Some physicochemical and rheological properties of

International Journal of Biological Macromolecules 86 (2016) 302–308

Contents lists available at ScienceDirect

International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac

Some physicochemical and rheological properties of starch isolated from avocado seeds Luis Chel-Guerrero a , Enrique Barbosa-Martín a , Agustino Martínez-Antonio b , Edith González-Mondragón c , David Betancur-Ancona a,∗ a Facultad de Ingeniería Química, Universidad Autónoma de Yucatán, Periférico Norte Km. 33.5, Tablaje Catastral 13615, Colonia Chuburná de Hidalgo Inn, 97203 Mérida, Yucatán, Mexico b Departamento de Ingeniería Genética, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Unidad Irapuato, Libramiento Norte Carretera Irapuato-León Km. 9.6, 36821 Irapuato, Guanajuato, Mexico c Universidad Tecnológica de la Mixteca, Carretera a Acatlima Km. 2.5, 69000 Huajuapan de León, Oaxaca, Mexico

a r t i c l e

i n f o

Article history: Received 27 May 2015 Received in revised form 19 December 2015 Accepted 15 January 2016 Available online 20 January 2016 Keywords: Avocado starch Physicochemical properties Rheological characteristics

a b s t r a c t Seeds from avocado (Persea americana Miller) fruit are a waste byproduct of fruit processing. Starch from avocado seed is a potential alternative starch source. Two different extraction solvents were used to isolate starch from avocado seeds, functional and rheological characteristics measured for these starches, and comparisons made to maize starch. Avocado seed powder was suspended in a solution containing 2 mM Tris, 7.5 mM NaCl and 80 mM NaHSO3 (solvent A) or sodium bisulphite solution (1500 ppm SO2 , solvent B). Solvent type had no influence (p > 0.05) on starch properties. Amylose content was 15–16%. Gelatinization temperature range was 56–74 ◦ C, peak temperature was 65.7 ◦ C, and transition enthalpy was 11.4–11.6 J/g. At 90 ◦ C, solubility was 19–20%, swelling power 28–30 g water/g starch, and water absorption capacity was 22–24 g water/g starch. Pasting properties were initial temperature 72 ◦ C; maximum viscosity 380–390 BU; breakdown −2 BU; consistency 200 BU; and setback 198 BU. Avocado seed starch dispersions (5% w/v) were characterized as viscoelastic systems, with G > G . Avocado seed starch has potential applications as a thickening and gelling agent in food systems, as a vehicle in pharmaceutical systems and an ingredient in biodegradable polymers for food packaging. © 2016 Published by Elsevier B.V.

1. Introduction Native to Mexico and Central America, the avocado tree (Persea americana Mill) belongs to the Lauraceae family. Its commercially valuable fruit has high monounsaturated oil-content. Easily adaptable to many tropical regions, it is currently distributed throughout the tropics and some areas in the subtropics. Commercial production centers mainly in Mexico, California, Chile, Israel, Australia and South Africa. Mexico is the largest worldwide avocado producer, with 3.4 million tons annual production; in 2009 it accounted for 32% of global avocado production [1]. Avocado fruit have a dark olive-green peel and thick pale yellow-green pulp rich in oils and prized for its sensory attributes [2]. Avocado production has grown rapidly in recent years and the avocado fruit processing industry has followed suit. The fruit is used

∗ Corresponding author at: Periférico Norte Km. 33.5, Tablaje Catastral 13615, Col. Chuburná de Hidalgo Inn, Mérida, Yucatán, Mexico. Fax: +52 9999460994. E-mail address: [email protected] (D. Betancur-Ancona). http://dx.doi.org/10.1016/j.ijbiomac.2016.01.052 0141-8130/© 2016 Published by Elsevier B.V.

in the food, cosmetics, and pharmaceutical industries in applications as diverse as ice cream, mayonnaise, and sandwich spreads. Research has also been done on dehydrating, freezing and canning avocado fruit [3]. Processing avocado fruit results in substantial waste, particularly from discarded seeds, which represent about 16% of fruit dry weight [4]. These by-products can cause environmental problems, particularly propagation of pests such as insects and rodents. They also generate financial losses due to the high cost of transport to disposal areas [5]. Efforts are ongoing to develop integrated use strategies for avocado fruit. Avocado seed proximate composition (wet base) is water (51–58%); starch (29%); sugars (2.21–3.50%)—mainly arabinose (2.04–2.15%); protein (2.38–2.45%); and ash (1.24–1.34%) [6]. It contains high levels of potassium and antioxidants, and is an excellent dietary fiber source. Indeed, the seed’s tannins and polyphenolic compounds contents provide it a higher antioxidant activity than its edible portion, and even higher than common synthetic antioxidants such as Trolox [4,6]. Starch is widely used as a functional ingredient in food systems. Its thickening, gelling and stabilizing properties are essential to

L. Chel-Guerrero et al. / International Journal of Biological Macromolecules 86 (2016) 302–308

imparting viscosity, texture and consistency, properties that make it useful in the manufacture of paper, adhesives and biodegradable packaging, among other products [7]. Rising demand for starch in food products and the manufacture of biodegradable materials is notably impacting the supply of natural starch sources normally used in human diets. Much current starch research focuses on identifying non-conventional starch sources that pose no competition to starches used in human diets and that can function as raw materials in industrial processes. Avocado seeds are a waste byproduct and have high starch content, making them a promising natural, alternative starch source. Starch from avocado seeds has received limited research attention, and starch isolation techniques are still being developed. Khan [8] isolated starch from avocado seeds by soaking seed slices in a sodium hypochloride solution, grinding the slices, and allowing the starch to settle out. The resulting avocado starch granules were oval-shaped with a relatively smooth surface, an average diameter of 5–35 ␮m, a B-type x-ray diffraction pattern, and were non-ionic and not waxy. Builders et al. [9] isolated starch from avocado seeds by first finely chopping the seeds, soaking this meal in a 0.075% w/v sodium metabisulphite solution for 24 h, washing and then filtering the mash. This suspension was allowed to stand for 12 h for starch granule sedimentation, the supernatant decanted, and the resulting starch cake washed and air-dried. In another technique, Lacerda et al. [10] isolated starch from avocado seeds using a sodium metabisulphite solution and then oxidized the starch in sodium hypochlorite solutions at 0.5, 1.0 and 2.0%. Multiple analyses showed the treated starch samples to exhibit decreases in gelatinization enthalpy, average roughness, degree of relative crystallinity and pasting properties. The present study objective was to identify, describe and compare some functional and rheological properties of avocado seed starch isolated using two different extraction methods. 2. Materials and methods 2.1. Seed powder preparation Chopped avocado seeds (P. americana Mill cv. Hass) were spread onto a tray and placed in an oven at 60 ◦ C until dry. The chopped seeds were turned periodically to ensure uniform drying. Once dry, the seeds were finely ground (20-mesh screen) using a Retsch® Ball Mill grinder (Retsch GmbH, Germany) for 10–20 s, depending on initial seed size. The resulting seed powder was stored at 4 ◦ C until use. 2.2. Starch isolation Starch was extracted from the avocado seed powder with two different wet fractionation techniques. The first technique was based on Khan [8] method. Using a Kitchen-Aid® blender (Benton Harbor, MI, USA), the seed powder was wet-milled in a solution containing 2 mM Tris (pH 7.0), 7.5 mM NaCl and 80 mM NaHSO3 (solvent A). The resulting slurry was passed through 80-mesh screens followed by two washings with solvent A to separate the fiber solids from the starch. The avocado starch residue was ovendried at 40 ◦ C for 12 h, and then milled in a Mykros impact mill (Infraestructura Inteligente, Mexico) until passing through a 0mesh screen. In the second technique, starch was isolated following de la Torre et al. [11]. modified as follows. Seed powder was suspended in a sodium bisulphite solution (solvent B, 1500 ppm SO2 ) at 1:5 (w/v), and the suspension left to soak under constant agitation for 1 h. It was then passed through an 80-mesh screen, producing a solid fraction containing fiber and a liquid fraction containing

303

starch. The liquid fraction was left to precipitate for 4 h, and the supernatant removed with a siphon. The settled starch fraction was washed three times by re-suspension in distilled water, and then centrifuged at 1100 × g for 12 min (Mistral 3000i, Sanyo MSE, UK) in the final wash to recover the starch. This was dried at 40 ◦ C for 12 h in a convection oven, weighed and milled in a Cyclotec mill (Tecator, Sweden) until passing through a 20-mesh screen. The resulting avocado seed starch powder was stored at room temperature in a sealed container. Physicochemical and rheological characterization was done of the isolated starches. All the properties were analyzed in triplicate and compared to commercial maize (Zea mays) starch (28% amylpose content; Maizena® , Unilever Food Solutions, Mexico). 2.3. Amylose content Apparent amylose content was estimated after iodine complexation following Morrison and Laignelet [12]. 2.4. Differential scanning calorimetry (DSC) Starch gelatinization was determined with a DSC-7 (PerkinElmer Corp., Norwalk, CT), using the technique described by Ruales and Nair [13]. The DSC device was calibrated with indium and the data analysed using the Pyris software program. Two milligrams (d.b.) of starch were weighed into an aluminum pan and the moisture level adjusted to 70% by adding de-ionized water. The pan was then hermetically sealed and left to equilibrate for 1 h at room temperature. Samples were scanned at temperatures between 30 and 120 ◦ C at a heating rate of 10 ◦ C/min. Gelatinization temperature was determined by automatically calculating onset temperature (To), maximum peak temperature (Tp), final temperature (Tf), and gelatinization enthalpy (H) from the resulting thermogram. 2.5. Solubility, swelling power (SP) and water absorption capacity (WAC) Solubility, water absorption and swelling power patterns were measured at 60, 70, 80 and 90 ◦ C following de la Torre et al. [11]. Briefly, 40 ml of a 1% starch suspension (w/v) was prepared in a previously tared, 50 ml centrifuge tube. A magnetic agitator was placed in the tube, and it was kept at a constant temperature (60, 70, 80 or 90 ◦ C) in a water bath for 30 min. The suspension was then centrifuged at 2120 g for 15 min, the supernatant decanted and the swollen granules weighed. From the supernatant, 10 ml were dried in an air convection oven (Imperial V) at 120 ◦ C for 4 h in a crucible to constant weight. Percentage solubility and swelling power were calculated using the following formulas: %Solubility =

dry weight at 120◦ C(g) × 400 Weight of sample(g)

Swelling power =

weight of swollen granules(g) (sample weights(g) × (100 − % solubility)

water absorption capacity was measured using the same conditions as above, but was expressed as weight of the gel formed per sample, divided by treated sample weight. 2.6. Pasting properties Pasting properties were evaluated following the method of Wiesenborn et al. [14]. using a viscoamylograph (Brabender PT100, Germany). Briefly, 400 ml of 6% (d.b.) starch suspension were heated to 95 ◦ C at a rate of 1.5 ◦ C/min, held at this temperature for 15 min, cooled to 50 ◦ C at the same rate, held at this second

304


temperature for another 15 min, and finally cooled to 30 ◦ C at the same rate. Maximum viscosity, consistency, breakdown and setback were calculated in Brabender Units (BU) from the resulting amylograms. 2.7. Rheological profile Avocado starch dispersions with 10% (w/v) total solids were used to determine the viscoelastic properties in an AR 2000 rheometer (TA Instruments, New Castle, DE). Oscillatory tests were run in triplicate. A cone and plate geometry was used with 60 mm diameter, 4◦ angle and a water-filled solvent trap. Before measurement, samples were homogenized by stirring at 30/s for 2 min at room temperature, and left to stabilize for 50 s. A vapor trap was attached to the geometry. Initial sample temperature was set at 25 ◦ C, and the starches pasted or gelled in situ by applying either heating or cooling at a constant rate of 2.5 ◦ C/min. The linear viscoelastic region (LVR) was identified by running strain amplitude sweeps (1 Hz) from 0.1 to 10% at 95 ◦ C and then at 25 ◦ C for heatingcooling kinetics (i.e., initial stage I, measured at 95 ◦ C and final stage F, measured at 25 ◦ C). Frequency sweeps (0.1–20 Hz) were then run at the same temperatures, but with a constant strain for the avocado starch and the corn starch (control). Storage (elastic) modulus (G ), loss (viscous) modulus (G and loss tangent (Tan ␦ = G /G ) were evaluated for each test, and all tests run in triplicate. Based on the amplitude sweep tests, dynamic viscosity versus strain data were analyzed with the equipment software [15]. 2.8. Statistical analysis Statistical analyses were done to calculate data central tendency and deviation. An ANOVA and a Duncan test (5% significance level) were applied to identify differences between means. All statistical analyses were run using the Statgraphics plus 5.1 package (Statpoint Technologies Inc., WA, USA). 3. Results and discussion The native starches isolated from the avocado seeds appeared as a light brown powder with a characteristic odor and a smooth texture. Seed starch yield by weight was 19.66 ± 0.58% with solvent A, and 20.13 ± 0.42% with solvent B. This value is similar to the 20.5% yield reported for avocado starch produced using chopped kernel and soaking in a 0.075% w/v sodium metabisulphite solution for 24 h [9]. 3.1. Amylose content Starch amylose content was 14.94 ± 0.03% with solvent A, and 15.78 ± 0.02% with solvent B. These contents are less than half the 32.5% reported by Builders et al. [9]. Analytical method may have influenced this difference since, when fully gelatinized, the native avocado starch produced in the present study was an opaque light-brown paste whereas other native starches form colorless translucent pastes. Paste color can interfere in the interaction between amylose and the iodine-potassium iodide complex. It is therefore crucial that the starch complex reach maximum stability. Amylopectin content – estimated by subtracting amylose content from total starch – was 85.06 ± 0.02% with solvent A, and 84.22 ± 0.11% with solvent B. The avocado seed starch amylose contents observed in the present study were lower than the 28.3% reported for maize starch, and the amylopectin contents were correspondingly higher than the 71.7% in maize starch [16]. Amylose and amylopectin contents will affect avocado seed starch functional

Table 1 Differential scanning calorimetry values of avocado and maize starches compared with potato and cassava starches. Starch Avocado (solvent A) Avocado (solvent B) Maize Potato1 Cassava2

To (◦ C)

Tp (◦ C)

Tf (◦ C)

H (j/g)

a

a

74.15a 73.91a 72.91b 80 75.4

11.60a 11.39a 10.18b 4.46 9.61

56.15 56.45a 62.19b 60 57.6

65.79 65.73a 66.32b 69 65.2

a–b Different letter superscripts in the same column indicate statistical difference (p < 0.05). 1 Pérez et al. [19]. 2 De la Torre et al. [13].

properties and consequently determine in which food systems it can be used to improve product characteristics and appearance. 3.2. Differential scanning calorimetry (DSC) Extraction solvent (A or B) had no effect (p > 0.05) on avocado seed starch gelatinization parameters. The avocado starches had high gelatinization temperatures (Table 1), with an onset granule gelatinization temperature of To = 56 ◦ C, a peak temperature of Tp = 65.7 ◦ C, and a final temperature of Tf = 74 ◦ C. All these temperatures are similar to those of maize starch (To = 62, Tp = 66 and Tf = 73 ◦ C) and cassava starch (To = 57.6, Tp = 65.2 and Tf = 75.4 ◦ C) [13]. The higher gelatinization temperature range of the avocado starch compared to other starches suggests a higher degree of association between molecular components (mainly amylose). Use of DSC confirmed the gelatinization values (62–75 ◦ C) reported by Kahn [8] for avocado starch, even though this previous data was generated with a Kofler microhot stage on a Zeiss polarized microscope. Lacerda et al. reported generally higher gelatinization temperature values (To = 73.77, Tp = 78.43 and Tf = 81.27 ◦ C) than observed in the present study [10]. These high temperatures occurred in starch isolated using a sodium metabisulphite solution, and with a higher amylose content (32.5%) than the 15–16% observed in the present study. This may account for the differences in amylose content in the present data. Starches are characterized by their crystalline and amorphous portions. The crystalline portion causes melting peaks to appear in a starch’s thermogram. Starch granules’ crystalline content is related to their amylose content while their amorphous content is related to their amylopectin content. Starches with high amylose content have a higher crystalline domain and characteristically higher melting and gelatinization temperatures as more energy is needed to initiate dislocation or breaking of the well-ordered, firmly-bonded glycosidic bonds [17,18]. The higher amount of heat required to attain melting in the avocado starch was probably due to its comparatively higher amylose content. The high gelatinization temperatures for avocado starch coincide with the higher energy required to gelatinize the avocado starches. Gelatinization enthalpy (H) was 11.4–11.6 J/g for the avocado starch, notably higher than H values for maize starch (10.18 J/g), potato starch (4.6 J/g), cassava starch (4.8 J/g), cocoyam starch (4.0 J/g) and arrowroot starch (4.4 J/g) [19]. The higher melting temperature of avocado starch compared to maize starch suggests it has greater crystallinity within the range evaluated by DSC. Differences in enthalpy values between the avocado and maize starches may also be due to their having different crystalline structures. X-ray diffraction (XRD) patterns for avocado starch are reported to be B-type [8] while those of maize starch are A-type [7]. Starch XRD patterns characterize crystal packing in native starch granules. Based on these patterns and their characteristics, starches

305

Temperature (°C)

30

30

25 20 15 10 5 0 60

70 80 Temperature (°C)

Avocado (solvent A)

90

Viscosity (Brabender units)

Water absorption (g water/g starch)


60

95

95

50

50

90

105

30

700 600 500 400 300 200 100 0

Avocado (solvent B)

0

15

30

45

60

75

Swelling power (g water/g starch)

Time (min) Avocado (solvent A)

40

Maize

Fig. 2. Viscoamylogram of avocado seed and maize starches.

30 20 10 0 60


Avocado (solvent A)

Solubility (%)

Avocado (solvent B)

90

Avocado (solvent B)

25 20 15 10 5 0

60


Avocado (solvent A)

90

Avocado (solvent B)

Maize Fig. 1. Water absorption capacity, swelling and solubility (%) patterns of avocado seed and maize starches.

exhibiting a semi-crystalline structure have different polymorphic forms that are classified into three types: (A) (rhomboid crystal); (B) (hexagonal crystal); and (C) (both rhomboid and hexagonal crystals) [20]. Hexagonal crystals have more compact structures, which require more energy to melt because their glycosidic bonds are less exposed than in rhomboidal crystals. Hexagonal crystals are also very stable and have a fusion temperature of approximately 150 ◦ C [21]. 3.3. Solubility, swelling power (SP) and water absorption capacity (WAC) Water absorption capacity (WAC), swelling power (SP) and solubility correlated directly to increases in temperature. When an aqueous suspension of starch granules is heated, these structures hydrate, causing swelling. The avocado seed starch water absorption and swelling power patterns (Fig. 1) show that its granules did not swell appreciably at temperatures below 70 ◦ C. A slight increase in swelling was observed between 70 and 80 ◦ C, and swelling increased notably from 80 to 90 ◦ C. This continuous rise in swelling is caused by rupture of intermolecular bridges in amorphous zones as temperature gradually increases, allowing progressive and irreversible water absorption. Starch granules’ molecular organization

is gradually and irreversibly destroyed during gelatinization, causing loss of birefringence and granule crystallinity, and extensive water absorption leading to swelling and increased volume. Some molecules solubilize, especially amylose, which diffuses toward water [22]. No differences (p > 0.05) were observed in water absorption and swelling power values between solvents A and B. In the avocado starches, both these properties rapidly increased above 80 ◦ C. In contrast, maize starch begins to swell and absorb water at temperatures as low as 60 ◦ C. Avocado seed and maize starches have similar granule sizes (avocado starch, 5–35 ␮; maize starch, 1–25 ␮) [8]. so this difference between the starches is probably due to differing amylose and amylopectin molecule organization patterns within their crystal structures. Solubility in all the starches increased in response to higher temperature. However, at 90 ◦ C the maize starch exhibited lower solubility values (15.8%) than the avocado starches (19.7–20.6%) (Fig. 1). The greater rate of change in solubility in the avocado seed starches was caused by amylose exudation from swollen starch granules. The higher solubility of the avocado seed starches may also be explained by structural differences (e.g., chain length distributions) compared to maize starch [7]. High starch solubility can provide good aqueous dispersion in food systems, as well as higher water absorption and retention. 3.4. Pasting properties Pasting refers to changes in starch in response to heating after gelatinization has occurred, including swelling, polysaccharide leaching from starch granules and increased viscosity [23]. No differences (p > 0.05%) in pasting properties were identified due to use of solvents A or B, but the avocado seed starches did differ (p < 0.05%) from maize starch in most of the pasting parameters (Table 2). The Brabender viscosity curves showed no pronounced pasting peak for the avocado seed starches (Fig. 2). Pasting peaks have been widely reported in starches from legumes such as garbanzos, peas, cowpea and lima bean [24], as well as breadnut [7]. Viscosity increases with heating in the presence of excess water, a phenomenon caused by further swelling of starch granules in response to heat and moisture transfer. In the avocado seed starches, maximum viscosity (380–390 BU) was reached at 95 ◦ C, while in maize starch maximum viscosity (252 UB) was reached at 92 ◦ C. Heating of the avocado seed starches at 95 ◦ C for 15 min led to slight increases in viscosity. As they were cooled to 50 ◦ C, viscosity increased notably and continued to increase slightly when kept at this temperature for a further 15 min. All the maize starch viscosities were lower than those of the avocado seed starches. Increased viscocity during cooling results from

306


Table 2 Pasting properties of avocado seed and maize starches. Parameter

Avocado starch (solvent A)

Avocado starch (solvent B)

Maize starch

Initial gelatinization temperature (◦ C) Maximum viscosity (BU) Viscosity at 95 ◦ C Maximum viscosity temperature (◦ C) Viscosity at 95 ◦ C for 15 min (BU) Viscosity at 50 ◦ C (BU) Viscosity at 50 ◦ C for 15 min (BU) Breakdown (BU)1 Consistency (BU)2 Setback (BU)3

72a 390a 390a 95a 392a 590a 598a −2a 200a 198a

72a 380a 380a 95a 382a 580a 588a −2a 200a 198a

76a 252b 245b 92a 237b 538b 520b 15b 286b 301b

a–b

Different letter superscripts in the same column indicate statistical difference (p < 0.05). Breakdown = maximum viscosity (BU)—viscosity at 95 ◦ C for 15 min (BU). 2 Consistency = viscosity at 50 ◦ C (BU)—maximum viscosity (BU). 3 Setback = viscosity at 50 ◦ C (BU)—viscosity at 95 ◦ C for 15 min (BU). 1

molecular re-association. In this process, released amylose forms three-dimensional networks known as entanglements as it interacts with water molecules via hydrogen bonds [25]. Breakdown

values were negative (−2 BU) for the avocado seed starches indicating that their viscosity decreases very slightly during processing stages. The maize starch had a positive value (15 BU) (Table 2). The

Fig. 3. Strain amplitude sweep of avocado seed starch dispersions with 5% (w/v) total solids, under two treatments: (a) initial state: test at 95 ◦ C; (b) final state: test at 25 ◦ C.


307

Fig. 4. Frequency sweep of avocado seed and maize starch dispersions with 5% (w/v) total solids, under two treatments: (a) initial state: test at 95 ◦ C; (b) final state: test at 25 ◦ C.

difference between the starches suggests that maize starch was less resistant to heat and mechanical shear, and therefore more susceptible to loss of viscosity upon holding and shearing. The avocado seed starches also had lower consistency (200 BU) and setback (198 BU) values than the maize starch (286 and 301 BU, respectively), meaning the avocado seed starches were more stable in heating and cooling processes. It also suggests they have high paste stability in mechanical processes, as occurs with sweet potato starches [26]. 3.5. Rheological profile Starch linear viscoelastic region (LVR) was shown in the heating (Fig. 3a) and cooling (Fig. 3b) phases. Starch G and G profiles were constant within the range of 0.1 to slightly above 1%, suggesting linear viscoelastic behavior. This indicates that the material maintained its structure. In the frequency sweeps at high temperature (Fig. 4a) and at room temperature (Fig. 4b), the linear viscoelastic behavior was not observed at low frequencies ( G ,

the gel systems were largely frequency insensitive, depending on biopolymer concentration and its nature. In essentially random coil systems, entanglement networks are formed by the simple topological interaction of polymer chains, rather than by cross-linking, when biopolymer concentrations are higher than total occupancy concentration [27]. This is probably due to the higher degree of association in molecular components as shown in the higher gelatinization temperature range of the avocado seed starches compared to other starch sources (Table 1). In the present data, the avocado seed starch systems oscillated between weak and strong gels. Based on polymeric classification [28,29], their tan ␦ values oscillated from 0.108 to 0.177 (solvent A) and 0.109 to 0.184 (solvent B) during the heating phase, and from 0.063 to 0.094 (solvent A) and 0.066 to 0.093 (solvent B) in the cooling phase. These values contrast with the starch maize tan ␦ values: 0.08 to 0.1 in heating phase; 0.05 to 0.12 in cooling phase. Solvent type had no observable effect since the heating and cooling phase tan ␦ values were very close, no matter the solvent used. However, temperature had a clear effect since the tan ␦ values were lowest in the cooling phase. This behavior can be explained by a reduction in kinetic energy that allows interaction between polymer chains. The elas-

308


tic modulus increases in response to gel structure reinforcement during cooling caused by enhanced reactions between the amylose and amylopectin molecules, producing double-helix aggregations noticeable at higher frequencies. The solvent and polymer entanglements then begin to contribute to material response, raising both the elastic and viscous moduli to higher levels. Similar tan ı values (0.1 to 0.6 at 95 ◦ C) in comparable starch dispersions have been reported in several tubers and root vegetables [15]. These values clearly indicate materials with an amorphous structure. Lower values (0.07–0.55 at 25 ◦ C) indicate reformation of crystalline structure during cooling [30,31]. The same as occurred in the avocado seed starches. The avocado seed starch pasting and rheological results suggest its elasticity could provide a smooth texture, remaining soft and flexible at low temperatures. It would also retain its thickening power at high temperatures, and maintain high shear values in heating and cooling processes. 4. Conclusions Avocado (P. americana) seeds are an un-conventional starch source. The functional and rheological properties of Avocado seed starch suggest applications as an ingredient in food systems and other industrial applications. Use of two different solvents (one containing Tris, NaCl and NaHSO3 ; the other only sodium bisulphite) in starch isolation had no influence on starch yield or properties. The isolated starches had functional properties similar to those of commercial maize starch. The avocado seed starch had a slightly lower gelatinization temperature (65.7 ◦ C) than maize starch (66.32 ◦ C) despite their differing amylose contents. Testing of pasting and rheological properties at a 5% (w/v) total solids level showed the avocado seed starches to have enhanced structure at room temperature (25 ◦ C). They remained useful at higher temperatures (95 ◦ C), with G values between 850 and 1000 Pa. Avocado seed starch has potential applications in products such as baby food, sauces, bread products, jellies, candies and sausages. Other possible uses are as a vehicle in pharmaceutical products, and in biodegradable polymers for food packaging. References [1] Secretaría de Economía Monografía del Sector Aguacate en México: Situación Actual y Oportunidades de Mercado (2012) http://www.economia.gob.mx/ files/Monografia Aguacate.pdf.

[2] J.J. Giffoni, E.H. Salles, R. Aguiar, R.S. Nogueira, J.J. Costa, L. Medeiros, S. Maia de Morais, M.F. Gadelha, Rev. Soc. Bras. Med. Trop. 42 (2013) 110–113. [3] S. Khatoon, Patent no: IN 229982, dt. 24.2.2009, Assignee: Council Sci and Ind Res India. (2009). [4] D. Dabas, R.J. Elias, J.D. Lambert, G.R. Ziegler, J. Food Sci. 76 (2011) 1335–1341. [5] R.A. Ferrari, F. Colussi, R.A. Ayub, Rev. Bras. Frutic. 26 (2004) 101–110. [6] F. Segovia-Goméz, S. Peiró-Sánchez, M.G. Gallego-Iradí, N.A. Mohd-Azman, P. Almajano, Antioxidants 3 (2014) 459–464. [7] E. Pérez-Pacheco, V.M. Moo-Huchin, R.J. Estrada-León, A. Ortiz-Fernández, L.H. May-Hernández, C.R. Ríos-Soberanis, D. Betancur-Ancona, Carbohydr. Polym. 101 (2014) 920–927. [8] V. Kahn, J. Food Sci. 52 (1987) 1646–1648. [9] P.F. Builders, A. Nnurum, C.C. Mbah, A.A.C. Attama, R. Manek, Starch—Stärke 62 (2010) 309–320. [10] L.G. Lacerda, T.A.D. Colman, T. Bauab, M.A. da Silva Carvalho Filho, I.M. Demiate, E.C. de Vasconcelos, E. Schnitzler, J. Therm. Anal. Calorim. 115 (2014) 1893–1899. [11] L. de la Torre, L. Chel-Guerrero, D. Betancur-Ancona, Food Chem. 106 (2008) 1138–1144. [12] W.R. Morrison, B. Laignelet, J. Cereal Sci. 1 (1983) 19–35. [13] J. Ruales, S. Valencia, B. Nair, Starch/Stärke 45 (1993) 13–19. [14] D. Wiesenborn, P. Orr, H. Casper, B. Tacke, J. Food Sci. 59 (1994) 644–648. [15] L. Chel-Guerrero, G. Cruz-Cervera, D. Betancur-Ancona, J. Solorza-Feria, J. Food Process Eng. 34 (2011) 363–382. [16] D.A. Betancur-Ancona, L.A. Chel-Guerrero, R.I. Camelo-Matos, G. Dávila-Ortiz, Starch/Stärke 53 (2001) 219–226. [17] G. Bultuosa, J.R.N. Taylor, Starch/Staerke 55 (2003) 304–3127. [18] T. Sasaki, JARQD 39 (2005) 253–260. [19] E.E. Pérez, W.M. Breene, Y.A. Bahnassey, Starch/Staerke 50 (1998) 70–72. [20] T.Y. Bogracheva, Y.L. Wang, T.L. Wang, C.L. Hedley, Biopolymers 64 (2002) 268–281. [21] M.V. Leloup, P. Colonna, G.S. Ring, J. Cereal Sci. 16 (1992) 253–266. [22] J. Torruco-Uco, D. Betancur-Ancona, Food Chem. 101 (2007) 1319–1326. [23] N. Singh, L. Kaur, R. Ezekiel, H.S. Guraya, J. Sci. Food Agric. 85 (2005) 1275–1284. [24] A. Lecuona-Villanueva, J. Torruco-Uco, L. Chel-Guerrero, D. Betancur-Ancona, Starch/Staerke 58 (2006) 25–34. [25] D.J. Thomas, W.A. Atwell, Starches, in: Practical Guides for the Food Industry St. Paul Minnesota, American Association of Cereal Chemists, Egan Press, USA, 1999. [26] F.O. Osundahunsi, N.T. Fagbemi, E. Kesselman, E. Simón, J. Agric. Food Chem. 51 (2003) 2232–2236. [27] D.R. Picoud, S.B. Ross-Murphy, Sci. World J. 3 (2003) 105–121. [28] S.B. Ross-Murphy, J. Rheol. 39 (1995) 1451–1463. [29] J.D. Ferry, Viscoelastic Properties of Polymers, 3rd edition, John Wiley & Sons, New York, 1980. [30] [30] A.M. Grillet, N.B. Wyatt, L.M. Gloe, Rheology, Juan de Vicente (Ed.) In Tech. Croatia (2012). [31] J.F. Steefe, Rheological Methods in Food Process Engineering, East Freeman Press East Lansing, MI, 1992.