Types of indices

1. Visual indices:

a) Size and shape:

• Maturity of fruits can be assessed by their final shape and size at the time of harvest.
• Fruit shape cane be used in some instances to decide maturity.
• Eg: Banana: Angular shape changes to round.

b) Colour:

• The loss of green color of many fruits is a valuable guide to maturity.

2. Physical indices:

a) Firmness:

• As fruit mature and ripen, they soften by dissolution of the middle lamella of the cell walls. Eg: Apple, pear, peach, pulm, etc.
• Penetrometer measure the pressure necessary to force a plunger of specified size into the pulp of the fruit and is measured in pounds and kilograms force.

b) Specific gravity:

• As fruit mature, their specific gravity increases.
• This parameter is rarely used in practice to determine when to harvest a crop.
• Fruit or vegetable is placed in a tank of water; those that float will be less mature than those that sink.

3. Chemical Measurement:

• The total soluble solids (TSS) of the fruit can be measured with refractometer, which indicate the harvest maturity of fruits.
• TSS/TA is better to judge maturity as it gives the sugar acid blending of product.

a) Total Soluble solid:

• Can be measured by using hand refractometer.
• The refractometer measures the refractive index, which indicates how much a light beam will be slowed down when it passes through the fruit juice.
• For larger fruits, these should be cut from stem to blossom end and to the center of the fruit to account for variability in TSS from top to bottom and inside to outside of the fruit.
• The fruit tissues should be macerated thoroughly in pastle motor and then from the macerated pulp, the juice is extracted by passing through muslin cloth.
• A drop of juice is the put on the prism of the refractometer and TSS content cane be read directly on the scale.

b) Titratable acidity:

• It can be determined by titrating a known volume of juice with 0.1 N NaOH to end point.
• The mililiters of NaOH needed are used to calculate the TA.

TA = (ml NaOH x N (NaOH) x acid meq. Factor x 100)/ Juice titrated.

4. Physiological method:

a) Respiration rate:

• Particularly on itagawaic fruits can accurately pin point the most appropriate time of harvest as there is climacteric rise in respiration.

b) Internal ethylene evolution:

• Like respiration rise, climacteric fruits also have ethylene rise.

c) Volatile production:

• Apple: Ethyl-2-methyl butyrate
• Banana: Eugenol
• Grapefruit: Nootaketone
• Lemon: Citral
• Orange: Valencene