How the milling of an apple affects the flavor and color profile of its juice and how we can avoid these effects and give recommendations

Apple Malus domestica is a common fruit consumed widely. It is a rich source of minerals, monosaccharides, fibers, and active compounds i.e. phenolic compounds and vitamin C. Apple juice extraction consists of several processes i.e. preparation, milling, pressing, clarification, filtration, concentration, pasteurization, additives, and finally packaging. Apple juice, cider has higher phenolic contents thus more chances of oxidation.   

Impact on Flavor of Apple Juice

Apple juice contains hundreds of compounds with are directly and indirectly affect the flavor and aroma profile of juices. Containing esters, alcohols, and aldehydes, and other compounds called damasceone, HMF, and furfural. During the milling of apples, undesirable compounds are formed due to enzymatic changes. Unsaturated fatty acids conversation takes place to form aldehydes and alcohols, through a series of lipoxygenase reactions, resulting in C6 compounds being synthesized. (ADH meditated this reaction). Esters are broken down into their successive components. Some compounds like hexanol and butanol provide flavor characteristics in apple juices Although some of the aroma-providing compounds are still unknown. Millard reaction affects the color and flavor of the juice adversely even during storage. Concentrated juices are more prone to undesirable changes if the temperature is above 5C. Poor storage conditions lead to compounds like HMF and furfural. A study has shown that under poor storage conditions of apple juices at 20C, HMF contents are high up to 100 mg per liter. Thus concentrated juices are stored in proper conditions to avoid such circumstances. However, polyphenols impart an astringency effect. Thus excess polyphenols result in undesirable flavor (astringent effect). Astringency and bitterness mainly depend on acidity and cultivar. Bitter apples have more flavan-3-ols contents than non-bitter apples. Juices have less degree of polymerization than fruit pulp itself because procyanidins extraction and solubility decreases for large polymers in juices.  

Avoid

• ·       Millard reaction is inhibited by the addition of Sulphur dioxide. It will effectively stop the very first reaction between the Glucose molecule and charged lysine. Thus as a result further undesirable compounds will not be formed. i.e. HMF, Furfural, Melanoidin
• ·       To prevent the Millard browning storing the juice at lower temperatures. i.e. Rapid cooling of juice is beneficial after heating

Impact on Color of Apple Juice

Apple Juice color relate to PPO on the phenolic contents. The final color of juice depends on the chlorogenic acid. Thus in coupled oxidation reaction secondary oxidation products are formed. Colored oxidation products are extracted from the juice during pressing which increases the golden color of the apple juices. The visual color of juice also depends on light scattering through cloud particles which is considered as turbid juice or hazy juice. Quinones formed during the oxidation of polyphenols are mainly responsible for brown pigments. Research has shown that heating of apple juice forms slight color products possibly due to precipitation bonded quinones and proteins. Thus presence of polyphenol causes turbidity in juices. These include not sediment substances, pectin, hemicellulose, proteins, and solubilized starch. Research has shown that the more phenolic content more the degree of turbidity. Apple primarily contains phenolic acids, catechins, procyanidins, and dihydrochalcones, etc. Moreover, the composition may vary among different cultivars. Complex formation between pectin and procyanidins affects the stability of the apple juice during storage and processing. After pressing apple juice is of pale color and after oxidation of polyphenols color of apple juice changes to yellow-brown color. Chlorogenic acid is mainly called phenolic acid.  However, other compounds like phloridzin and epicatechin impart yellow-orange color in apple juices. A study has shown that these compounds impart 25% of color in juice. Procyanidins are responsible for the other 50% and give a brown color to the apple juices. However, there is a possibility that the chlorogenic acid might break into its further components i.e. caffeic and quinic acid. This will happen in extreme conditions by depectinizing enzymes. Its level may fall up to 10 mg per liter which is undesirable.

Avoid

• ·            Some undesirable color changes may be reduced by the addition of salt i.e. Ascorbate
• ·            Reducing the temperature below 2°C prevents the activity of polyphenol oxidase PPO enzyme
• ·           High Acidic conditions would also be beneficial as they will lower the pH up to 3
• ·      Reducing the substrate concentration will also be helpful. i.e. Tanning process will mask the active sides of procyanidin thus reducing its activity
• ·          Formation of big size polymers i.e. Polymerization
• ·       Oxidized products removal which eventually causing color changes. i.e. by adsorption onto the pulp
• ·       Avoid procyanidins to produce insoluble compounds by adding enzymes. i.e. amylolytic and pectinolytic. Thus haziness or cloudiness is avoided in the juice and thus forms a clear apple juice

Recommendation

In order to avoid the non-enzymatic browning (PPO polyphenol oxidases) different methods are proved beneficial i.e. chelating agents react with metal thus forming stable and water soluble compounds (EDTA, Carboxylic acid), Acidulates that will lower the pH (Citric and Malic acids). An antioxidant will prevent binding the oxygen (Ascorbic Acid and Maclurin) etc. Moreover, using unripe grape and lemon juice is also helpful in preventing undesirable reactions and complex brown pigments.