Object of Experimentation:

The continuous HTST pasteurisation of raw milk. Including Clean in Place procedure

Summary of Theory

The high temperature short time (HTST) process is a continuous process which has been developed in industry to offset the problems of the batch (holder) process. If a higher temperature is used for pasteurisation the kill of bacteria can be achieved in a much shorter time and damage to the flavour and nutritive value of the milk will be less

A temperature of 71.7ºC is used for 15 seconds to achieve HTST pasteurisation. This process has very little damaging effect with the exception of the loss of about half the vitamin C, and about 10% of thiamine, and cyancocobalamin which reacts with breakdown products of vitamin C. Protein of the membrane surrounding the milk fat globules is slightly denatured and this allows for globules to rise to the surface and form the characteristic cream layer

Operating Procedure for Pasteurising Milk

Set up the pasteuriser ready for milk processing by following the following instructions.
Prior to processing the foodstuff, it is necessary to follow a strict cleaning and disinfecting procedure at the beginning and end of every throughput of milk

Disinfection immediately precedes the processing run:

To begin the processing run, fill the feed tank with 3 litres of 1% hypochlorite solution. The corrosive effect of chlorine is accelerated by increase in temperature and the solution should only be applied to cooled plant. To disinfect the plant, set the diverter valve temperature set point to 30ºC. Set the pasteurising set point to 71.7ºC. Switch on the heater. Initial settings on the flow rate could be set at 1000ml per minute. Switch on the feed pump variable speed drive to position 20 on the graduations - ensure that air is removed from all tubes

Allow the hypochlorite solution to circulate and then when the temperature reaches 30ºC, to be expelled from the apparatus, through the regenerator and cooler sections. When all hypochlorite has been expelled, fill feed tank with 3 litres of distilled water. Set the diverter valve set point at 50ºC. Allow the distilled water to circulate and then to be expelled when the temperature reaches 50ºC

Test the discarded solution for free chlorine using starch iodine paper. Continue washing with distilled water until no free chlorine can be demonstrated

Fill the feed tank with 1 litre of clean distilled water. Set the diverter valve set point to 69ºC. Turn the pump variable speed down to give a throughput of 167 mls per minute or 10 litres per hour. Allow the water to circulate. The temperature will rise and allow the water to be expelled when it rises above 69ºC

Fill the feed tank with raw milk, just as the last drop of hot water leaves the feed tank. The milk will chase the hot water through the system and by observing the output, it will be noticed when hot water, cloudy water then milk is being expelled and then the milk can be collected in a suitable container

The feed tank can be continuously replenished if required. Be sure not to let it empty or air will be drawn into the system. When the final drop of milk is leaving the feed tank, pour in 3 litres of cold water. Wait for the water to get through the equipment and then direct to drain. Switch off heater and turn pump drive speed to 20. Also turn diverter set point to minimum. Keep replenishing the feed tank with rinse water until it becomes clear at the outlet. The equipment will rapidly cool, preventing the drying on of milk residues and there will be removal of soil from contact surfaces. As the water empties from the feed tank add 3 litres of 1% hypochlorite solution

When the hypochlorite solution is used, follow with enough clean water to flush it all out of the system as checked by starch iodide paper. Allow the pasteuriser to drain. Switch off

If required the heat exchanger could be split at this point so that the condition of the heat transfer surfaces can be inspected. This will also serve to drain any water still trapped inside the exchanger although it is not essential to do this before starting a new process run with the disinfecting operation

It is also possible to take swab samples from plates in each of the heat exchanger sections in order to carry out tests to determine the effectiveness of the clean in place procedure

Rinse water can also be analysed taking samples of fresh water and the water being circulated during cleaning and the final wash water as it is leaving the heat exchanger