Vegetables and pH of water

Addendum Nov. 2, 2011:  Here is another good set of experiments and explanation on keeping greens green!:
http://www.whycook.org/2011/09/green-vegetables-green/

The pH of the water you cook with can affect the quality of the final boiled vegetables (baking or stir-frying do not count here since the vegetables are not surrounded by water) I have noticed this most in the leafy greens (eg. kale and collards) and other green veggies. Kale's vibrant green colour can be preserved better if you add a tiny bit of vinegar to the cooking water especially near the end. 

The cell walls and membranes of vegetables contain proteins, cellulose, hemicelluloses and pectins.

During cooking some of the proteins are denatured by heat and the celluloses and pectins are lost based on the pH of the water – the hemicelluloses are dissolved by basic water and the pectins by more acidic water. 

Denatured and reformed proteins

from: Geoffrey Cooper, The Cell - The Molecular Approach, 2nd ed, p.

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cooper&part=A249

 Pectins aid in giving fruit and veggies solidity – loss of pectin makes them mushy. We use pectin in jam making to create the gell (i nearly spelt that "Jell" - the omnipresence of brand names..) – it does the same thing within the plant cells. 

Cellulose is not easily dissolved – it is a one of the basic building blocks of plants and not digestible by our stomachs (excellent source of fibre though!) - it is a chain of sugar molecules but because of the way it is formed it is very resistant to being broken down by chemicals.

The hemicelluloses and pectins also form parts of the cell walls and they can be dissolved so their loss will make the veggies softer.

Experiment 1:

Cooked green beans in salt water (left) and acidic water - added vinegar (right) for the exact same length of time.

 The beans cooked in the acidic water lost their colour significantly and tasted vinegary (probably used too much vinegar!)

Experiment 2:
Cooked sugar snap beans in salt water (1/2 tsp), baking soda water (1/2 tsp) and vinegar water (1 tsp)

acidic water
the sugar snap beans retained their colour and crispness without colouring the water

basic water
still crisp but outer skin a bit slimy  - flavour still good - water was quite green (removal of vitamins

salt water
colour retained - flavour good - water coloured like the basic water (salt is a bit basic too)

So if you want to keep the veggies crisp add a little bit of vinegar to the water - seems to also help retain the colour and therefore probably the vitamins - although heating any food will break down the vitamins to a certain extent so there is always loss.

Another idea to explore later - do we traditionally use a vinegar based dressing on salads to aid the digestion in addition to tasting really nice??

Reference:
Harold Mcgee 1984. On Food and Cooking, p.147-180.