Me and My Celery

Me and My Celery……

Recently I decided to have a celery snack.  The celery from my refrigerator was not crisp, it was limp. 

 After finding my “how to do everything in the kitchen” book, I soaked the celery in freshwater overnight.  The next morning it was crisp.

WHY does the celery become crisp after soaking over night?      When I placed the celery in water,  the higher vapor pressure of the liquid water  refilled the celery’s cells.    Natural chemicals inside the cells (carbohydrates, lipids, proteins, etc. ) cause reduced water vapor pressure.   The vapor pressure difference is the driving force for refilling the cells with water. In biologic terms, the filled cells are called the turgid.   

Raoult (1882) discovered the effect of solutes on vapor pressure.    He argued that the solutes block some of the water molecules to effectively lower water vapor pressure.

Why did the celery become limp in the refrigerator?     Refrigerators are designed to remove heat and moisture from items stored on its shelves.   Cold refrigeration coils remove moisture.  

Next, I purchased some crisp lettuce.  I put a few pieces into a salt water solution and a few pieces into a sugar water solution on the kitchen counter.   I left both overnight. Nothing happened.   Both samples were crisp the next morning.   I added more salt and more sugar.   The next day, the celery in the salt water was limp while the sugar water solution had no effect on the celery.

I added much more sugar to the sugar solution.   Finally, the celery became limp in the sugar solution also.  

Back to Raoult’s Law.    The reduction of vapor pressure due to the sugar and salt is related to the mole concentration.  180 grams of sugar make one gram-mol.  Only 58 grams of salt are required for a gram-mol.  A smaller quantity of salt produces a larger vapor reduction than the same quantity of sugar.  See  Table 1.     Three tablespoons of salt caused a salt water concentration of 8% mol/mol  and the same quantity of sugar only a 3% concentration.  The maximum concentration for room temperature water is 8% for salt and 5% for sugar.

Table 1    for 1 cup water at room temperature, Celery soak overnight

Solute  #          tablespoons     result   mole %

Salt                  1                “                             no effect              3 %

Sugar               1                “                             no effect              1 %

Salt                  3                “                             limp celery          8 % *

Sugar               3                “                             no effect              3 %

Salt                  5                “                             limp celery          8 % *

Sugar               5                “                             limp celery          5 % *

Salt                  saturated         molar concentration =    8 % *

Sugar               saturated         molar concentration =    5 % *

Conclusion:     Higher concentrations of salt and sugar reduce the water vapor pressure and draw water from the celery cells.   Vapor pressure difference is the driving force for adding or removing water from plant cells.

©   Larry Howlett     HTMD Engineering     2018

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