CONDENSED MILK - How to prepare?
The recipe to condesne milk
The basic principle underlying the production of condensed and evaporated milk is that milk of high quality is filtered or clarified, standardized, forewarmed, and condensed / evaporated to the desired level. The preservation of condensed milk is by the addition of sugar whereas the evaporated milk is preserved by heat sterilization.
The quality of the raw milk is the single most important criterion that determines the quality of the end product. The quality of the raw incoming milk in turn is dependent on the sanitary procedures followed during the milk production and transportation.
Healthy cows, milk produced under clean environment, clean and hygienic utensils, freedom from colostrum, prompt cooling of milk soon after milking and transport under refrigerated state are the factors that determine the number of microorganisms.
The received milk is thoroughly checked for organoleptic qualities like colour, odour, taste (raw milk is seldom tasted at the entry point) etc. along with other platform tests like clot on boiling (COB), alcohol / ethanol test to determine the suitability of milk for heat processing.
Take 5 ml of milk in a test tube and an equal quantity of 70% alcohol is added and mixed well. Formation of clots and / or flakes denotes a positive test and the milk is tested for its susceptibility for heat coagulation by COB. The coagulation of milk by alcohol is due to disturbed salt balance. This test detects abnormal milk including colostrum which has higher ash content. The sensitivity of the test in detecting mastitic milk is very good.
Alcohol Index Test
The test is done by placing absolute alcohol in the burette and taking 10 ml milk in a porcelain dish. Alcohol index is defined as the number of ml of alcohol required to induce flake formation in milk and when the value is 7 or more, it indicates high heat stable milk while values less than 3 can be rejected outright. The values falling between 3 and 7 requires further study.
Alcohol Alizarin Test
This test has twin advantages in that not only the heat stability but also the pH of milk can be determined.
Clot on Boiling Test
Take 5 ml of milk in a test tube and place it in a boiling water bath for 5 minutes. Presence of curd or flake like particles adhering on the surface of test tube shows a positive test and it is interpreted that the milk is unfit for processing due to developed acidity.
Filtration / Clarification
This procedure is adopted to remove visible foreign matter, which is not only unsightly but also cause consumer complaints. It is a common practice to preheat the milk before clarification in the warm clarifiers in order to increase the efficiency of the operation. After clarification, the milk is invariably cooled if further processing is delayed.
This is done to conform to the legal standards in the end product. The standardization of milk meant for condensed milk preparation is done in three stages.
The first standardization is done to bring the Fat : SNF ratio to the desired level (1:2.44).
The second standardization attempts to establish the desired ratio between the added sugar and total milk solids.
The third and final standardization adjusts the concentration of the finished product to the desired percentage of total solids.
Forewarming / Preheating
This is the heating of milk before it is condensed and serves
- To ensure that the end product is free of microorganisms and spoilage causing enzymes
- To ensure uninterrupted boiling in the vacuum pan and
- To effectively control a defect known as “age thickening” in the end product – condensed milk.
The temperature time combination of preheating / forewarming varies between 82 - 93°C for 5-15 minutes or in some cases, it may be heated up to 116 - 149°C for 0.5 to 5 minutes. The modern trend calls for heating the milk to 115 - 118°C for no hold or a fraction of a second. Fore warming is usually done in tubular heat exchangers.
Addition of sugar
The objective of adding sugar is to preserve the condensed milk in the absence of sterilization of the product by heat. Sucrose is the sugar of choice for addition and is usually added as refined cane or beet sugar. There are other alternatives to sucrose and sweetening agents such as corn syrup, glucose, dextrose have been tried with replacement levels up to 25% with varying success. The problem with these replacements is that their reduced sweetening power along with adverse effect on colour and changes in the rate of thickening especially during storage of the finished product.
Generally sucrose in granular or syrup form of good quality is used. Sugar syrups at concentration levels of 65% shall be subjected to high pasteurizing temperatures in order to destroy pathogenic and spoilage causing microorganisms before being added to the condensed milk.
The temperature level at which the sugar is added to the milk has a definite effect on the physical stability and keeping quality of the finished product. Usually sugar is added at the end of the condensing process and it is mixed with the least quantity of potable water. The addition of sugar at the end of condensing process avoids an increase in viscosity and thereby alleviates the greater difficulty in the evaporation of moisture. Apart from this, presence of sugar during condensing process affords protection to pathogenic and spoilage causing organisms and thereby influence the keeping quality of the finished product. The sugar syrup is refined by passing it through pressurized filter or centrifugal clarifier.
The principle underlying the condensing operation is the removal of water from the standardized milk by boiling it at low temperature under partial vacuum till the desired concentration is reached. The advantages of using the condenser for evaporation of moisture are
- rapidity of evaporation
- economy in the scale of operation
- protection of nutrients of milk against heat damage.
Apart from these, evaporation under vacuum achieves a product which is free from cooked flavour defect but at the same time, the product can be readily reconstituted into the original milk.
The evaporators used in the dairy industry may vary and can be classified into
1. Vertical short tube evaporator
2. Vertical long tube evaporator
3. Forced circulation evaporator
4. Plate evaporator
5. Multiple effect evaporator
6. Centrifugal evaporator
7. Expanding flow evaporator
8. Vapor recompression evaporator
9. Low temperature evaporator
Important steps involved in evaporating the milk
- The evaporator may be operated on a batch or continuous method depending on the type of the machine and volume of milk processed.
- Sanitization of the evaporator is must before beginning the operation
- The product (milk) should cover the heating tubes containing the coils before admitting the steam and this prevents scorching.
- The quantum of the product needs to be maintained at recommended level and this is made possible by controlling the fresh product intake so that the volume of water removed is replaced.
- Quick and rapid boiling inside the evaporator is avoided as it leads to entrainment.
- Air leaks in the evaporating system shall be strictly controlled as it reduces the efficiency of the operation.
Stopping of the evaporator shall be followed with coordinated sequences and it is usually in the following order : Turn off the steam, turn off water to the condenser, stop the vacuum pump, and finally open the vacuum relief.
Generally dry saturated steam is considered more desirable for vacuum pan operation than wet or super heated steam.
Striking the batch
When the boiling milk approaches the desired concentration, it is visually indicated by settling down of milk to a quite boil, the surface of milk assumes glossy and glistening luster, heavy roll of milk from periphery to the centre etc. These are the visual signs that warn the operator as to the right time for “striking the batch”.
The term “striking the batch” indicates the correct concentration (as determined by specific gravity / density tests) has been reached. But the sampling of condensed milk shall begin sufficiently early to permit taking and testing for density several successive samples without the risk of objectionable over condensing. The pan temperature slowly drops to 49°C, which is the standardized testing temperature.
What are the practical density tests available to check the condensed milk?
1. Pycnometer test
2. Hydrometer test
3. Refractometer test
4. Viscosimeter test.
Finishing the batch
On “striking the batch”, when the desired density has been reached, the condensing operation is stopped. The entire steam to the condenser is stopped and the steam to the pan is shut off. The valve in the waterline to the condenser is closed. Then the vacuum pump is stopped and the vacuum relief is opened. These operations are done in a sequence as told in order to avoid milk from burning on to the heating surface and condenser water from flooding the pan. Only when the vacuum is dissipated, the condensed milk is drawn from the pan.
It is a common practice to slightly over condense the milk and then standardize it back to exact concentration required to meet the legal standards by adding correctly calculated quantum of water.
Throughout the world the hot condensed milk during its manufacture, is invariably homogenized before being cooled and crystallized. The objective of homogenization is to uniformly distribute the fat globules and reduce the fat separation during storage. A specially designed homogenizer, capable of handling highly viscous products is used for this purpose and it is operated at a pressure of 2000 PSI in the first stage and 500 PSI in the second stage.
Cooling and crystallization
The cooling of hot condensed milk is very important in its manufacture so that the end product is delicious and marketable. Prompt cooling is essential to delay age thickening and changes in color; prolonged high temperature favors color darkening. Quick cooling helps in the production of smooth texture of the finished product and its freedom from objectionable sugar deposit.
Importance of lactose
Lactose in condensed milk helps in controlling the texture of the product, which is present in a highly concentrated state. At room temperature, a majority of the lactose is present in crystal form and their size determines the relative smoothness of the product. It is the size of the lactose crystal that determines smoothness.
Effect of cooling
The type of cooling the hot sweetened condensed milk receives during the manufacture that determines the number and permanent size of lactose crystals. If the number of lactose crystals is in the range of 300,000 to 400,000 per cubic millimeter with the length of the longest edge of the crystal is approximately 9 micron, then the product will be regarded as excellent in smoothness.
How lactose crystallizes?
Alpha lactose hydrate will crystallize under the temperature conditions that prevail in the manufacture of condensed milk. The rate of lactose crystallization is impeded by presence of milk colloids and high viscosity which prevents the rate of diffusion. The temperature of maximum rapidity of crystallization for an average compositioned condensed milk product is about 30°C or 86°F.