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Keywords:

  • bioplastics;
  • bloodmeal;
  • peracetic acid;
  • solubility

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Section
  5. 3 Results and Discussion
  6. 4 Conclusion

Peracetic acid is used to remove the color and odor from bloodmeal to produce a new bioplastic feedstock. The effects on bloodmeal molecular mass, crystallinity, thermal stability, solubility, product color and smell is investigated. 3 wt% PAA is the lowest concentration to sufficiently remove the odor from bloodmeal. Protein molecular mass is unaffected by PAA concentration. The crystallinity decreases from 35 to 31–27% when treated with 1–5 wt% PAA. Treating bloodmeal with 1–5 wt% PAA also reduces the protein's thermal stability, glass transition temperature (from 225 down to 50 °C) and increases its solubility in PBS, SDS, and sodium sulfite.mame201200447-gra-0001

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Section
  5. 3 Results and Discussion
  6. 4 Conclusion

Renewable and compostable bioplastics can be produced from biopolymers such as proteins.[1-6] Animal blood is a by-product from meat processing and is rich in protein. It is dried into low value bloodmeal and is used as animal feed or fertilizer. Bloodmeal can be converted into a thermoplastic called Novatein thermoplastic (NTP) using water, urea, sodium dodecyl sulfate (SDS), sodium sulfite, and triethylene glycol (TEG).[7-9] To increase NTP's range of applications and acceptance from consumers, its color and odor must be removed without compromising its ability to be processed into a bioplastic using common thermoplastic processing techniques.[4, 10, 11]

Treating blood products with oxidizing agents reduces the color and smell.[12-19] Treating proteins with oxidizing agents can also have undesirable outcomes such as amino acid side chain modification, protein back bone cleavage and cross link formation.[20, 21] The extent and type of damage varies depending on the type, strength, and concentration of the oxidant as well as other secondary reactions (e.g. repair mechanisms and antioxidants).[22]

Peracetic acid (PAA), also known as peroxyacetic acid, is a clear liquid with a strong acetic acid smell and low pH (less than pH = 2).[23] It is produced commercially by reacting acetic acid with hydrogen peroxide in the presence of a catalyst and sold as an equilibrium mixture containing PAA, acetic acid, hydrogen peroxide, and water. Non-equilibrium mixtures can be produced using distillation to remove acetic acid, hydrogen peroxide and water although this is more expensive.

Due to its effectiveness and growing concerns over the environmental impact of chlorine use, PAA has been suggested as an alternative for use in waste water treatment and pulp and textile bleaching.[23] When used to bleach linen, PAA gave better brightness and less fiber damage.[24] PAA has also been used as a food and surface sanitizer as well as removing odors from wastewater sludge.[23] PAA is a protein denaturant and is known to react quickly with proteins.[25, 26] PAA does not produce any harmful disinfection by-products, decomposes if discharged into the environment and bioaccumulation is unlikely to occur.[27, 28]

Previous studies involving PAA and proteins had variable results in terms of its reaction with amino acids. When PAA was used to oxidize wool it exclusively oxidized disulfide crosslinks and tryptophan side chains while no significant oxidation of tyrosine occurred.[29] A separate study showed that PAA readily oxidized cysteine, methionine, histidine, glycine, and lysine but not tryptophan.[30] A third study involving PAA and dairy proteins suggested tryptophan and methionine were most vulnerable to oxidation and lysine was not affected.[26] The variable results could be due to the different reaction conditions, proteins used and PAA composition.

Consolidation of a biopolymer into a bioplastic involves a combination of heating and chemical treatment to overcome physical and chemical interactions between polymer chains allowing them to flow under pressure and the subsequent reformation of interactions upon cooling to form a new solid homogeneous material. Proteins are heteropolymers made from up to 20 different amino acids in chains of varying lengths. Each amino acid has a different side group that may be hydrophobic, acidic, basic, or neutral, and participate in hydrogen bonding, hydrophobic interactions, covalent crosslinks or electrostatic interactions. Specific sequences of amino acids that participate in hydrogen bonding are responsible for secondary structures in proteins such as alpha helices and beta sheets. Hydrophobic, electrostatic, and covalent interactions are responsible for folding the protein chain into their final shape and determine their interaction with other proteins. This means that there are a large number of different possible interactions between protein chains that can make processing difficult with only a small range of operating conditions available for successfully processing them into bioplastics.[4]

Protein oxidation can have negative effects on the properties and processing characteristics of bioplastics. If amino acid side groups are modified they may not be able to contribute to stabilizing interactions, which will lead to a decrease in mechanical properties such as strength, stiffness and elongation. Backbone fragmentation can also lead to reduced mechanical properties as well as poor consolidation because the smaller chains would have reduced entanglements and fewer stabilizing interactions. If disulfide crosslinking between proteins occurred then processing would also be inhibited because the crosslinks would prevent the protein chains from flowing past each other. This can result in processing equipment blocking or high shear rates which can lead to protein degradation.

No previous studies were identified which focused on using PAA to decolorize blood or blood products for use in bioplastics. One of the disadvantages with PAA is its relatively high cost. However, as demand and global production increases its cost may decrease.[23] PAA was used to remove color and odor from bloodmeal to produce a bioplastic feedstock because it was faster, more effective, and more environmentally friendly than other treatment methods.[12, 31, 32] The effect of PAA treatment strength on chain architecture was investigated by using changes in molecular mass, crystallinity, chain mobility, and solubility. These were chosen to assess the degree to which PAA has changed chain interactions that could ultimately affect bioplastic properties.

2 Experimental Section

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Section
  5. 3 Results and Discussion
  6. 4 Conclusion

2.1 Peracetic Acid Treatment of Bloodmeal

Bloodmeal (98 wt% solids) was supplied by a local rendering company. Aqueous PAA equilibrium mixture (5 wt% PAA, 7.5 wt% acetic acid, 25 wt% hydrogen peroxide) was obtained from Solvay Interox and diluted to the required concentration using distilled water.[33]

Decolored bloodmeal powders were produced by treating 100 g bloodmeal with 300 g PAA solution (1–5 wt% PAA). After 5 min of continuous mixing, 300 g distilled water was added to form a slurry which was then filtered to remove acetic acid and unreacted PAA. Any remaining acetic acid was neutralized by submerging the treated and filtered bloodmeal in 300 g distilled water and adjusting to pH = 7 by adding 1 mol · L−1 sodium hydroxide solution. The slurry was filtered, washed, frozen and freeze-dried overnight using a Labconco Freezone 2.5 freeze dryer to 5–8 wt% moisture. They were then ground using a bench top grinder and sieved using a 700 µm sieve. All analysis techniques were repeated at least twice.

2.2 Powder Color

Powder color was analyzed using a Minolta Chroma Meter CR-200b set in L*a*b* (CIE 1976) mode using D (6504K) illuminant conditions as these settings closely represent what is seen by the human eye. The L*a*b* values were converted to RGB and the percentage whiteness calculated using

  • display math(1)

2.3 Molecular Mass

Molecular mass was determined by dissolving 8–9 mg protein in 1.5 mL of 0.02 mol · L−1 phosphate buffer at pH = 7 containing 2% SDS, 0.5% sodium sulfite and 0.1 mol · L−1 sodium chloride, heated to 100 °C for 5 min. After centrifuging the solution at 11 000 rpm for 3 min to remove any remaining solid particles, a 0.5 mL aliquot was applied to a Superdex 200 gel filtration column (GE Healthcare) connected to an Akta Explorer 100 (GE Healthcare). The column was calibrated with a low molecular mass size exclusion calibration kit (GE Healthcare). Two column volumes of 0.02 mol · L−1 phosphate running buffer at pH = 7 containing 0.1% SDS and 0.1 mol · L−1 sodium chloride was applied at a flow rate of 0.5 mL · min−1. Protein concentration was measured at 215 nm using an inline ultraviolet detector. The number-average molecular mass (inline image), weight-average molecular mass (inline image) and polydispersity index (PI) was calculated using Equation 2–4,

  • display math(2)
  • display math(3)
  • display math(4)

where Ni is equal to the number of molecules with molecular mass Mi.[34]

2.4 Thermogravimetric Analysis

Thermal stability of selected decolored powders was assessed using a thermal gravimetric analyzer (SDT 2960, TA Instruments). Approximately, 10 mg of sample was used. The mass loss was recorded while the sample was heated from room temperature to 800 °C at a rate of 10 °C · min−1. The data was normalized at 150 °C to account for water loss. The first derivative of percentage mass change versus temperature was also calculated to investigate temperature regions where mass loss was occurring.

2.5 X-Ray Diffraction (XRD)

The XRD pattern of decolored powders was measured using wide-angle powder X-ray scattering (WAXS). XRD was carried out using a Philips X-ray diffractometer at a low angle configuration of 2θ = 2 to 60°, with a scanning rate of 2θ = 2° min−1, operating at a current of 40 mA and a voltage of 40 kV using CuKα1 radiation.

Crystallinity was investigated by baseline correcting the data from 5–60° and fitting an amorphous halo to this region (Figure 1). Peaks identified at 9.5 and 20° were assigned to inter-helical packing and the alpha helical backbone based on values found in literature.[35] In addition, peaks were also identified at 24 and 40°. These were assigned to beta sheets based on calculations for expected structural spacing and previous values identified in literature.[36, 37] The percent crystallinity was calculated by finding the area between the corrected XRD plot and the amorphous halo then dividing by the total area.

image

Figure 1. Example of a corrected XRD plot showing the amorphous halo and assigned peak locations.

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2.6 Dynamic Mechanical Analysis (DMA)

DMA of PAA treated powders was carried out using a Perkin-Elmer DMA8000. Approximately 50 mg of PAA treated powder was placed inside a stainless steel powder pocket (Perkin-Elmer) and mounted in single cantilever mode. The sample was cooled to below −150 °C using a cryo gun and liquid nitrogen, then heated from −150 °C to +250 °C at 2 °C · min−1 and oscillated at 1 Hz using a displacement of 0.05 mm. Glass transition temperatures were indicated by a peak in tan δ.

2.7 Solubility

The solubility of PAA treated bloodmeal was tested in phosphate buffer (pH = 7), phosphate buffer with 1% SDS, phosphate buffer with 1% sodium sulfite and phosphate buffer with combined 1% SDS and 1% sodium sulfite. PAA (1 g) treated bloodmeal was dissolved in 5 mL of buffer and heated to 100 °C for 80 min. The samples were centrifuged at 4000 rpm for 10 min and decanted. The pellets were washed with 20 mL of distilled water, centrifuged and decanted again. They were then dried at 110 °C overnight and the soluble fraction calculated based on change in solids content. Solubility trials were repeated in triplicate and the average calculated.

3 Results and Discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Section
  5. 3 Results and Discussion
  6. 4 Conclusion

3.1 Color Change

Color removal occurred almost instantaneously and longer reaction times had no additional color removal. A standard reaction time of 5 min was used in all trials. During the reaction using 1 and 2 wt% PAA the temperature of the mixture reached 60–70 °C, when using higher concentrations the temperature reached up to 100 °C. However, it was found that the temperature could be controlled without slowing the rate of color removal. Increased swelling of bloodmeal was also observed at 5 wt% PAA and it was difficult to filter the decolored slurry.

PAA has been shown to remove color by breaking the double bonds in color compounds.[38] Its action on heme is probably similar; it would first have to break the tetrapyrrole structure and then remove one of the double bonds resulting in a yellow color. Untreated bloodmeal is 19% white, treating bloodmeal with 1 and 2 wt% PAA resulted in 47 and 55% white powders, respectively, but the odor was not removed. Odor was removed above 3 wt% PAA and at this concentration percentage whiteness reached a plateau (Figure 2). This indicated that above 3 wt% PAA treatment most of the heme and odor compounds were degraded and it is the lowest possible treatment strength able to be used for bioplastic production.

image

Figure 2. Percentage whiteness of bloodmeal treated with 1–5% PAA.

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3.2 Molecular Mass

Figure 3 shows the elution profile of bloodmeal treated with PAA using gel permeation chromatography. No significant changes in peak location were observed for the different treatments suggesting no apparent significant change in molecular mass. At 4 and 5 wt% PAA, a small peak appeared that was approximately 1.13 kDa, but this should be disregarded as it fell outside the calibration range for the column and appeared at a retention volume greater than the column volume.

image

Figure 3. Elution profile for bloodmeal treated with 1–5 wt% PAA.

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By calculating the number-average mass and weight-average molecular mass (Table 1), treated bloodmeal appeared to have a slightly higher number-average molecular mass than bloodmeal. Treatment strength did not have an obvious effect on molecular mass, but it is noteworthy that there was no significant evidence of low molecular mass fractions forming, thereby confirming that PAA did not lead to chain scission.

Table 1. Number-average molecular mass, weight-average molecular mass, polydispersity index, and molecular mass distribution of 1–5 wt% PAA treated bloodmeal
SampleMolecular mass distribution [weight fraction]
 inline image [kDa]inline image [kDa]PI1490-560 kDa560-215 kDa215-80 kDa80-30 kDa30-11 kDa11-4 kDa4-1.6 kDa1.6-0.6 kDa
bloodmeal1394553.30.070.110.210.380.090.060.050.03
PAA (1 wt%)1905422.80.120.150.200.310.070.060.060.03
PAA (2 wt%)2235902.60.150.170.190.290.070.060.050.02
PAA (3 wt%)2065492.70.120.180.200.300.070.060.050.02
PAA (4 wt%)2105562.70.130.180.190.250.070.070.080.03
PAA (5 wt%)2145552.60.130.190.190.260.060.070.060.04

It is unlikely that the apparent increase in molecular mass was due to crosslink formation (or aggregation) as a decrease in solubility in phosphate buffer and an increase in thermal stability would be expected if this was the case (results presented later). The results presented here are consistent with literature where it has been shown that treating casein with PAA did not lead to an increase in molecular mass nor did it cause protein fragmentation.[26] One would therefore expect that PAA preferentially reacted with other readily available compounds in bloodmeal such as heme or caused side reactions such as acetylation.

Comparing the molecular mass of bloodmeal (139 kDa) with that of hemoglobin (64 kDa), it is clear that bloodmeal must be partially crosslinked, leading to a higher average molecular mass. However, the results also highlight a deficiency of the test; that only the soluble fraction's molecular mass was determined. It stands to reason that with bloodmeal the fraction that could be dissolved would be less aggregated and would therefore have a lower average molecular mass than treated bloodmeal, which had a higher solubility.

The narrower distribution in chain length (smaller PI) would further support this argument. If only chains of shorter average length were dissolved from bloodmeal a broader apparent molecular mass distribution would be expected. As disulfide crosslinks are broken during oxidation more chains could be solubilized leading to a slightly more uniform distribution of chain lengths and an apparently higher molecular mass.

3.3 Crystallinity

Using WAXS, the crystallinity of untreated bloodmeal was calculated to be 35%. This was reduced to 31–27% when treated with 1–5 wt% PAA (Table 2). This could indicate a reduction in interactions such as hydrogen bonding, which are responsible for stabilizing protein structure.

Table 2. Calculated crystallinity of 1–5 wt% PAA treated bloodmeal
SampleCrystallinity [%]
bloodmeal35
PAA (1 wt%)31
PAA (2 wt%)31
PAA (3 wt%)29
PAA (4 wt%)28
PAA (5 wt%)27

Methionine is one of the most common amino acids in alpha helices.[36] Alpha helices are common in bloodmeal and contribute to its crystallinity. Due to its structure, methionine is highly susceptible to oxidation.[20, 21, 39] Furthermore, proline is usually found on the outside of alpha helices and is responsible for turns.[36] Its structure and location mean that it could also be susceptible to PAA oxidation. It contributes to the overall alpha helical structure but not to stabilization because it cannot form hydrogen bonds. As treatment concentration increased, crystallinity decreased most likely due to the modification of methionine side groups and removing the alpha helix stabilizing hydrogen bonds. Oxidation of proline may be contributing a small amount to structure loss due to loss of turns or bends.

The XRD peak associated with alpha-helical structures was reduced after PAA treatment (Figure 4), whereas the peak associated with beta sheets is not significantly affected. It appears that PAA treatment mostly led to a reduction in alpha helical structures, but did not affect beta sheets significantly and led to an increase in amorphous structure.

image

Figure 4. Corrected XRD plots for bloodmeal and 3 wt% PAA treated bloodmeal showing peak locations.

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3.4 Thermal Stability

Degradation of decolored bloodmeal occurred in three stages. These were 0–150 °C, 230–400 °C, and above 400 °C. The first stage was attributed to the loss of bound water and the second stage to the breaking of S[BOND]S, O[BOND]N, and O[BOND]O linkages, above 400 °C thermal decomposition is occurring.[7] There was no significant difference in total mass loss at different treatment concentrations (Table 3). However, the maximum rate of mass loss occurred at lower temperatures as PAA treatment strength increased (Figure 5). This suggests some de-aggregation or loss of intermolecular interactions as PAA treatment strength increased.

Table 3. Mass loss of 1–5 wt% PAA treated bloodmeal at different temperature regions during thermal degradation
SampleMass loss [%]
150–230 °C230–400 °C
bloodmeal440
PAA (1 wt%)737
PAA (2 wt%)636
PAA (3 wt%)739
PAA (4 wt%)639
PAA (5 wt%)538
image

Figure 5. Thermal stability of 1–5 wt% PAA treated bloodmeal.

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The mass loss profile for 1 and 2 wt% PAA treated bloodmeal had a gradual change from bloodmeal (Figure 5). Oxidation of disulfide crosslinks and amino acid side chains have been shown to cause loss of protein stability.[30] Above 3 wt% PAA concentration, rate of mass loss continued to increase showing loss of stabilizing interactions probably due to disulfide crosslink and side chain modification.

No significant loss in bloodmeal molecular mass was detected due to PAA treatment from TGA. If small molecular mass compounds had been produced, a significant loss of thermal stability would have been observed evident from an additional region of mass loss between 150–230 °C.[12] The absence of this supported earlier results using gel permeation chromatography.

3.5 Solubility

Previous studies have used protein solubility in SDS as an indication of crosslinking.[40] Treated bloodmeal's solubility in phosphate buffer increased linearly to about 30% with increasing PAA concentration (Figure 6), indicating that the amount of cross/links and stabilizing interactions that were removed by PAA treatment was proportional to PAA concentration. However, treated bloodmeal was still mostly insoluble in phosphate buffer, which suggested that the protein was stabilized by hydrophobic interactions or possibly covalent crosslinks.

image

Figure 6. Solubility of 1–5 wt% PAA treated bloodmeal in phosphate buffer, phosphate buffer with SDS, phosphate buffer with sodium sulfite and phosphate buffer with SDS and sodium sulfite at 100 °C.

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Using sodium sulfite increased solubility by approximately 5% for treatments below 3 wt% PAA. This was probably due to breakage of residual disulfide crosslinks.[41] Above 3 wt% PAA concentration a plateau was reached and solubility in sodium sulfite was roughly equal to solubility in phosphate buffer. This indicated that 3 wt% PAA may have broken the majority of disulfide crosslinks and suggests that sodium sulfite may not be required for processing these powders into bioplastics.

Solubility could also be affected by hydrophobic interactions between protein chain segments. SDS is an anionic surfactant and interacts with hydrophobic regions of proteins making them more soluble. Untreated bloodmeal and 1–2 wt% PAA treated bloodmeal only showed a small increase in solubility in the presence of SDS which indicated that they also had other interactions such as disulfide crosslinks, preventing them from becoming solubilized.

Above 3 wt% PAA, a large increase in solubility was seen in the presence of SDS. This indicated that the majority of disulfide crosslinks were removed at this point. The large increase in solubility in the presence of SDS indicated that the remaining interactions were most likely hydrophobic interactions, which could be reduced with SDS. Some of these interactions could have been formed by the drying process applied after the decolorizing process.

When considering the results for solubility in phosphate buffer, SDS and sodium sulfite, the increase in solubility for bloodmeal treated with PAA concentrations up to 3 wt% was mainly due to disulfide crosslink breakage. Previous studies showed that PAA reacted preferentially with conjugated systems and disulfide crosslinks.[26, 42] At low concentrations, PAA also appeared to react preferentially with the readily available heme porphyrin and odor compounds, which are also conjugated or ring structures. Above 3 wt% PAA concentration, most of the crosslinks, heme and odor compounds were removed and the increase in solubility in buffer was due to the PAA concentration being high enough to increase its reaction with side chains causing it to reduce other interactions such as hydrogen bonding or hydrophobic interactions.

Untreated bloodmeal and 1–3 wt% PAA treated bloodmeal showed large increases in solubility in the presence of SDS and sodium sulfite. In these cases, the solubility was almost equal to the solubility in SDS and sodium sulfite combined suggesting a cooperative effect between SDS and sodium sulfite at PAA treatment concentrations up to 3 wt%.

Above 3 wt% PAA treatment concentration, solubility in SDS and sodium sulfite plateaued. Disulfide crosslinks are broken by the action of the sulfite ion (inline image).[4] If there were no disulfide crosslinks for the sulfite ion to react with, it would have remained in solution. As a result, it may interact electrostatically with SDS (both have negative charge) or act as a salting-out agent. This result indicated that combining both SDS and sodium sulfite may be unsuitable for processing 3–5 wt% PAA treated bloodmeal into a bioplastic.

PAA treated powders had the highest solubility in SDS solutions and it was observed that the pellets and supernatants from these trials (above 3 wt% PAA treatment) formed a consolidated film when dried, whereas pellets from sodium sulfite solubility trials remained a powder. Successful processing of PAA treated bloodmeal into a bioplastic could therefore be more dependent on SDS than sodium sulfite because PAA treated bloodmeal is stabilized by more hydrophobic interactions than disulfide crosslinks.

3.6 Glass Transition Temperature

DMA analysis showed bloodmeal and PAA treated bloodmeal powders had three peaks in Tan Delta indicating a γ-transition, β-transition, and glass transition temperature (Figure 7). The γ transition peak was constant for bloodmeal and all PAA treatment strengths as it was independent of protein chain interactions. 1 and 2 wt% PAA treated bloodmeal showed a similar profile to untreated bloodmeal, although the original glass transition temperature peak associated with untreated bloodmeal became smaller and the new glass transition temperature peak became larger. This indicated that the protein was becoming more mobile possibly due to a reduction in stabilizing forces.

image

Figure 7. DMA plots for 1–5% PAA treated bloodmeal.

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At 3 and 4 wt% PAA treatment, the new glass transition temperature peak became much more significant and more distinct indicating a new lower glass transition temperature. Reduction in glass transition temperature can be an indication of a reduction in disulfide crosslinks and other intermolecular interactions. The larger new glass transition temperature peak of 3 and 4 wt% PAA treated bloodmeal could be due to regions of low disulfide crosslinking, hydrogen bonding and hydrophobic interactions. Some regions still containing stabilizing interactions may still have been present and could be responsible for the remainder of the original glass transition temperature peak.

At 5 wt% PAA treatment, the original glass transition temperature peak was completely removed indicating that a significant amount of the original stabilizing interactions in bloodmeal had been reduced leading to a new lower glass transition temperature and effectively moving the complete graph to the left when compared to untreated bloodmeal.

These results indicated that PAA treated bloodmeal contained less stabilizing interactions than untreated bloodmeal. Most interactions were removed when bloodmeal was treated with PAA concentrations above 3 wt% as indicated by a lower glass transition temperature. Processing untreated bloodmeal into a bioplastic required disulfide crosslinks, hydrogen bonding and hydrophobic interactions are reduced using urea, SDS and sodium sulfite. It appeared that PAA treatment was fulfilling the role of urea, SDS and sodium sulfite by reducing these interactions. The acetic acid in the PAA equilibrium mixture could also be acting as a plasticizer, contributing to the reduction in glass transition temperature.

3.7 Overall Effects of PAA Treatment on Bloodmeal Properties

By considering the results from gel chromatography, thermogravimetric analysis, DMA, WAXS and solubility it was concluded that at up to 3 wt% PAA treatment concentrations, the PAA reacted preferentially with the heme, odor causing compounds, and disulfide crosslinks. Above 3 wt% PAA, the concentration was high enough to increase the amount of reactions with the amino acid side chains further reducing other stabilizing interactions such as hydrogen bonding as well as hydrophobic and electrostatic interactions. This resulted in an overall loss of regular structure and stability. PAA showed limited reactivity with the protein backbone with high concentrations (5 wt% PAA) having a limited effect on molecular mass. The effect of different PAA treatment concentrations on bloodmeal was summarized in Table 4.

Table 4. Effect of different PAA treatment concentrations on bloodmeal properties
 PAA treatment
1 wt%2 wt%3 wt%4 wt%5 wt%
mainly reacted withhemehemehemehemeheme
odor compoundsodor compoundsodor compoundsodor compoundsodor compounds
  disulfide crosslinksdisulfide crosslinksdisulfide crosslinks
   side chainsside chains
   peptide backbonepeptide backbone
amount reduction of     
colorlowlowhighhighhigh
odornonoyesyesyes
crosslinklowlowhighhighhigh
crystallinitylowlowmediumhighhigh
total stabilizing interactionslowlowmediumhighhigh
molecular masslowlowlowlowlow
potential to be processed?nonoyesyesyes

PAA (3–5 wt%) treated bloodmeal could potentially be used for producing a bioplastic. However, the additives, processing conditions, and material properties for each will be different as they had different amounts of stabilizing interactions.

4 Conclusion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Section
  5. 3 Results and Discussion
  6. 4 Conclusion

Treating bloodmeal with PAA resulted in increased whiteness of bloodmeal accompanied by smell removal above 3 wt% PAA. Change in color and smell was accompanied by other changes in protein properties.

After treatment, a small increase in molecular mass was observed as well as a narrower distribution of chain lengths. PAA treatment reduced crystallinity mostly as a result of decreasing alpha-helical structures in bloodmeal. Amino acids stabilizing this structure were mostly oxidized after PAA treatment, preventing hydrogen bonding. A significant reduction in intermolecular interactions was evident from reduced thermal stability as well as a reduction in the glass transition temperature. It was concluded that PAA treated bloodmeal is stabilized mainly by hydrophobic interactions evident from a very large increase in SDS soluble fraction after treatment.

Producing a bioplastic from PAA treated bloodmeal would likely not require sodium sulfite and urea as most crosslinks would have been reduced above 3 wt% PAA and SDS would be sufficient to reduce remaining hydrophobic interactions that need to be overcome during processing.