Oxygen and carbon dioxide

Altering the concentrations of oxygen and carbon dioxide around products can inhibit respiration and increase storage life due to reduced yellowing, decay and softening. 

The previous section discussed the principles of respiration. Respiration follows the equation:    

Carbohydrates + O2      CO2 + H2O + Energy

As products respire they use O2 and release CO2.

This suggests that reducing the concentration of O2 and/or increasing the concentration of CO2 can inhibit respiration and extend storage life. While this can be true, O2 often needs to be reduced to quite low concentrations before respiration is greatly inhibited. The figure below shows a typical relationship between respiration rate (CO2 production) and the O2 concentration. Respiration is not significantly inhibited until oxygen falls below 5%. 

The effect of O2 concentration on respiration rate, measured as CO2 production. Respiration is not greatly inhibited until O2 falls below 5%. However, if O2 falls below 1%, then anaerobic respiration occurs, and CO2 production actually increases. 

Responses to altered atmospheres vary widely between different vegetables. In many cases respiration may be less affected than other factors that impact on storage life. Potential effects include:

  • Reduced production of ethylene.
  • Reduced sensitivity to ethylene, effectively slowing down colour change and ripening in fruit and vegetables.
  • Improved retention of green colour due to reduced breakdown of chlorophyll.
  • Reduced softening due to retention of the pectins that hold cell walls together.
  • Less breakdown due to rots and diseases; growth of some fungal diseases is reduced when CO2 is 10% or more.

Altering the atmosphere around products can also have negative effects. If O2 falls too low then products can turn anaerobic. Anaerobic respiration induces undesirable changes in texture and flavour. It also results in storage reserves (sugars and carbohydrates) being used up many times faster than during normal aerobic respiration. Depletion of reserves can affect flavour and reduce storage life.

Anaerobic conditions can also allow the growth of some human pathogenic bacteria. These may not spoil the product, but pose a significant danger to human health.