Crop Process Engineering

Lesson 2. Methods and Principles of Food Processing (Part I)

The fruits and vegetables are comparative higher value than cereals and more perishables. Losses in the fruits and vegetables are high and chances to reduce the waste and enhancing the employability through post harvest processing are more. The processing includes pre-processing of fruits and vegetables before these are fit to final conversation into processed foods. The food preservation and processing industry has now become of a necessity than being a luxury. It has an important role in conservation and better utilization of fruits and vegetables. In order to avoid the glut and utilize the surplus during the season, it is necessary to employ modern methods to extend storage life for better distribution and also processing techniques to preserve them for utilization in the off season on both large scale and small scale.

 Food processing therefore refers to the application of techniques to foods in a systematic manner for preventing losses through preservation, processing, packaging, storage and distribution, ultimately to ensure greater availability of a wide variety of foods which would help to improve the food intake and nutritional standards during the periods of low availability. The main objective of fruits and vegetables processing is to supply wholesome safe, nutritious and acceptable food to consumers throughout the year.

 Prerequisites of preservation are cleaning, grading and sorting as per maturity.

 1. Cleaning: It is a unit operation in which contaminating materials are removed from the food material and separated to leave the surface of the food in a suitable condition for further processing. In vegetable processing, blanching also helps to clean the product. In addition, the early removal of small quantities of food contaminated by micro-organisms prevents the subsequent loss of the remaining bulk by microbial growth during storage or delays before processing.

 Wet cleaning: Wet cleaning is more effective than dry methods for removing soil from root crops or dust and pesticide residues from soft fruits or vegetables. It is also dustless and causes less damage to foods than dry methods.

 Dry cleaning: Dry cleaning procedures are used for products that are smaller, have greater mechanical strength and possess lower moisture content (for example grains and nuts). After cleaning, the surfaces are dry, to aid preservation or further drying. The main groups of equipment used for dry cleaning are;

- air classifiers

- magnetic separators

- separators based on screening of foods

 Removing contaminants and foreign bodies: Physical separation of contaminants from food is possible when the food has regular well defined shape. e.g. round foods are separated from contaminants by exploiting their ability to roll down in inclined, upward moving conveyor belt.

2. Sorting: Sorting is the separation of foods into categories on the basis of a measurable physical property. Like cleaning, sorting should be employed as early as possible to ensure a uniform product for subsequent processing. The four main physical properties used to sort foods are size, shape, weight and color.

 Shape and size sorting: The particle size distribution of a material is expressed as either the mass fraction of material that is retained on each sieve or cumulative percentage of material retained. Size sorting is the separation of solids into two or more fractions on the basis of differences in size.

 Color sorting: Small particulate foods may be automatically sorted at high rates using microprocessor controlled color sorting equipment. Particles are fed into chute one at a time. The angle, shape and lining material of chute are altered to control the velocity of pieces as the pass a photo detector. Photo detectors measure the reflected color of each piece and compare it with preset standards, and defective foods are separated by a short blast of compressed air.

 Weight sorting: Weight sorting is more accurate than other methods and is therefore used for more valuable foods. Aspiration and flotation sorting use differences in density to sort food and are similar in principle and operation to aspiration and flotation cleaning.

3. Grading: This term is often used interchangeably with sorting but strictly means ‘the assessment of overall quality of a food using a number of attributes’. Grading is carried out by machines or operators who are trained to simultaneously assess a number of variables.

 The basic principles of preserving bio materials and their respective methods are given in the following table.

Sr. No.	Principle	Method
1	Reducing temperature so that deteriorating reactions occurring within the bio materials are minimized	Evaporating Cool ChamberRefrigerated StorageCold storageFreezing
2	Creation of an environment of gases such deteriorating bio chemicals reduce their activities.	Hermetic StorageControlled Storage and Modified Packaging
3	Reduce chemical potential of water (water activity reduction) Application of heat Addition of solute so that water is strongly bound	Drying and Dehydration, Convective microwave drying, pasteurization and sterilizationOsmo-dehydration
4	Production of  chemicals through fermentation which will be detrimental to the microbes causing food spoilage	PicklingControlled fermentationAerobic, Anaerobic
5	Innovative methods	Irradiations, Dielectric, infrared and Ohmic heating