Unit Operations in Food Processing

Material quantities, as they pass through food processing operations, can be described by material balances. Such balances are statements on the conservation of mass. Similarly, energy quantities can be described by energy balances, which are statements on the conservation of energy. If there is no accumulation, what goes into a process must come out. This is true for batch operation. It is equally true for continuous operation over any chosen time interval.

Material and energy balances are very important in the food industry. Material balances are fundamental to the control of processing, particularly in the control of yields of the products. The first material balances are determined in the exploratory stages of a new process, improved during pilot plant experiments when the process is being planned and tested, checked out when the plant is commissioned and then refined and maintained as a control instrument as production continues. When any changes occur in the process, the material balances need to be determined again.

The increasing cost of energy has caused the food industry to examine means of reducing energy consumption in processing. Energy balances are used in the examination of the various stages of a process, over the whole process and even extending over the total food production system from the farm to the consumer's plate.

Material and energy balances can be simple, at times they can be very complicated, but the basic approach is general. Experience in working with the simpler systems such as individual unit operations will develop the facility to extend the methods to the more complicated situations, which do arise. The increasing availability of computers has meant that very complex mass and energy balances can be set up and manipulated quite readily and therefore used in everyday process management to maximise product yields and minimise costs.

BASIC PRINCIPLES

If the unit operation, whatever its nature is seen as a whole it may be represented diagrammatically as a box, as shown in Fig. 2.1. The mass and energy going into the box must balance with the mass and energy coming out.

Figure 2.1. Mass and energy balance

The law of conservation of mass leads to what is called a mass or a material balance.

Mass In = Mass Out + Mass Stored

Raw Materials = Products + Wastes + Stored Materials.

Sm_R = Sm_P + Sm_W + Sm_S
(where S (sigma) denotes the sum of all terms).

Sm_R = m_R1 + m_R2 + m_R3 +.....    = Total Raw Materials.

Sm_P = m_P1 + m_P2 + m_P3+ ....    = Total Products.

Sm_W = m_W1 + rn_W2+ m_W3 + ....= Total Waste Products.

Sm_S = m_S1 + m_S2 + m_S3 + ...     = Total Stored Materials.

If there are no chemical changes occurring in the plant, the law of conservation of mass will apply also to each component, so that for component A:

m_A in entering materials = m_A in the exit materials + m_A stored in plant.

For example, in a plant that is producing sugar, if the total quantity of sugar going into the plant in sugar cane or sugar beet is not equalled by the total of the purified sugar and the sugar in the waste liquors, then there is something wrong. Sugar is either being burned (chemically changed) or accumulating in the plant or else it is going unnoticed down the drain somewhere. In this case:

(m_A ) = (m_AP + m_AW + M_AS+ m_AU)

where m_AU is the unknown loss and needs to be identified. So the material balance is now:

Raw Materials = Products + Waste Products + Stored Products + Losses

where Losses are the unidentified materials.

Just as mass is conserved, so is energy conserved in food processing operations. The energy coming into a unit operation can be balanced with the energy coming out and the energy stored.

Energy In = Energy Out + Energy Stored
SE_R = SE_P + SE_W + SE_L + SE_S

where:

SE_R = E_R1 + E_R2 + E_R3 + ….….   = Total Energy Entering
SE_P = E_P1 + E_P2 + E_P3 + ……..   = Total Energy Leaving with Products
SE_W = E_W1 +E_W2 + E_W3 + …......= Total Energy Leaving with Waste Materials
SE_L = E_L1 + E_L2 + E_L3 + ……....   = Total Energy Lost to Surroundings
SE_S = E_S1 + E_S2 + E_S3 + …..….. = Total Energy Stored

Energy balances are often complicated because forms of energy can be interconverted, for example mechanical energy to heat energy, but overall the quantities must balance.

Material & Energy Balances > MATERIAL BALANCES

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