Lecture 5 Flashcards
what do most chemical reactions with complex lipids involve
acyl residues
For thermolytic reactions what are the main sites for reactions
The main reaction site is unsaturated double bonds specifically the unsaturated acyl residues.
Why does a radical intermiadte have to form in order to initiate an oxidative reaction
Because a direct reaction between oxygen molecule and double bonds is thernodynamically unfavorable
what does full and partial hydrogenation depend on?
the cataylyst used
what does fully hydrogenated fat result in
hard and brittle fats which is obvi unwanted
what are the advantages and disadvantages of hydrogenation
- increase in metling and smoking points
- Increase shelf stability and flavor stability
- disadvantages of trans fat or fatty acids
explain in one sentance the different types of oxidation
autooxidation: is a spontaneous reaction influenced by temperature
photooxidation: is a light induced oxidation process which involves a singlet oxygen
enzymic oxidation: is enzyme catalyzed by lipoxygenase
Atmospheric oxidation: results from oxygen in the air and is a direct reaction between oxygen and lipids. It forms free radicals and is incresesd in the presence of light, heat or metal
Explain the steps of atmospheric oxidation
- initiation: hydrogen atom from a fatty acid forms a free fatty acid radical called alkyl radical (L*) This results in doouble bond shifting
which can make cis or trans isomers - propagation: In this step an oxygen is added to the alkyl radical which results in peroxyl radical (LOO*). When you add hydrogen to the peroxyl radical resulting in a fatty acid hydroperoxide (LOOH)
- termination: In this step two radicals forms a nonradical species. This happens when peroxyl and alkoxyl radicals react in atmospheric conditions. When oxygen is low tho alkyl radicals will react with each other fomring fatty acid dimers
explain the phases of atmospheric oxidation
- lag phase: this is the phase where rancidity will not be detected and is signifcant to food process in terms of length
- Exponential phase: this is where oxidation is exponetial and it will produce off aroma compounds very quickly
What happens to oxidation rate when you increase methylene interrupted carbon
increases oxidation
What occurs when lipids are heated and in the presence of oxygen?
Lipids will undergo oxidation resulting in the breakdown of fatty acids and the formation of free radicals. These free radicals can react with protien, DNA, Vitamins and other food parts which could reduce the nutritional quality of the food
what occurs during the frying process
The formation of trans fats. But also there is an exchange between the frying medium and the foods, the oils will absorb 5-35% by weight of the food and foods will release some of their fat components into the oil this results in composition and stability of the fat used for frying to be altered
what is important to control during frying and why?
It is important to control frying temperature, time and oil replacement to optimize sensory qualities of the fried foods as well as to decrease the accumilation of oxidative and thermal spoilage products
Describe and define these two reactions: hydrolysis and saponifcation
- Hydrolysis: Breaks down triglycerides (TAG) into glycerol and free fatty acids.
- Saponification: the formation of a metalic salt of a fatty acid (soap) through the addition of sodium hydroxide (alkaline)
explain what the three chemical indices of lipids determine/measure
- Iodine Value (IV): Measures unsaturation in fats and oils. which can be used to indicate adulteration and determinng the lipids sutabiluty for hydrogrenation process and to monitor said process
- Acid Value (AV): Measures free fatty acid content, indicating purity.
- Saponification Value (SV):Determines the average molecular weight of fatty acids.
What is the general rule when it comes to density and lipids
If you increase the solid fat content→ your density will increase. This can be explained due to the packing effeciency of saturated fatty acids contained in SFC. Also increase molecular weight of FA component→ increase density, or increase proportion of hydroxy FA→ increase density
whats one exception to the density rule
If the lipid systems have a high concentration of pure TAGS that crystalize over a narrow temperature range. The density will decrease on crystalization because voids will form
Define the following thermal properties (melting point and entalpy of fusion)
- Melting Point (Tm): Solid-to-liquid transition.
- Enthalpy of Fusion (ΔHf): Heat required to melt lipids. The amount of heat needed to change unit of mass
define the following thermal properties (smoke point, flash point and fire point)
- smoke point: is the temperature where the sample begins to smoke
- flash point: is the lowest temperature where volatile products generated by the lipid could be temporally ignited if flame is added but cant keep the combustion
- fire point: temperatyre where continous combustion can be kept after applying the flame
how do various structural factors of FA and TAGS affect melting point and enthalpy of fusion? (chain length, saturation, branched, symetry, trans vs cis,polymorphic)
- Longer Chain Length Increases Melting Point (Tm) and Enthalpy of Fusion (ΔHf)
- As the length of the fatty acid chain increases, the melting point (Tm) and enthalpy of fusion (ΔHf) also increase.
- Why? Longer fatty acid chains have stronger van der Waals forces, requiring more heat to break them apart.
- Saturated vs. Unsaturated Fatty Acids
- Saturated fatty acids (with no double bonds) have higher melting points and enthalpy of fusion than unsaturated fatty acids.
- Why?
- Saturated fats have straight chains that pack tightly together, making them more stable and harder to melt.
- Unsaturated fats have kinks in their structure due to double bonds, preventing tight packing and making them easier to melt.
- Straight-Chained vs. Branched-Chain Fatty Acids
- Straight-chained fatty acids have higher melting points than branched-chain fatty acids.
- Why?
- Straight chains can align closely, increasing interactions.
- Branched chains disrupt packing, lowering the melting point.
- Symmetrical TAGs Have Higher Melting Points
- If a triacylglycerol (TAG) has a more symmetrical distribution of fatty acids, it will have a higher melting point and enthalpy of fusion.
- Why?
- Symmetrical molecules pack more efficiently into a crystalline structure, increasing stability.
- Trans Fats vs. Cis Fats
- Trans fatty acids have higher melting points than cis fatty acids.
- Why?
- Trans fats have a linear structure, similar to saturated fats, allowing tight packing.
- Cis fats have bends in their structure, making them harder to stack, lowering the melting point.
- More Stable Polymorphic Forms Have Higher Melting Points
- More stable fat crystal structures (polymorphs) have higher melting points.
- Why?
- Some fats can exist in multiple crystal forms, and the most stable forms require more heat to melt.
What does refractive index indicate or give info on?
it inidcates the molecular structure of fatty acids giving info about average molecular weight or degree of unsaturation. The RI will increase with increasing chain length, number of double bonds and the conjugation of double bonds
what are the steps of crystalization of fat?
- supercooling: first you have a supersaturated solution in which molecules will come into contact and orient themselves in a way which they can interact and form nuclei
- nucleation: stable nuclei will then form
- crystal growth: the nuclei grow into crystals
- post crystalization evnts: changes will occur to change crystals into more stable forms and there will be an increase in crystal size
what are factors which affect the morphology of crystals both internally and externally
- internal: molecular structure, composition, packing and interaction
- external factors: temperature-time profile, mechanical agitation and impurities
whats the difference between rapid and slow cooling of liquid oil in terms of crystal size and explain why
Rapid cooling→ large number of small cyrstals
Why?
- High nucleation rate: Rapid cooling causes the formation of many crystal nuclei.
- Low crystallization rate: Since there is little time for crystal growth, the crystals stay small.
slow cooling→ small number of large cooling
- Low nucleation rate: Fewer initial crystal nuclei form because cooling is gradual.
- High crystallization rate: The few crystals that form have time to grow larger.
