Oxidative stress is mediated by free radicals. Free radicals are generated when the weak bond between atom splits. They are very unstable and highly reactive. Free radicals cause DNA damage, lipid peroxidation and protein damage. Due to their reactivity, they in turn generate free radicals from stable atoms. This chain reaction continues and leads to irreparable damage to the cells. Damage to cells accumulates over time and thus the effects of free radicals' can be seen more and more with aging. Anti-oxidants are compounds that neutralize free radicals by donating and electron. Thus the antioxidants protect the cells against the free radical mediated damage. Anti-oxidants are found very frequently in nature.

Flavanoids are compounds that are found in many fruits and vegetables such as apples and blueberries. 'Flavanoid' is a broad term used to describe certain types of water-soluble plant pigments.  Each flavanoid has a fifteen-carbon ring structure.  Based on the saturation or unsaturation of C ring and hydroxylation pattern, flavanoids are divided into flavones, flavonol, flavanone, flavanol and anthocyanin.  Flavanoids are known to possess very high anti-oxidative properties and free radical scavenging activities.  All fruits and vegetables contain different flavonoids in different combinations.  The three cultivars of apples studied in this experiment contained had a variety of flavonoids among them. These flavonoids may contain neuroprotective properties as well.  By gathering free radicals, the flavonoids are able to reduce the levels of oxidative stress within the body.

Many different neurodegenerative diseases are caused by oxidative stress, such as Parkinson's disease and Alzheimer's disease.  Parkinson's disease occurs due to the loss of striatal dopaminergic neurons from oxidative stress. Oxidative stress is also implicated in Alzheimer's disease.  The neuroblastoma cell line, SH-SY5Y has been used as an in vitro model to study neurodegenerative disease.  SH-SY5Y cell line was originally derived from the metastatic tumor from a neuroblastoma patient. This cell line has been used extensively in vitro to study the molecular mechanisms involved in oxidative stress.

An MTT Assay is a cell viability test.  In metabolic active cells, the enzyme succinate dehydrogenase breaks down MTT to purple blue formazan particles.  All viable cells treated with MTT turn purple blue in colour.  All treated cells that have died, are unable to break down the MTT and therefore, their colours do not change. The rate of colour change, which is a measure of the amount of formazan particles, can be measured by reading the absorbance using a plate reader. The absorbance was read using a plate reader that assigned a numerical value to each well of a 96 well plate including the blank well. The viability was expressed as percentage of control.