CONDENSED AND DRIED MILKS

Module 4. Manufacturing techniques

Lesson 12
RECOMBINED CONCENTRATED MILKS - PROCESSING

12.1 Introduction

Processes for manufacturing recombined concentrated and dried milks vary widely and basic principles are well described. Usually the powder is dispersed and fat is mixed to give a level of total solids slightly above that required in final product. Some plants prepare recombined milk at solids level of normal milk and follow the evaporation process. The main stages are

12.2 Mixing

• Powder or powders for Evaporated Milk preparation are mixed to get 18-20% SNF concentration in water, heated to 45-55°C and dispersion is effected using a high speed blade, a powder funnel and centrifugal pump, a powder/liquid blender or number of other devices designed for the purpose. The non-fat milk powder used shall confirm the standards as specified. But especially heat stability of SMP used is of prime importance and hence WPNI of < 1.5 is recommended for recombined evaporated milk preparation. Care should be taken to minimize air incorporation or a deaerator can be used.

• Liquid fat (55-60%) may be added before, after or with powder to minimize foaming. Uniform dispersion of fat to maintain protein/fat ratio at homogenization is required otherwise fat separation results.

• The ingredients for SCM preparation are weighed. The SMP is dissolved in water heated to 40-50°C first to give solids concentration of 44%. For this a high energy, high shear mixer, most commonly the "Cowles' Dissolver" type is required. Introduction of good quality powder to mixing vat through a pneumatic conveying blow line ensures sufficient dispersion. Also inline solid-liquid blenders of Tri-blender type are used for mix preparation.

• After hydration time of about 15 min during which the dispersed powder completes its dissolution, the sugar can be added and dissolved (agitation stopped to minimize air incorporation and to save power). During sugar addition, mix is heated to maintain temperature at 40-50°C. Then previously melted fat (50 °C) is added while mixing and the mix is further heated to 50-60°C to facilitate filtration.

• When full cream powder is used, highest possible level of TS (52%) is used at mixing stage to achieve maximum economy.

12.3 Filtration

Filtration is carried out on duplex-type filters using a range of filter materials to remove extraneous material from milk prior to homogenization.

12.4 Preheating

Preheating to homogenization temperature, 50-60°C is done. Higher temperature adversely affects heat stability of recombined evaporated milk.

12.5 Homogenization

From several systems, high pressure unit using shear type valve is commonly used for recombined EM preparation. Usual pressures are in range of 140 – 175 kg/cm² for first stage, and 35 kg/cm² on second stage. Increasing the pressure in first stage may reduce heat stability of protein. Prior to homogenization, recombined SCM is deaerated but it may not be followed if vacuum cooling is applied at the end.

Homogenization of recombined SCM is usually carried out at a temperature range of 50-60°C in single stage (70 kg/cm² / 7 MPa) prior to pasteurization. However better control of product viscosity is possible if two stage homogenization (pressure up to 105 kg/cm² /10 MPa) is done after pasteurization. Use of full cream milk powder requires low pressure (2.8-0.7 MPa) two stage homogenization to control viscosity. Unhomogenized product gives high initial viscosity and accelerated rate of age thickening.

12.6 Pasteurization and Cooling

Recombined EM may be heated to homogenization temperature or pasteurized to extend its storage life and cooled to < 5°C. Pasteurization conditions used for recombined SCM vary in range of 80-90°C for 30 sec to 2 min. Use of 91°C for 30 sec gives adequate safety to product from yeasts without adverse effect on color or flavour. However, holding the product at higher temperature for longer period gives discoluration due to Maillard reaction.

The recombined SCM must be handled aseptically here on. Cooling in plate heat exchanger up to 50°C and then by vacuum cooling to 30°C is carried out.

12.7 Lactose Seeding

In recombined SCM, formation of greatest number of small crystals (<10 µ) is required to control large crystal formation giving sandy mouth feel. Addition of very finely ground seed lactose powder (300 mesh, ~ 1-10 µ size) with high microbiological standards is done at a rate of about 0.05 % either as powder or as slurry. Alternatively, skim milk powder or sweetened condensed milk of previous batch can be used as seed material. After crystallization, further cooling under vacuum leads to about 10 % moisture evaporation and hence suitable allowances are made in original mix.

12.8 Storage, Stabilization and Standardization

The viscosity of EM is determined and solids level is adjusted to required specification. Addition of phosphates, citrates and sometimes calcium is required to stabilize proteins against heat effects during sterilization but their use should be kept to a minimum to avoid adverse effect on flavour of the product. The level of stabilizer is determined by taking trials and then whole batch can be standardized for canning.

12.9 Canning and Sterilization

The essentials in canning recombined EM are clean, well made cans, minimum foam, appropriate head space, correct weight and efficient closure. Any type of sterilization method employed should be efficiently performed and rapid cooling should follow to avoid any defect.

Careful packaging to avoid any contamination from external sources is required for recombined SCM along with minimum headspace while filling to prevent growth of any surviving moulds.