Hts Tvs Batch
1
CHEESE REPORTER 4 February 13, 2009 Cheesemaking Practice HTST vs. Batch: The Ongoing Pasteurizer Debate Neville McNaughton FROM OUR ARCHIVES 50 YEARS AGO Feb. 13, 1959: St. Paul, MN— Some important clues in studies on “cold storage” bacteria called “psy- chrophiles” – the kind that cause milk, cottage cheese and other dairy products to spoil even though refrigerated, were recently reported by the University of Minnesota. Madison—Bacteria used for cheese starter can be spray-dried effec- tively, according to UW-Madison bacteriologists. This would be a simpler and less expensive process than the freeze-drying methods now used for making dried lactic starters. 25 YEARS AGO Feb. 17, 1984: Alexandria, VA— Dairy industry representatives from across the US gathered here this Batch: Batch Pasteurizer HTST: High Temperature Short Time Pasteurizer Before any cheese maker makes his or her first pound of cheese he or she will make this decision: raw or pas- teurized. The next decision will be HTST or Batch. The debate is ongo- ing so I will attempt to explain with- out bias how the two systems differ. Batch pasteurizers are commonly used by smaller cheese makers 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 US is one that may mirror the small farmstead operator in Europe who used to make raw milk cheese and has been forced by regulation to first pasteur- ize 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 infra- structure. 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 sur- face. In the case of cavity vessels hot water may be heated with an electric element, steam or the hot water maybe circulated. The temperature of the water in these systems is unlikely to be greater than 212° F and is typically between 180° F and 200° F. The water heats the stainless steel liner of the batch pasteurizer which in turn heats the milk. The temper- ature differential across the stainless steel wall ranges from 150° F when the milk is cold to 40° F when the milk is at 145° F in the pasteurizer at the end of heating. In one installation viewed recently I observed that this particu- lar batch pasteurizer was being heated with steam above the con- densate causing extensive burn on the wall of the tank at the milk to air interface; this is because the temper- ature on the stainless steel wall of the vessel was being exposed to a temperature approaching 300° 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° F to 150° F. Low Delta T is better. This difference in temperature across the stainless steel wall is referred to as the Delta 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 batch is constantly agitated during the heating and cooling cycles designed 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 con- trolled manner between closely spaced and matched plates. The milk enters the HTST first passing through a series of stainless steel plates called the regenerative sec- tion; flowing on the other side of these plates is milk returning from the heating sections. To pasteurize a fixed volume of milk in one hour a batch system requires double the heating energy of an HTST; add to this the energy to cool and you see that the operational cost to operate a batch pasteurizer is two to three times, not a green option. The plates are placed very close to each other, creating a thin film of milk millimeters thick. In addition the plates are not flat but ribbed, causing the milk to remain in very close contact with the plate and causing very efficient heat transfer. The milk entering the regenera- tive section at 35° F will be exposed to milk on other side of the plate at approximately 140° F, a Delta T of 105° F. This milk then enters the heating section where it will likely be exposed to a plate backed by counter flowing hot water at a tem- perature of between 5 and 10 degrees above the desired temperature of the milk. Where the desired temperature is 162° F the water temperature may be as low as 167° F but for this exercise let us assume 163° F is the desired temperature and 169° F is the water temperature, a Delta T of 6° F for not less than 16 seconds. In an HTST there are three brief temperature treatments given to the milk, warming up, final heating and cooling down. The milk is cooled down to the desired temperature required for culture and coagulation in the cheese vat. In an HTST the milk is exposed to less elevated temperature for less time than a batch system. The con- sequences of this are less denatura- tion of the whey proteins when milk is treated with an HTST than a batch. The negative consequences of protein denaturation are: poor coag- ulation in sweet curd cheeses and poor drainage characteristics for acid drained curd such as Quark, Chevre and ladled acid curds. When milk is heated whey proteins form an attachment to the casein in the milk in such a way as to interfere with the action of rennet on the casein and prevent it from forming a good set. 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 en route to the cheese vat and counter flow cold water from an ice bank or chiller. Through correct siz- ing it is possible to have the milk in the cheese vat in say 10 minutes at the desired temperature. Many operators of batch pasteuriz- ers 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, have milk that often has an elevated sweetness and lightly caramelized flavor due to mild break- down of the sugar and mild Maillard reaction, a reaction between lactose and milk protein, and have an increased energy cost in the form of gas or electricity, and considerably more labor cost. All systems have their pros and cons, and HTST may cost more but not much more than a good batch system setup with appropriate cool- ing. To pasteurize a fixed volume of milk in one hour a batch system requires double the heating energy of an HTST; add to this the energy to cool and you see that the opera- tional cost to operate a batch pas- teurizer is two to three times, not a green option. The hidden capital cost in a batch system is the greater size of the steam or hot water heating system which must be twice as large as that for an HTST. One further comment that should be made is a comment on milk han- dling. 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 and a fine tolerance, low sheer centrifugal as the booster pump to minimize damage. Pumping is also an issue with batch systems; the use of centrifugal pumps for pumping cold raw milk causes churning and fat damage which can lead to off flavors in the cheese. This situation would arise when transfer- ring cold milk to the batch system from a bulk tank or silo. The answer is to use a positive pump; these are avail- able and affordable. The Bottom Line An HTST gives you milk that is more like the raw milk you started with than a batch, for me this is the over- whelming reason to use an HTST. Final Comment Myth : Batch Pasteurizers are more gentle on the milk because they only heat to 145° F. Simply, technically not true: 145° F for 30 minutes is equal to 162° F for 15 seconds. The heating damage done on the shoulders of the heating and cooling cycle is many magni- tudes greater with a batch system than an HTST. 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 pasteuriz- ers are used as the whey had under- gone too much denaturation. r
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Hts Tvs Batch
Transcript of Hts Tvs Batch
C H E E S E R E P O R T E R4 February 13, 2009
C h e e s e m a k i n g P r a c t i c e
HTST vs. Batch:The Ongoing Pasteurizer Debate
Neville McNaughton
FROM OUR
ARCHIVES
50 YEARS AGOFeb. 13, 1959: St. Paul, MN—Some important clues in studies on“cold storage” bacteria called “psy-chrophiles” – the kind that causemilk, cottage cheese and otherdairy products to spoil even thoughrefrigerated, were recently reportedby the University of Minnesota.
Madison—Bacteria used for cheesestarter can be spray-dried effec-tively, according to UW-Madisonbacteriologists. This would be asimpler and less expensive processthan the freeze-drying methodsnow used for making dried lacticstarters.
25 YEARS AGOFeb. 17, 1984: Alexandria, VA—Dairy industry representatives fromacross the US gathered here this
Batch: Batch PasteurizerHTST: High Temperature ShortTime PasteurizerBefore any cheese maker makes hisor her first pound of cheese he or shewill make this decision: raw or pas-teurized. The next decision will beHTST or Batch. The debate is ongo-ing so I will attempt to explain with-out bias how the two systems differ.
Batch pasteurizers are commonlyused by smaller cheese makers andthe number one criteria that carriesthe most weight is capital cost; abatch pasteurizer may cost less thanan HTST. In many cases the modelthat is emulated in the US is onethat may mirror the small farmsteadoperator in Europe who used tomake raw milk cheese and has beenforced by regulation to first pasteur-ize their milk before making cheese.
On this scale a batch pasteurizercan likely be introduced into theiroperation with the least amount ofmodification to the existing infra-structure.
Principle Of Heat TransferA batch pasteurizer is in essence atank surrounded by a cavity of wateror a tank with very small film ofwater being pumped across the sur-face.
In the case of cavity vessels hotwater may be heated with an electricelement, steam or the hot watermaybe circulated. The temperatureof the water in these systems isunlikely to be greater than 212° Fand is typically between 180° F and200° F.
The water heats the stainless steelliner of the batch pasteurizer whichin turn heats the milk. The temper-ature differential across the stainlesssteel wall ranges from 150° F whenthe milk is cold to 40° F when themilk is at 145° F in the pasteurizer atthe end of heating.
In one installation viewedrecently I observed that this particu-lar batch pasteurizer was beingheated with steam above the con-densate causing extensive burn onthe wall of the tank at the milk to airinterface; this is because the temper-ature on the stainless steel wall ofthe vessel was being exposed to atemperature approaching 300° F.
I believe this to be an incorrectuse of steam and changes will bemade. The Delta T in this situationranged from 260° F to 150° F. Low
Delta T is better.This difference in temperature
across the stainless steel wall isreferred to as the Delta T. Delta T isthe driving force for the energytransfer and the reduction in DeltaT as the milk temperature rises is thereason why the rate of temperatureincrease slows as you get closer tothe temperature of the heatingmedium.
The batch is constantly agitatedduring the heating and coolingcycles designed to limit the amountof laminar flow but this still has aless efficient rate of heat transfer andhigher localized temperatures thancan occur in the HTST.
In an HTST the milk is notplaced inside a vessel like the batchbut pumped through in a very con-trolled manner between closelyspaced and matched plates. Themilk enters the HTST first passingthrough a series of stainless steelplates called the regenerative sec-tion; flowing on the other side ofthese plates is milk returning fromthe heating sections.
To pasteurize a fixedvolume of milk in onehour a batch systemrequires double theheating energy of an
HTST; add to this theenergy to cool and yousee that the operationalcost to operate a batchpasteurizer is two to
three times, not a greenoption.
The plates are placed very close toeach other, creating a thin film ofmilk millimeters thick. In additionthe plates are not flat but ribbed,causing the milk to remain in veryclose contact with the plate andcausing very efficient heat transfer.
The milk entering the regenera-tive section at 35° F will be exposedto milk on other side of the plate atapproximately 140° F, a Delta T of105° F. This milk then enters theheating section where it will likelybe exposed to a plate backed bycounter flowing hot water at a tem-perature of between 5 and 10 degreesabove the desired temperature of themilk.
Where the desired temperature is162° F the water temperature may beas low as 167° F but for this exerciselet us assume 163° F is the desiredtemperature and 169° F is the watertemperature, a Delta T of 6° F fornot less than 16 seconds.
In an HTST there are three brief
temperature treatments given to themilk, warming up, final heating andcooling down. The milk is cooleddown to the desired temperaturerequired for culture and coagulationin the cheese vat.
In an HTST the milk is exposedto less elevated temperature for lesstime than a batch system. The con-sequences of this are less denatura-tion of the whey proteins when milkis treated with an HTST than abatch.
The negative consequences ofprotein denaturation are: poor coag-ulation in sweet curd cheeses andpoor drainage characteristics for aciddrained curd such as Quark, Chevreand ladled acid curds. When milk isheated whey proteins form anattachment to the casein in the milkin such a way as to interfere with theaction of rennet on the casein andprevent it from forming a good set.
In the batch system the milk thenhas to be cooled down to the desiredtemperature for arrival at the cheesevat. This step, unlike the HTST,requires additional equipment in theform of a source of cool water.
Initial cooling may be done withcool ground water and finished witha chilled water source which willhave required the use of refrigerationchiller or ice bank system. If thisstep is completed in the batch it willbe quite slow and energy intensive.
A good modification for coolingmilk after batch pasteurizing is to runit through a dedicated cooling plateen route to the cheese vat andcounter flow cold water from an icebank or chiller. Through correct siz-ing it is possible to have the milk inthe cheese vat in say 10 minutes atthe desired temperature.
Many operators of batch pasteuriz-ers fail to add the cooling equipmentnecessary to get the temperaturedown quickly. They save some capitalcost but have a milk that is moredamaged (denatured) by heat than anHTST, have milk that often has anelevated sweetness and lightlycaramelized flavor due to mild break-down of the sugar and mild Maillardreaction, a reaction between lactoseand milk protein, and have anincreased energy cost in the form ofgas or electricity, and considerablymore labor cost.
All systems have their pros andcons, and HTST may cost more butnot much more than a good batchsystem setup with appropriate cool-ing. To pasteurize a fixed volume ofmilk in one hour a batch systemrequires double the heating energyof an HTST; add to this the energyto cool and you see that the opera-tional cost to operate a batch pas-teurizer is two to three times, not agreen option.
The hidden capital cost in a batchsystem is the greater size of the steamor hot water heating system whichmust be twice as large as that for anHTST.
One further comment that should
be made is a comment on milk han-dling. There is a trend in HTSTdesign to move away from positivepumps and use two centrifugal pumpsand a Mag Meter (a flow controldevice); the combination of these twopumps can have a negative impact onmilk and its components which cannegatively impact cheese quality. Mypreference is to use a positive pump asthe timing pump and a fine tolerance,low sheer centrifugal as the boosterpump to minimize damage.
Pumping is also an issue with batchsystems; the use of centrifugal pumpsfor pumping cold raw milk causeschurning and fat damage which canlead to off flavors in the cheese. Thissituation would arise when transfer-ring cold milk to the batch systemfrom a bulk tank or silo. The answer isto use a positive pump; these are avail-able and affordable.
The Bottom LineAn HTST gives you milk that is morelike the raw milk you started withthan a batch, for me this is the over-whelming reason to use an HTST.
Final CommentMyth: Batch Pasteurizers are moregentle on the milk because they onlyheat to 145° F.
Simply, technically not true: 145°F for 30 minutes is equal to 162° Ffor 15 seconds. The heating damagedone on the shoulders of the heatingand cooling cycle is many magni-tudes greater with a batch systemthan an HTST.
Closing comment: Tera Johnson,CEO of the new whey plant beingconstructed in Reedsburg, WI, saidthey cannot use the whey fromcheese plants where batch pasteuriz-ers are used as the whey had under-gone too much denaturation. rr
