIVE STUDYOF EFFECT OF PEF PROCESSINGON SHELF LIFE OF MILK :
• It is suggested that the application of PEF at mild temperatures is more effective than
immediately after thermal pasteurization or even as second preservative treatment applied
after bulk-shipping, to extend shelf life of milk.
To study this effect, a experiment was conducted…
Materials and Methods :
• Types of milk monitored :
HTST pasteurized milk
Both were subjected to a combination of mild heat treatment with PEF.
• 2 processing strategies were used in case of HTST pasteurized milk :
mild thermal treatments and PEF applied just after
HTST thermal pasteurization
mild thermal treatments and PEF applied after 8 days of
HTST thermal pasteurization
• Studied milk was immediately cooled after treatment and stored at 4 C
• Samples were taken after every two days for quality control purposes.
We look out for…
Quantification of mesophilic and psychotropic bacteria
determination of the titratable acidity to determine the presence of
Enteric bacteria quantification
visual and olfactory (smell) inspection of the samples
Coagulation, thickening, curdling, changes of colour, sedimentation, and bubbling were
some of the parameters considered in the visual inspection.
Understanding PEF’s 2 processing strategies :
I. The first processing strategy involved the application of PEF at mild temperature
immediately after HTST thermal pasteurization.
Such that to increase the shelf life of retail-bottled product so it would not need to be .
. processed after arrival at the point of consumption.
II. The second processing strategy was designed to be employed as a second preservation
. treatment applied on pasteurized milk at the point of consumption after bulk shipping.
HTST thermal pasteurization milk , bulk-stored for eight days at 4 C,
After it was treated with PEF in combination with a mild thermal treatment before .
….. retail bottling.
Applying pulsed electric field…
• Maximum electric field : 35 kV/cm
• Approx. pulse width : 2.3 microsecond.
• Input flow rate of milk : 72 L/h
• Inlet temperature : 50 C.
• Relevant parameters such as inlet and outlet temperatures, thermal
regeneration efficiency, pulse shape, voltage, current, and energy
expenditure were monitored
Result and discussions :
• The total mesophilic bacterial population present in treated samples was considered as the
principal index defining the shelf life of milk in this study.
106–107 cfu/ml cause sensory changes that can be detected as spoilage
• Figure shows the total mesophilic bacterial population of controls and treated samples
throughout the refrigerated storage period.
• It can be seen that processing strategies effectively extended the shelf life of HTST
A point to note …
• Commercially HTST pasteurized milk usually lasts between two and three weeks before
However, in this study it took 44 d for the mesophilic bacterial population in thermally
pasteurized milk (HTST) to exceed the established bacterial limit, probably due to the extreme
sanitary conditions employed in this study that are not attainable during commercial
processing, distribution, and consumption of HTST pasteurized milk.
• Levels of Enteric bacteria remains low ; as presence of enteric bacteria in pasteurized milk is
normally an indication of post-pasteurization contamination. (absent in our study)
• the end of the shelf life, although detected in the mesophilic total plate count, must also be
considered as the result of the multiplication of ‘‘weak’’ psychotropic bacteria.
Composition of flora present in raw
milk stored at 4 C for
• all treated samples were free of
Salmonella spp., Listeria
monocytogenes and Bacillus
cereus, which are pathogenic
bacteria with the potential to grow
at refrigeration temperatures that
may cause health concerns if present
• PEF and HTST thermal pasteurization in combination with storage at low temperatures act
in a hurdle fashion, not only causing repeated stresses on the microorganisms, but also
targeting different weaknesses
• titratable acidity of all treated samples
remained below the sensory detection level
(0.2% of lactic acid)
• It is possible that other flavour defects such as
bitterness may have been detectable at the
end of the shelf life
Shelf life of milk : PEF immediately after pasteurization : 60 days
Shelf life of milk : PEF after 8 days of HTST pasteurization : 78 days
No apparent changes in the olfactory or visual characteristics of the treated milks.
No significant changes observed in total titratable acidity throughout the study.
Thermoduric and spore-forming bacteria appear to be the main microorganisms
responsible for spoilage of extended shelf life PEF processed milk.
Total eradication of enteric and psychotropic bacteria.
This technique ensures long lag phases in the growth curves of the microbial populations
surviving the application of HTST and PEF treatments, translating into long shelf lives.