Editing Orange juice (section)

==Physical and chemical properties==
===Molecular composition===
[[File:Orange juice UHPLC UV chromatogram.png|thumb|right|UV 280 nm chromatogram after UHPLC separation of commercial orange juice, showing, amongst other peaks, [[narirutin]] and [[hesperidin]].]]

On a molecular level, orange juice is composed of organic acids, sugars, and phenolic compounds. The main organic acids found in orange juice are citric, malic, and ascorbic acid. The major sugars found in orange juice are sucrose, glucose, and fructose. There are approximately 13 phenolic compounds in orange juice including hydroxycinnamic acids, flavanones, hydroxybenzoic acids, [[hesperidin]], [[narirutin]], and ferulic acid.<ref>{{cite journal |last1=Kelebek |first1=Hasim |last2=Selli |first2=Serkan |last3=Canbas |first3=Ahmet |last4=Cabaroglu |first4=Turgut |title=HPLC determination of organic acids, sugars, phenolic compositions and antioxidant capacity of orange juice and orange wine made from a Turkish cv. Kozan |journal=Microchemical Journal |date=March 2009 |volume=91 |issue=2 |pages=187–192 |doi=10.1016/j.microc.2008.10.008 }}</ref>

====Composition of the cloud====
The cloud is the portion of suspended particles that range in size from 0.05 micrometers to a few hundred micrometers in orange juice. The cloud is responsible for several sensory attributes in orange juice including color, aroma, texture, and taste.<ref name=":1">{{cite journal |last1=Kyriakidis |first1=N.B |title=Use of pectinesterase for detection of hydrocolloids addition in natural orange juice |journal=Food Hydrocolloids |date=November 1999 |volume=13 |issue=6 |pages=497–500 |doi=10.1016/S0268-005X(99)00034-X }}</ref> The continuous medium of the cloud consists of a solution of sugars, pectin, and organic acids while the dispersed matter is formed through cellular tissue comminuted in fruit processing.<ref name=":3">{{cite book |doi=10.1007/978-0-387-75430-7_29 |chapter=Zeta-Potential as a Way to Determine Optimal Conditions During Fruit Juice Clarification |title=Food Engineering: Integrated Approaches |series=Food Engineering series |year=2008 |last1=Filippi |first1=M. V. |last2=Genovese |first2=D. B. |last3=Lozano |first3=J. E. |pages=391–397 |isbn=978-0-387-75429-1 }}</ref> Specifically, the cloudiness of the juice is caused by pectin, protein, lipid, hemicellulose, cellulose, hesperidin, chromoplastids, amorphous particles, and oil globules.<ref name=":2">{{cite journal |last1=Mizrahi |first1=Shimon |last2=Berk |first2=Zeki |title=Physico-chemical characteristics of orange juice cloud |journal=Journal of the Science of Food and Agriculture |date=May 1970 |volume=21 |issue=5 |pages=250–253 |doi=10.1002/jsfa.2740210508 |bibcode=1970JSFA...21..250M }}</ref> In particular the chemical composition of the cloud consists of 4.5-32% pectin, 34-52% protein, 25% lipids, 5.7% nitrogen, 2% hemicellulose, 2% ash, and less than 2% cellulose.<ref name=":1" />

=== Physical structure ===
Orange juice is a suspension that consists of heterogeneous particles in a clear serum. A serum is the clear [[precipitation (chemistry)|supernatant]] after the precipitation of the cloud through [[centrifugation]]. The previously mentioned cloud makes up a large part of the suspension.<ref name=":1" />

If the suspension in orange juice is not stable, the cloud particles can [[flocculation|flocculate]] which causes the suspension to physically decompose. The cloud can break apart and the citrus juice will clarify if the suspension becomes unstable.<ref name=":2" /> The activity of pectin methyl esterase increased the interaction between pectin and cloud proteins, which led to protein-pectin flocculation. The insoluble material of the cloud clumps in conditions above {{cvt|70|C}} and at a pH of 3-4 at which proteins coagulate and flocculate. Cloud flocculation is enhanced at pH 3.5 and can result in clarification, which is undesirable in orange juice.<ref name=":5">{{cite thesis |last1=Ackerley |first1=Jennifer Lynn |title=Clarification of Valencia orange juice is influenced by specific activity of thermolabile pectinmethylesterase, inactive PME-pectin complexes and the changes in serum soluble components |date=August 2002 |hdl=10724/29473 |hdl-access=free }}</ref>

The [[suspension (chemistry)|suspension]] is unstable when the zeta potential is less than 25 mv in magnitude. [[Zeta potential]] is a measure of the magnitude of electrostatic forces between particles, which affect repulsion, and attraction between particles. A low zeta value signifies that the repulsive forces will not be able to overcome [[Van der Waals force|Van der Waals attractions]] between cloud particles and thus begin to agglomerate. Agglomeration of cloud particles will prevent free flow characteristics, which is essential in the juice. A high zeta potential will inhibit particle-particle agglomeration and maintain the free flowing nature as well as uniform dispersion in orange juice.<ref name=":3" />

The oil globules adsorbed to the cloud particles stabilize the suspension by decreasing the average density of particles to bring it closer to that of the serum. However, large amounts of oil can be problematic as they cause complete breakdown of suspensions by causing cloud particles to float to the surface. The particles in the cloud have a negative charge that decreases with decreasing pH. In accordance with cloud stability, the hydration of particles is more significant than their electrical charge.<ref name=":2" />

==== Heat treatment ====
When orange juice is heat treated there is an increase in the number of fine particles and decrease in that of coarse particles. The fine particles in particular are responsible for the appearance, color, and flavor of orange juice. Heat treatment plays a vital role on pulp volume, cloud stability, serum [[turbidity]], and serum [[viscosity]]. Heat treatment stabilizes the cloud through enzyme inactivation and enhances the turbidity of a stable cloud formation. The increase in serum viscosity is due to the extraction of pectic substances into the serum. Based on Stokes' law, the increase in serum viscosity is the cause for the enhanced cloud stability. In relation to pulp volume, the pulp from heated juices was finer and more compact than unheated juice pulp, which was voluminous and fluffy.<ref name=":2" />

==== Properties of pulp ====
In orange juice, pulp is responsible for desirable flow properties, taste, flavor, and [[mouthfeel|mouth feel]]. However, pulpy orange juice precipitates based on a rate dependent on the diameter, density, and viscosity of the suspended particles as well as the suspending juice. In order to remain suspended in orange juice, pulp particles must have appropriate particle size, charge, and specific gravity. Depending on type of processing method, the size of pulp particles ranges from {{convert|2|–|5|mm|sp=us|sigfig=1}}. Those that are smaller than {{cvt|2|mm|sigfig=1}} are known to be more stable, so it is beneficial to reduce the size of particles by incorporating hydrocolloids to the juice product. Hydrocolloids would decrease the rate of sediment formation and decrease the falling rate of pulp particles.<ref name=":4">{{cite journal |last1=Bagheri |first1=Leila |last2=Mousavi |first2=Mohammad E. |last3=Madadlou |first3=Ashkan |title=Stability and Rheological Properties of Suspended Pulp Particles Containing Orange Juice Stabilized by Gellan Gum |journal=Journal of Dispersion Science and Technology |date=2 September 2014 |volume=35 |issue=9 |pages=1222–1229 |doi=10.1080/01932691.2013.834422 |s2cid=95127674 }}</ref>

==== Hydrocolloids ====
Hydrocolloids are long-chain polymers that form viscous dispersions and gels if dispersed in water. They have a number of functional properties in food products including emulsifying, thickening, coating, gelling, and stabilization. The main reason hydrocolloids are used in foods is their capability to modify the rheology of food systems. Hydrocolloids impact viscosity through flow behavior and mechanical solid properties like texture.<ref>{{cite book |doi=10.5772/32358 |chapter=Hydrocolloids in Food Industry |title=Food Industrial Processes - Methods and Equipment |year=2012 |last1=Milani |first1=Jafar |last2=Maleki |first2=Gisoo |isbn=978-953-307-905-9 }}</ref> Some common hydrocolloids that are used to stabilize juice products include gellan gum, sodium carboxymethylcellulose, xanthan, guar gum, and gum Arabic. The aforementioned hydrocolloids are generally used in the production of imitation orange juices and are often referred to as synthetic hydrocolloids. Pectin is the hydrocolloid found in natural orange juices.<ref name=":4" />

==== Properties of pectin ====
{{Main|Pectin}}
Pectin is the soluble polymeric material in the pulp of oranges, which contains 75% of carboxyl of [[arabinose]] and [[galactose]]. Pectic compounds are complex [[polysaccharide|heteropolysaccharides]] in that their chemical composition includes a chain structure of axial-axial α-1.4-linked [[D-Galacturonic acid|d-galacturonic acid]] unit along with blocks of [[rhamnose|L-rhamnose]] regions that have side chains of arabinose, galactose, and [[xylose]]. Pectin methyl-esterase is the enzyme responsible for hydrolyzing carboxymethyl esters and liberating free carboxyl groups and methyl alcohols. The free carboxyl groups interact with cations to form insoluble pectic acid divalent metal ion complexes. These metal ion complexes precipitate in the juice and carry all the colloids in orange juice with it. The enzyme would [[flocculation|flocculate]] the cloud and clarify the orange juice. Thus, in order to keep the orange juice cloud intact, it is vital to inactivate [[pectinesterase]]. Pectinesterase is inactivated by heating the juice for 1 minute at {{cvt|90|C}}.<ref name=":1" />

==== Interactions of pectin ====
The solution behavior of pectin is strongly influenced by a number of factors including hydrogen bonding, ionic character, and hydrophobic character. Hydrogen bonding is favored when pH is less than pKa while the ionic character is favored when pH is greater than pKa. Ionic character relies on free carboxyl content, the presence of cations, and is favored at a high water activity. Charge-charge repulsions along with the presence of neutral side chains are essential in inhibiting intermolecular association among pectin molecules. The methyl ester content in orange juice determines hydrophobic character, which is favored at low water activity.<ref>{{cite journal |last1=Klavons |first1=Jerome A. |last2=Bennett |first2=Raymond D. |title=Preparation of Alkyl Esters of Pectin and Pectic Acid |journal=Journal of Food Science |date=May 1995 |volume=60 |issue=3 |pages=513–515 |doi=10.1111/j.1365-2621.1995.tb09815.x }}</ref>

There is a specific interaction between pectin and hesperidin through the sugar moieties in the hesperidin molecule. Through acid hydrolysis, the rhamnose and glucose sugar moieties are removed from hesperidin, which breaks the interaction between hesperidin and pectin. Hydrogen bonding plays a role in the specific interaction of neutral sugars of pectin and the sugar moiety of hesperidin. A polymer that has a high structural content of neutral sugar branches interacts with hesperidin more tightly and strongly than that of a low content of neutral sugar branches. The interaction between pectin and hesperidin is one of the factors that enable the colloidal suspension in orange juice to be stable.<ref name=":5" />