Lesson 9. MANUFACTURE OF EVAPORATED MILK
MANUFACTURE OF EVAPORATED MILK
Evaporated milk is sterilized, concentrated, homogenized milk. The product can be kept without refrigeration and has a long shelf life; it is completely safe for the user. After dilution, flavor and nutritive value of the product are not greatly different from that of fresh milk. A major problem with sterilization is the heat stability; the higher the concentration of the milk, the lower its stability. That is why concentrating cannot be by more than about 2.6 times, which corresponds to a level of about 22% solids-not-fat in the evaporated milk.
Currently, bottled evaporated milk is often used in coffee in certain countries. It can be added while cold because a fairly small amount is involved as compared to non-evaporated milk. After the bottle has been opened, the milk can be kept in a refrigerator for up to 10 days because it initially contains no bacteria at all and because contaminating bacteria grow somewhat more slowly owing to the reduced water activity, which is about 0.98.
The condensed milk may pass to homogenizer without the use of a drop tank but the availability of a drop tank may prove helpful in case of accidental interruption in the flow of the milk between pan, homogenizer and storage tanks and especially when using the batch system of condensing.
From the drop tank, the condensed milk passes through homogenizer over cooler into storage tank where it is standardized for total solids and held cold until ready to be tinned. The standardized milk made homogeneous through agitation in the storage tanks is then forced from these tanks by pump through evaporated milk warmer into a supply tank that feeds the filling machines by gravity. From the filled and sealed tins reconvened through a leak detector to the sterilizer where they are subjected to steam under pressure destroying the bacteria contained in the milk and giving the evaporated milk necessary keeping quality and desired consistency.
9.2 Inspection and Treatment of Milk before Manufacture
Besides inspection by the senses of smell, taste and sight, the use of rapid chemical test adopted to the routine of the receiving platform, such as the acidity test, the direct microscopic count and M.B.R. test, the alcohol test and the phosphate test have proved helpful. Variation of the quality of the milk has a direct effect on heat stability of evaporated milk and thus indirectly also influences the viscosity of the finished product.
9.3 Standardization of Fat to SNF desired
The milk that has passed inspection is then dumped into the weigh tanks and after sampling and recording weights; it is dropped into supply tank from which it is pumped over or through a cooler into holding tanks. There, it is tested for fat and TS and standardized to the ratio of Fat to SNF desired in the evaporated milk. The addition of fat in the form of cream or melted butter to the cooled homogenized evaporated milk in the storage tank (if standardization is done after manufacture) would require re-homogenization after such standardization in order to prevent objectionable fat separation after sterilization. (Also see Sweetened Condensed Milk)
9.4 Addition of Casein Stabilizers
In factories where casein stabilizers such as disodium phosphate or Na-citrate are used for control of heat coagulation in the sterilizer, it is considered good practice to add at least a portion of the required quantity anticipated to the fluid milk before manufacture.
9.5 Fore-Warming of Milk for Evaporated Milk
Preheating serves to enhance the heat stability of the evaporated milk, inactivate enzymes, and kill microorganisms, including a significant proportion of the bacterial spores present. Here the destruction of biological agencies injurious to health or damaging to keeping quality, that might be present in the raw milk, is the function of the process of sterilization by steam under pressure. However, aside from this, the fore-warming is indispensable for satisfactory vacuum pan operation also because
(1) The fore-warming process is one of the most important steps in the manufacture, upon which the heat stability of evaporated milk depends.
(2) Through their effect on heat stability, variation in fore-warming procedures has a definite effect on the viscosity of the evaporated milk.
(3) Increasing the temperature of fore-warming and limited increase of the period of exposure to the fore-warming temperature diminishes viscosity and increases the stability of the milk towards sterilizing heat.
9.5.1 Temperature of fore-warming
The heating temperature–time relationship is usually selected on the basis of heat stability. Formerly, a long heat treatment (e.g., 20 min) at a temperature below 100°C was often applied. Currently, UHT treatment is generally preferred. It reduces the number of spores in the milk considerably, and therefore a less intensive sterilization suffices.
The fore-warming temperature commonly used in USA ranges from ~ 93°C to boiling with an approximate exposure of 10-25 minutes. However, an increase in milk solids lowers the stability of evaporated milk. In addition, the higher concentration definitely increases the danger of objectionable cooked flavour and darkening of colour. It also tends to cause deposits of crystals of Ca-citrate and of other mineral salts in the container.
Low heat stability can no longer be considered a factor which might limit evaporated milk to 26% solids content. From a heat stability point view point, it should be commercially possible to manufacture evaporated milk of ~ 32% TS content. A good grade of milk should be quickly fore warmed to about 120°C and held 2-4 minutes, then drawn into the vacuum pan and handled in the usual way. For the usual type of normal milk supply, fore-warming temperature below the boiling temperature (within the approximate range of 87-99 °C for 10-15 minutes) are considered adequate to produce desired heat stability in evaporated milk and sterilized by the conventional process of 115°C for 18-20 minutes.
It should be clearly recognized that each milk supply has its own peculiarity as to optimum forewarming temperature or treatment for accomplishing the viscosity and desired body.
9.5.2 Seasonal adjustment of forewarming temperature
The milk coming in flush season has highest heat stability and is lower during early stage and later stage of milkings due to change in salt content and albumin content (higher). Therefore, it becomes necessary to change forewarming temperature. A higher forewarming temperature (120°C, coming up time to seconds and holding time 3 to 4 minutes) will make up the lowering of heat stability due to late and early lactation milk.
9.5.3 Equipment for forewarming Evaporated milk
9.6 Condensing of Evaporated Milk
The forewarmed milk is condensed to desired concentration in the vacuum pan or evaporator in a similar manner as done in the case of SCM. Evaporated milk boils at some what more vigorously and needs somewhat closer watching because of absence of the stabilizing influence of sugar. Its rate of evaporation especially toward the end of condensing period is slightly faster than that of more sluggish sweetened product. Preparation, starting and operation of the pan is quite similar as in the case of SCM.
9.7 Ratio of Concentration
9.8 Striking the Batch
The striking or sampling and testing for density of evaporated milk is more easily accomplished than that of viscous syrup SCM. When evaporated milk approaches the desired concentration, its consistency resembles that of rich milk or thin cream, its fluidity, therefore, ensures ready equilibrium of the hydrometer.
The Baume’ hydrometer, with a scale of range from 0 to 15°Be subdivided into 1/10 degrees is commonly used for testing the pan sample. Establishment of standard striking temperature helps to avoid misleading results. A convenient temperature is the pan temperature which near the finish of the batch usually ranges from approximately 48.9 – 54.4°C.
9.9 Temperature Correction Factor
If the temperature of the sample used for the density test varies from the adopted standard temperature, correction of Baume’ reading to standard temperature may be made by multiplying the temperature deviation by the factor 0.031 for each degree of deviation. If the temperature of the sample is above the standard temperature the product is added to the observed Baume reading. If the sample is below the standard temperature the product is deducted from the observed Baume reading.
The temperature at which the Baume reading was taken was 130º F and the reading was 6.7° Be. What is the Be reading at 120º F?
Answer : 6.7 + (0.031 * (130-120) = 7.01° Be at 120°F.
Formula for Calculation of correct Hydrometer Reading for Desired Concentration:
The specific gravity of evaporated milk of any desired % composition may be calculated by the following formula:
Sp. gr. values that were adopted for the milk constituents of SCM. also apply to evaporated milk. They are: Fat = 0.93 & SNF = 1.608
Evaporated milk with a composition of 7.9% fat and 26% Total solids is desired. What is the correct Baume hydrometer reading at 120º F?
SNF = 26 – 7.9 = 18.1%
Water = 100 – 26 = 74.0%
Baume reading at 60º F = 145 – (145 / 1.0667) = 9.07
Baume reading at 120º F = 9.07 – (0.031 * (120 – 60)) = 7.21
What will be the Baume reading at 120°F if desired TS content is 31% & Fat is 9%.
Baume reading at 60º F = 145 – (145 / 1.955) = 13.71
Baume reading at 120º F = 13.71 - (0.031 * (120 – 60)) = 11.85
The milk is condensed to desired composition which is evaluated by Baume reading. If the milk is standardized before it is condensed than there is no problem but some standardize it after condensing because they over condense the previously standardized milk and then add calculated amount of water to get the desired composition. All standardizing is done in storage tank.
Homogenization serves to prevent creaming and coalescence. It should not be too intensive because the heat stability becomes too low. Precisely following is the effect of homogenization on the milk:
1. The homogenizer reduces the fat globules to such small size that majority of globules are no longer in a position to respond to force of gravity.
2. Homogenization increases the viscosity of evaporated milk. This further impedes particle movement other than Brownian movement, enhancing emulsion stability of fat globules.
Product should be homogenized at 49°C or above to avoid fat clumping. Two stage homogenizer is advisable at first stage 140 – 175 kg/cm2 and at second stage 35 kg/cm2.
9.11 Effect of Homogenization on Viscosity of Evaporated Milk
Homogenization increases the viscosity of evaporated milk. This means that the resulting smaller fat globules must overcome a greater resistance if they are to move upward. The cause of the increased viscosity of homogenized milk has not been fully explained, but it is believed to be due to the increased absorption of casein to the much larger surface area of the finely dispersed fat globules thereby considerable augmenting the volume of the dispersed phase and diminishing the volume of the continuous phase. A change in the physical properties of the casein may also be involved in the effect of homogenization on viscosity. Homogenization definitely increases the viscosity of evaporated milk and does so increasingly with increasing pressure.
9.12 Effect of Homogenization on Heat Coagulation of Evaporated Milk