I appreciate to receive any publications on the subject.

Dear Prof. Rajeswara Rao,

As you know, the pH will also change according to the type of the distillation used besides the diversity of locations. The basic definition of oil is that it is a triester of glycerol so when the ester is hydrolyzed by water & heat, then it will give carboxylic acid & alcohol. The acid itself will catalyze further hydrolysis of the acidic type. However, both hydrolysis steps will soon reach a state of equilibrium which renders their effect to be very limited upon a yield which is often low. The little decrease of yield is accompanied by a slight decline in quality.

The low yield, the lower than optimal quality, and the change of quality upon "wrong" storage did not have a significant effect on the high prices of essential oils, in general. Few days ago, I saw an advertisement in the internet for sage oil & the company offered to sell 11 ml. of this oil for 26 $. A colleague told me that the price of red rose oil is much higher than that. I am  convinced, now, that research into this area is more worthy than I thought before.

Unfortunately, the reference (in the link) is most probably known by you because your name is given in the 7th line of the introduction BUT I think it will benefit other RG colleagues.

http://www.academia.edu/5578671/Water_balance_in_Hydrosol_Production_Via_Steam_Distillation_Case_Study_using_Lavandin_Lavandula_x_intermedia

Dear Prof. Matar

Thanks for your elaborate and informative contribution. When aromatic crops are hydro-distilled it is known that artifacts are produced due to the hydrolysis of esters. In addition, the aromatic plant biomass contains other water-soluble chemical components some of which are thermo-sensitive and undergo chemical changes. These chemicals may alter the pH of the distillation water during the distillation process. My question is, is there any research on the effect of initial pH of the water used for distillation? I have published several papers on the yield and composition of many essential oils but never addressed this aspect nor I came across any paper dealing with the aspect. 

I have the copy of the paper you kindly attached as I have worked on hydrosols.

Dear Prof. Russo

Thanks for your informative post and the publication.

This is an interesting publication. Unfortunately I do not have a copy of the manuscript.

Shankaranarayana, K.H. and Kamala, B.S. 1989. Six new essential oils from waste plant material. Indian Perfumer 33: 40-43.

I do not have even the abstract. As far as I remember the authors re-distilled previously distilled aromatic biomass of a plant in Clevenger apparatus by adding an acid to the water in the Clevenger distillation tank and got 6 different kinds of essential oils with different compositions. If you can get a copy of the paper it will be good reading.

The acidification of the distillation water prior to re-distilling the distilled biomass resulted in the recovery of essential oil. The conversion of glycoside-bound volatiles into free volatiles that can be distilled through acidification of distillation water could be one of the reasons for the result.

I get through Dr.Nizar paper and i see its interesting one

I feel that previous answers do not meet your question. Essential oils are never triglycerides (these are fatty oils). And your question was to the pH change of local water, so the change of the pH is not too big, may be from 3 to 8 depending on the source and quality of the water. Generally, only those components in a water/essential oil mixture participate in steam distillation which are not in solution.Glycosides are usually well water soluble; acidic components form salts in alkaline water, alkaline components form salts in acidic water.

An acidic pH of 3 does not split the ß-glycosidic bond; terpene- or phenylpropanoid glycosides are thus stable. Only few essential oil components are acidic; if so they are not very acidic (carboxylic acids, phenolics). Their dissociation is more or less reduced in an acidic surrounding; consequently they are better steam distilled. On the other hand, in a slightly alkaline water the acidic components form salts and may be hindered to participate in steam distillation. I do not know alkaline constituents of essential oils (which  -if realized-  form salts in an acidic water and thus are so well soluble, that they are not steam distilled).  

For a usual steam distillation the pH of the water should be around 5, which is realized in most waters. Another problem may be waters rich in Ca ions. these may form complex compounds with polyphenolic aromatics, agin negatively influencing the steam distillation.

Essential oils are usually distilled from parts of plant (including the leaves, flowers, bark and roots). These oils may not contain triglycerides but ester content cannot be ruled out. The essential oils have a concentrated aroma & never go rancid.  Essential oils are not usually applied “neat” (or undiluted) but they are blended with carrier oils because it is necessary to dilute the neat oil. Most carrier oils or fixed oils usually have minimal aroma & minimal color.  This is due to being pressed from the fatty portions of botanicals such as the nuts & seeds.  Some carriers go rancid with time and therefore have a shorter life time. Here comes the pH effect in more pronounced way & this is a question that has not been settled by research (as far as I know).

On improper storage, essential oils get polymerized forming long-chain compounds and become useless. Therefore proper storage of essential oils is essential.

I agree with you dear Professor Rajeswara Rao & this example shows:

Eugenol (extracted component of essential oils of e.g. cloves, nutmeg, cinnamon, basil and bay leaf) is also called  " 4-Allyl-2-methoxyphenol " & belongs to the phenylpropanoids class of chemical compounds. These compounds are susceptible to undesirable polymerization.

Dear Prof. Adolf

You are right that the answers did not give a direct reply. I agree that during distillation the pH change of distillation water will not be significant and in some cases there may not be any change at all. But my question is about the pH of the water used for distillation before the start of the distillation process. Let us say we use tap water (in several nations tap water is not treated and may directly come from a bore/tube well or an open well or a pond etc.), distilled water, water from different rivers or salt water or acidic water, what will be effect of the pH of the water used for distilling aromatic crops on the resulting yield/recovery of essential oil and its chemical composition? Put it in other words if I use waters with different pH values (say ranging from 3 to 10 or more) for distilling aromatic crops how this pH will affect the result?

Dear Professor Rao,

thank you for your answer. I now understand more clealry what you want to know. Although I do not have experimental data, I believe that the yield and composition of the essential oil obtained will vary only slightly. The most strong effect will have salted water, as it reduces the solubility of the more or less lipophilic essential oil components - in former German pharmacopoeias there were instructions by which you had to add 1% NaCl to the distillation water in order to optimize the yield. However, when you have a pH of 10, I believe that the yield from Thymus plants (the phenolics thymol, carvacrol as main constituents) will be reduced. 

Please allow me to give my 2 cents comment:

When steam distillation is applied to produce essential oils, the steam may be externally generated or internally generated.  When the steam is externally generated & then directed to the vessel which contains the plant, steam "distilled water" (with pH of ~ 7) will act as a carrier for essential oils. Adding a salt to the steam-generating vessel will cause boiling point elevation for few degrees since in most countries "above sea level", the temperature of steam in the distillation is around 95 0C.

The effect of pH of water becomes more pronounced when steam is internally generated, i.e. when steam distillation becomes co-distillation [placing the plant & the water which will turn into steam in the same vessel].

During teaching organic chemistry lab, we usually adopt co-distillation to cope with the large number of the students & to be on the safe side. When there is research or a scientific exhibition, we usually apply steam distillation of the external generation type.  

Dear Prof. Adolf and Prof. Matar

Thanks for your informative and useful posts. The effect of distillation water pH will differ on different crops as pointed out by Prof. Adolf based on the composition of the essential oil of that crop. In steam distillation method this problem will not arise as the plant material is not in contact with the water.