Research paper

PROCESSING OF PASTEURIZED MILK BY PULSED ELECTRIC FIELD

RAJAN D JAYKAR 11 FET 1015 T Y B Tech FOODS PRESERVATION TECHNOLOGYINSTITUTE OF CHEMICAL TECHNOLOGY , MUMBAI , MAHARASHTRA

WHY PROCESS FOOD ?

1. Extend shelf life2. Maintain sensory properties3. Maintain nutritive properties4. Ensure safety5. Make more convenient. 6. Economic value

food processing can be classified into 2 groups

Thermal processing

Non-Thermal processing

Examples-

- high hydrostatic pressure - ultra sonication- PULSED ELECTRIC FIELD

WHY PULSED ELECTRIC FIELD IS NEEDED ?

LET’S FIND THE ANSWER …… BUT FIRST, LET’S SEE :

Importance of Thermal 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.

Limitations :• 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-High Temperature Pasteurization (UHT) or Ultra pasteurization (UP) which involves use of very high temperatures but again alters sensory qualities of milk.

INTRODUCTION OF PEF APPLICATION TO SOLVE THIS 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 qualities.

• 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 Does This Technology 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 PEF Work ?• 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 cell

Unruptured cell

Continuous PEF diagram

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.

SHORTCOMINGS OF 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 conductivity.

COMPARITIVE STUDY OF EFFECT OF PEF PROCESSING ON 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 : Raw milk 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 spoilage bacteria 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 pasteurized milk.

A point to note …• Commercially HTST pasteurized milk usually lasts between two and three

weeks before spoiling. 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

eight days.• 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 in milk

• 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

CONCLUSIONS :

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.