There are number of factors which affect the qualitative and quantitative characteristics of the essential oils. Some of them are listed as bellow;
1. Agricultural factors
2. Storage condition of the herb
3. Comminution of the charge
4. Drying of plant material prior to distillation
5. Quantity of the charge
6. Type of hydrodistillation
7. Influence of distillation period
8. Influence of other constituents
9. Effect of temperature/pressure in the still
10. Problem of condensation of constituents
11. pH of the distillation water
12. Rate of distillation
13. Redistillation of hydrosol (addition of recovered oil to the direct distilled oil)
14. Steam quality
15. Design of distillation units
A book chapter can be written on each of the above factors.
besides of 15 factors which other researcher mentioned on Jun 27, 2014, i can list some else :
1-the ratio of organs in biomass such as leaf, stem, flower, etc
2- maturity stage of organs
3- purity of variety
4- season of harvesting
5- time of harvesting
.........
You may read some of my articles uploaded in Researchgate on the subject and understand how different factors mentioned above (+ admixture of weeds with the biomass being distilled) affect different aromatic or essential oil yielding crops.
Dear Patel
If you are using a field level steam distillation equipment, proper packing of the aromatic plant biomass in the distillation tank is important. Improper packing results in formation of steam channels and loss of essential oil (cutting the biomass into small pieces helps resolve this issue and improves oil recovery by esposing more volume to the action of steam). Some crops agglutinate or form lumps during distillation (rose-scented geranium), this results in poor recovery. For such crops cohobation or recirculation of condensate water is recommended. The water circulation in the condenser is also important. The condensate should be warm and not hot. Any visible steam escaping from the condenser is an indication of improper condensation and loss of oil. Steam temperature, pressure, amount etc also influence the oil yield. You may read Guenther's classical books on essential oils for an understanding on the principles and mechanisms of distillation. You may also personally contact Er. Garikapati Babu (the first respondent to your question), who is an expert in this field.
No precaution, unless you want to avoid the essential oil oxidation you must work in a nitrogen atmosphere.
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