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  Definitions 
  Batch:  Batch Pasteurizer
  HTST:  High Temperature Short Time Pasteurizer
  
  
  Before any Cheesemaker makes his first pound of Cheese, he or she will make this decision:  Raw or Pasteurized? 
   
  The next decision will be:  HTST or Batch?
  • • • The debate is ongoing so I will attempt to explain without bias how

      the two systems differ.  Batch pasteurizers are commonly used by smaller Cheesemakers, and the number one criteria that carries the most weight is capital cost.  A batch pasteurizer may cost less than an HTST.  In many cases the model that is emulated in the USA is one that may mirror the small farmstead operator in Europe who used to make raw milk cheeses, but have been forced by regulation to first pasteurize their milk before making cheese. On this scale, a batch pasteurizer can likely be introduced into their operation with the least amount of modification to the existing infrastructure.

       

      Principle of Heat Transfer

       

      • • • A Batch pasteurizer is, in essence, a tank surrounded by a cavity of water or a tank with very small film of water being pumped across the surface.  In the case of cavity vessels, hot water may be heated with an electric element, steam. or the hot water may be circulated over the surface.  The temperature of the water in these systems is unlikely to be greater than 212 degrees F, and is typically between 180 and 200 degrees F.  The water heats the Stainless Steel liner of the Batch Pasteurizer, which in turn heats the milk.  The temperature differential across the Stainless Steel wall ranges from 150 degrees F when the milk is cold,  to 40 degrees F when the milk is at 145 degrees in the pasteurizer at the end of heating.  At a recent installation I observed that this particular Batch Pasteurizer was being heated with steam injected above the condensate causing extensive burn on the wall of the tank at the milk to air interface.  This is because the temperature on the Stainless Steel wall of the vessel was being exposed to a temperature approaching 300 degrees F.  I believe this to be an incorrect use of steam and changes will be made.  The Delta T in this situation ranged from 260  to 150 degrees F.  Low Delta T is better.*

             

            *This difference in temperature across the stainless steel wall is referred to as the Delta T (∆T).  Delta T is the driving force for the energy transfer, and the reduction in Delta T as the milk temperature rises is the reason why the rate of temperature increase slows as you get closer to the temperature of the heating medium.  The Round Batch Pasteurizer is constantly agitated during the heating and cooling cycles. This design is to limit the amount of laminar flow, but this still has a less efficient rate of heat transfer and higher localized temperatures than can occur in the HTST.

             

In an HTST the milk is not placed inside a vessel like the batch but pumped through in a very controlled manner between closely spaced and matched plates.  The milk enters the HTST through a series of stainless steel plates called the "regenerative section."  Flowing on the other side of these plates is milk returning from the "heating section."  The plates are placed very close to each other, creating a film of milk only millimeters thick.  The plates are not flat but ribbed, causing the milk to remain in very close contact with the plate and creating a very efficient heat transfer due to the turbulent flow.  (Running milk over these plates at rates below the manufacturer's recommended minimum flow rates will produce more heat damage than when operating at the correct flow rate).  The milk entering the regenerative section at 35 degrees F will be exposed to milk on other side of the plate at approximately 90 degrees F (a Delta T of 55 degrees F upon entry into the regen).  By the time the incoming milk leaves the regen at approximately 140 degrees F,  the Delta T may be less than 5 degrees.  This milk then enters the heating section where it will likely be exposed to a plate backed by counter flowing hot water at a temperature of between 2 to 10 degrees above the desired temperature of the milk.  Where the desired temperature is 162 degrees F, the water temperature maybe as low as 164  - 166 degrees F, but for this exercise let us assume 163 degrees is the desired temperature and 165 degrees is the water temperature (a Delta T of 2 degrees F for not less than 16 seconds--Achieving such low Delta T numbers is a function of the number of plates installed). 

In an HTST,  there are three brief temperature treatments given to the milk:  warming up, final heating and cooling down.  

In the Batch system, the milk then has to be cooled down to the desired temperature for arrival at the Cheese Vat.  This step, unlike the HTST, requires additional equipment in the form of a source of cool water.  Initial cooling may be done with cool ground water and finished with a chilled water source which will have required the use of refrigeration chiller or ice bank system.  If this step is completed in the Batch, it will be quite slow and energy intensive.  A good modification for cooling milk after batch pasteurizing is to run it through a dedicated cooling plate on route to the Cheese Vat and counter flow cold water from an ice bank or chiller.  Through correct sizing it is possible to have the milk in the Cheese Vat in approximately 10 minutes at the desired temperature. 

Many operators of Batch Pasteurizers fail to add the cooling equipment necessary to get the temperature down quickly.  They save some capital cost, but have a milk that is more damaged (denatured) by heat than an HTST.  They have milk that often has an elevated sweetness and lightly caramelized flavor due to mild breakdown of the sugar and mild mallard reaction; a reaction between lactose and milk protein; have an increased energy cost in the form of gas or electricity; and considerably more labor cost.

All systems have their pros and cons.  HTST may cost more but not much more than a good Batch system setup with appropriate cooling (a 10% premium). 

One further comment that should be made is a comment on milk handling:  There is a trend in HTST design to move away from positive pumps and use two centrifugal pumps and a Mag Meter (a flow control device).  The combination of these two pumps can have a negative impact on milk and its components, which can negatively impact cheese quality.  My preference is to use a positive pump as the timing pump with fine tolerances and a low sheer centrifugal as the booster pump to minimize damage.

The decision to use a steam boiler or hot water set will also be a big decision.  Note:  boilers require constant treatment with chemical to avoid scale.  These chemicals are expensive and highly toxic and not good for the environment, not a good green option or sound commercial thinking.  Green is good business.

Pumping is also an issue with Batch systemse. The use of centrifugal pumps for pumping cold raw milk causes churning and fat damage, which can lead to off flavors (rancidity) in the cheese.  This situation would arise when transferring cold milk to the batch system from a bulk tank or silo.  The answer is to use a positive pump, as these are available and affordable.   The next opportunity for damage is when you pump from the batch via a plate to the cheese vat; excessive back pressure will cause a centrifugal pump to partially homogenize the milk again damaging fat.

The Bottom Line:    An HTST will give you milk that is more like the raw milk you started with than a Batch.  For me, this is the overwhelming reason to use and HTST.

It is now five years since the Batch Pasteurizer in this discussion was installed and a conversion to circulated water where the steam is injected into the circulated water.  The vessel is now atmospheric, 70 psi steam is injected into the water loop, the excess volume generated by the steam injection is returned to the boiler feed.  It added approximately 10 minutes to the heat up time; the unit is now heated in 45 - 50 minutes.  There is no burn on, the milk smells different; the cheese curds have lost their cooked aroma.  Time will tell us how much the cheese from this pasteurizer has changed.  The wash cycle has been reduced by 20 minutes and I speculate that less detergent is required to remove the soft soil Vs the burn on that existed before.

Final Comment

Myth: Batch Pasteurizers are gentler on the milk because they only heat to 145 degrees F. 

Simply, technically not true: 145 degrees F for 30 minutes is equal to 161 degrees F for 15 seconds.  The heating damage done on the shoulders of the heating and cooling cycle is many magnitudes greater with a Batch System than an HTST.  The high Delta T numbers and presence of laminar flow are unacceptable trade-offs in what should be a world of excellence.

Closing comment: Tera Johnson, CEO of the new whey plant being constructed in Reedsburg, WI said they cannot use the whey from cheese plants where Batch Pasteurizers are used as the whey had undergone too much denaturation.

Summing Up:

HTST cuts labor cost, energy cost, pollution, chemical usage, produces more consistent product and produces milk that is more like "milk" than a Batch system.  Walk into the plant, start the hot water boiler, sanitize the HTST and move into forward flow, might take 10 minutes.

Batch Systems are slow (minimum two hours to begin to get product out), energy intensive, highly variable product.

Compare a 500 gallon batch pasteurizer that can produce 1,500 gallons of pasteurized milk in three hours and 10 minutes of running with a batch of 500 gallons that will have produced 500 gallons of pasteurized milk and you will have completed a wash cycle ready to start the process again.