Antibacterial Activity against Foodborne Bacteria and Antioxidant

Article pubs.acs.org/JAFC

Tannins and Extracts of Fruit Byproducts: Antibacterial Activity against Foodborne Bacteria and Antioxidant Capacity Petri Widsten* Scion, Private Bag 3020, Rotorua 3046, New Zealand

Cristina D. Cruz, Graham C. Fletcher, and Marta A. Pajak The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand

Tony K. McGhie The New Zealand Institute for Plant & Food Research Limited, Private Bag 11600, Palmerston North 4442, New Zealand ABSTRACT: The shelf life of fresh fish and meat transported over long distances could be extended by using plant-based extracts to control spoilage bacteria. The goals of the present study were to identify plant-based extracts that effectively suppress the main spoilage bacteria of chilled fish and lamb and to assess their antioxidant capacity. The phenolic compounds in woodbased tannins and extracts isolated from byproducts of the fruit processing industry were identified and/or quantified. The total phenol content, but not the flavonoid to total phenol ratio, was strongly associated with higher antibacterial activity against several fish and lamb spoilage bacteria in zone of inhibition and minimum inhibitory concentration assays as well as greater antioxidant capacity in the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical assay. The most promising compounds in both cases, and thus good candidates for antibacterial packaging or antioxidant dietary supplements, were mango seed extract and tannic acid containing mostly polygalloyl glucose type phenols. KEYWORDS: active packaging, antibacterial, antioxidant, dietary supplement, extract, fruit, shelf life, tannin

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INTRODUCTION Fruit and wood processing industries produce large amounts of processing residues which may be sources of valuable bioactive compounds. Specifically, many plant extracts have antimicrobial and antioxidant properties that could be useful in food or dietary supplement applications. The bioactive (antimicrobial and/or antioxidant) phenols in fruit-based extracts are typically different types of flavonoids, phenolic acids, and gallo- and ellagitannins. They are mostly nonvolatile and concentrated in the skin and seeds, from which they can be extracted with aqueous and organic solvents.1−8 Condensed tannins such as mimosa and pine tannins, which are flavonoid-based polymers, are mainly derived from wood bark.9 Commercial tannic acid, a hydrolyzable gallotannin, is obtained from leaves, gallnuts, and pods of certain plants and comprises mainly gallic acid esters such as pentagalloyl glucose.10,11 The transport of highly perishable foods such as fresh (refrigerated/chilled) meat and fish over long distances by sea requires a careful control of their bacterial populations to provide adequate shelf life. Although most meats such as beef are frozen for transport, and thus well protected from bacterial spoilage, lamb is required to age during transport and is therefore chilled. In order to save fuel, cargo ships tend to sail more slowly nowadays, but the consequent longer shipping times require greater storage life. This has prompted interest in novel types of antimicrobial packaging solutions for lamb, which is commonly shipped 19,000 km from New Zealand (NZ) to Europe. © XXXX American Chemical Society

Active packaging (AP) refers to functional packaging that plays some active role (control of temperature, moisture, microbial growth, etc.) rather than merely providing an inert barrier against external conditions.12−16 Modified atmosphere packaging (MAP), a form of AP, and vacuum packaging are often employed for packaging fresh meat and fish for transport. An emerging AP strategy is the inclusion of antimicrobial agents in packaging materials from which they are released to the food surface in a controlled manner, inhibiting bacterial growth by various mechanisms. Fresh fish and lamb are among the sectors where antibacterial agents have a great potential to extend product shelf life: the food items are highly perishable, transport times can be long, and close contact between the food and packaging material is possible. Antimicrobial agents could be applied as additives17 or in antimicrobial packaging.12−16 Films containing nonvolatile antibacterial agents would work well with vacuum packaging where there is direct contact with the food surface. They would be unsuitable for most modified atmosphere packaging or other packaging systems where food is not in direct contact with the packaging. Organoleptic change is a common reason why many volatile antimicrobial agents such as essential oils from common spices and herbs are unsuitable for food packaging applications.16 Received: April 21, 2014 Revised: October 22, 2014 Accepted: October 23, 2014

A

dx.doi.org/10.1021/jf503819t | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry

Article

Table 1. Major Spoilage Bacteria in Fresh Chilled Fish and Lamb22−25 occurrence/spoilage potential white fish (Atlantic cod)

NZa salmon bacterium

vacuum or N2

MAP (CO2)

air

vacuum or N2

MAP (CO2)

air

NZ lamb

Shewanella putrefaciens Pseudomonas f luorescens Photobacterium phosphoreum Serratia liquefaciens Serratia proteomaculans Rahnella aquatilis Brochothrix thermosphacta Clostridia spp. Haf nia alvei Lactobacillus spp.

occasb/very high freq/high

occas/very high occas/high

occas/very high domd,e/high

freqc/very high

freq/very high

freq/very high freq/high

occas/very high freq/high

a

b

freq/high infreqf/high

dom/high

freq/high freq/high occas/high occas/high

occas/high occas/high dom/modg freq/low c

occas/high

occas/high

freq/mod dom/mod

freq/low

d

e

f

g

New Zealand. Occasional. Frequent. Other Pseudomonas spp. also significant. Dominant. Infrequent. Moderate.

Table 2. Extracts and Reference Compounds Used in This Study species information common name ApSk AvPu AvSd AvSk BaSk FeSk GrSd KiSk LeSk MaSd MaSk

apple (Braeburn) avocado (Hass) avocado (Hass) avocado (Hass) banana feijoa grape (Sauvignon blanc) kiwifruit (Green) lemon mango (Tommy Atkins) mango (Tommy Atkins) mandarin mangosteen

scientific name

yield, % of dm

common name

34

BoJu

Persea americana

pulp (waterless)

13

BluJu

Persea americana

seed powder

11

RaJu

Persea americana

skin powder

13

StJu

Musa sp. Feijoa sellowiana Vitis vinifera

skin powder skin powder seed powder

22 37 n/a

ToJu

Actinidia deliciosa

skin powder

34

Citrus limon Mangifera indica

skin powder seed (incl endocarp) powder skin powder

35 19

Mangifera indica

CaTa MiTa PiTa mTaAc

40 TaAc

BlaJu BlcJu

blackcurrant

OrSk PaSk PiSk PwSk

extracted material

Skin and Seed Extracts Malus domestica skin powder

Citrus reticulata Garcinia mangostana orange Citrus sinensis passionfruit Passif lora edulis pineapple Ananas comosus pawpaw Asimina triloba Extracts of Fruit Juicing blackberry Rubus f ruticosus

MdSk MnSk

species information

extraction

Ribes nigrum

skin powder skin (pericarp) powder skin powder skin powder skin powder skin powder Residues whole fruit juicing residue whole fruit juicing residue

extracted material

yield, % of dm

Extracts of Fruit Juicing Residues boysenberry Rubus ursinus × whole fruit 15 Rubus idaeus juicing residue blueberry Vaccinium sp. whole fruit 45 juicing residue raspberry Rubus idaeus whole fruit 25 juicing residue strawberry Fragaria sp. whole fruit 16 juicing residue tomato Solanum whole fruit 16 lycopersicum juicing residue full name scientific name extracted material cationic mimosa tannin mimosa pine modified tannic acid tannic acid

37 37 ref compds

33 22 28 22

scientific name

extraction

ascorbic acid catechina

Tannins Acacia spp.

bark

Acacia spp. Pinus radiata Acacia spp.

bark bark see TaAc

C. spinosa pods, R. Caesalpinia spinosa, semialata and R. typhina Rhus coriaria, Rhus gallnuts, Q. infectoria semialata, Rhus and R. coriaria leaves typhina, Quercus infectoria full name ref compds full name ascorbic acid (+)-catechin

gallic acid Trolox

gallic acid Trolox

a

Anhydrous (+)-catechin was used for antioxidant capacity tests and (+)-catechin hydrate for ZI studies.

25 34

Thus, although spices and herbs are typically rich in bioactive essential oils containing simple phenols, terpenoids, and other bioactive compounds,16 such volatile compounds were excluded from the scope of the present work because of their strong flavors and odors. In the present study a major goal was to identify natural extracts that could be used for shelf-life extension of fresh (refrigerated/chilled) fish and lamb by controlling their

spoilage bacteria. Extracts from different materials have heterogeneous compositions and thus tend to differ in regard to their abilities to inhibit different bacteria. We focused on nonvolatile extracts from natural products in order to minimize cost and toxicology compliance issues as well as to mitigate any possible changes in the organoleptic properties (the effects on organoleptic properties of different extracts will depend on the amount migrating to the food and the type of food item and B



Article

England. Grape seed extract was obtained from NZ Extracts Ltd., NZ. African mango seed extract (not the mango seed extract in Table 2, which was produced by the authors and used for antibacterial and antioxidant tests) came from Hanzhong TRG Biotech Co. Ltd., Hanzhong City, China. Mimosa tannin and cationic mimosa tannin were supplied by Christian D. Markmann GmbH, Hamburg, Germany. Pine tannin was an in-house product of Scion, NZ (see Processing). Processing. For most of the fruits, skins and seeds were separated from pulp manually, then scraped, and rinsed clean of most pulp and juice/oil residues. Berry fruits (frozen) and tomatoes were juiced at room temperature with a low-temperature Ultem auger juicer (Oscar DA1000, Oscar Juicers, NZ); the juicing residues consisted of the skins and seeds of the fruits. After segregation of the fruit components, the parts to be extracted were immediately frozen at −82 °C, then lyophilized, and ground into powder through a 1 mm mesh using a Wiley mill. Tannic acid is extracted from the leaves, gallnuts, and pods of a range of wood species (Table 2) using water, organic solvents, or their mixtures; the grape seed extract originated from a water-based extraction process; mimosa tannin is commercially extracted from the bark of various acacia species; cationic tannin is a flocculant for pulp mill effluents produced from mimosa tannin by the Mannich reaction; pine tannin originated from alkali extraction of radiata pine (Pinus radiata) bark. Extraction. A portion (12−100 g) of fruit skin or seed powder was placed in a Schott bottle (0.5 or 1 L) and extracted with acetone− water (80:20, v/v) at a consistency of 10% for 90 min at 50 °C while the bottle was shaken at 180 rpm. The mixture was then centrifuged (3000 rpm, 5 min) and the supernatant collected by decanting and/or pipetting. The particle residue was washed twice with water, and all the supernatants were combined. The acetone was evaporated from the supernatant under reduced pressure at room temperature, after which the water phase was frozen at −82 °C and lyophilized. The particle residue from the extraction was then re-extracted as described above, and the lyophilized extract powders from both extractions were combined to give the final extract. The extracts were stored in a freezer at −20 °C pending use. Thermal Modification of Tannic Acid (TaAc). For the thermal modification of TaAc,26 10.0 g of TaAc was dissolved in 90.0 g of deionized water. The solution was decanted into the Teflon insert of a calorimetric bomb, which was then halfway immersed into an oil bath at 200 °C. After 260 min, the bomb was removed from the bath and allowed to cool overnight. The minor amount of insoluble material found at the bottom of the inset was discarded, and the solution was lyophilized, yielding a brown powder similar in appearance to the starting material. Chemical Assays. Total Phenol Content (TPC). Total phenol content of extracts was determined by the Folin−Ciocalteu method according to Kähkonen et al.27 Absorption was measured at 765 nm using a Cary 300 Bio UV−visible spectrophotometer. A calibration curve (linear fit R2 = 0.9985; 0.005−0.018 mg/mL gallic acid; A765 = 0.5−1.6) was constructed with gallic acid, and the sample TPCs (mean of three or more replicates) were expressed as gallic acid equivalents (mg GAE/g) on dry extract weight basis. Anhydrous (+)-catechin was used to check the accuracy of the method. Total Flavonoid Content (TFC). Total flavonoid content was determined using the colorimetric assay described by Dewanto et al.28 A Cary 300 Bio UV−visible spectrophotometer was used to measure the absorbance at 510 nm. (+)-Catechin hydrate was used for the calibration curve (linear fit R2 = 0.9995; 0.01−0.04 mg/mL catechin; A510 = 0.34−0.92), and the results were expressed as catechin equivalents (mg CE/g) on dry extract weight basis. Samples were analyzed in three or more replicates and the results averaged. Identifying Phenolic Compounds. Eight of the extracts with the highest phenol content, excluding CaTa, were also analyzed by LCMS. To extract the phenolic compounds, 1.0 mL of ethanol/Milli-Q water/ formic acid (80/20/1, v/v/v) was added to approximately 50 mg of extract, and the mixtures were vigorously agitated using a vortex mixer. Samples were then stored overnight at 4 °C, centrifuged, and diluted 10-fold with methanol before analysis by ultrahigh performance liquid chromatography−high resolution mass spectrometry (LCMS) using a

should be tested as part of shelf-life trials). The search was narrowed down to extracts from byproducts of fruit processing industries2,3,5−8 and tannins derived from different wood components that could be made readily available in New Zealand. The wider bioeconomy would benefit from adding value to agricultural byproducts while health-conscious consumers also tend to prefer natural antioxidants and antibacterial agents. The major spoilage organisms of chilled fish and lamb are psychrotolerant bacteria. Shewanella putrefaciens is a major white fish and chilled lamb spoilage bacterium; very low numbers produce enough H2S and other sulfur volatiles for a highly offensive rotten-egg odor, as well as trimethylamine and other off-flavor compounds.18 Photobacterium phosphoreum and Pseudomonas f luorescens are also able to rapidly spoil fish and/or lamb, when present. The facultative anaerobe Brochothrix thermosphacta is a concern for aerobically stored, vacuumpacked or MAP chilled red meats. In anaerobic conditions psychrotrophic Clostridia spp. such as Clostridium estertheticum can spoil these products as well.19 Lactobacilli typically produce a fermentative yeasty odor, but their impact on sensory properties depends also on the food. In some cases lactobacilli may be considered beneficial if they inhibit other, more potent bacteria such as Clostridia. The main spoilage bacteria for fresh New Zealand fish18,20,21 and lamb22 are summarized in Table 1. In addition to the bacteria already mentioned, they include several Enterobacteriaceae species, and all were included in the current study except for Rahnella. In selecting the target bacteria we aimed to include the food-borne bacteria that are mostly linked to outbreaks worldwide, as well as those associated with spoilage of different sources of food and having different phenotypic characteristics which might be expected to have differences in susceptibility (e.g., Gram-positive and Gram-negative bacteria). A second goal of the study was to determine the potential suitability of the natural extracts for use in dietary antioxidant supplements. Manufacturers typically claim that antioxidant dietary supplements possess antiaging properties and offer protection against various diseases. The potential benefits of such supplements are uncertain, and high doses of some antioxidant supplements such as vitamin A, vitamin E, and betacarotene have even been linked to increased all-cause mortality.23 On the other hand, the many health-promoting effects of fruits are seldom contested and are largely attributed to the antioxidant properties of their phenolic compounds.24,25 In this study we evaluated the antibacterial and antioxidant properties of nonvolatile extracts from byproducts of fruit processing industries2,3,5−8 and tannins derived from trees. To assist with future regulatory acceptance, we have also identified the main phenolic compounds in eight of the most promising extracts.

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MATERIALS AND METHODS

Materials. The raw materials and reference compounds used for this study are listed in Table 2. Most of the fruit skins and seeds came from ripe fresh whole fruits obtained from local fruit vendors. The berry fruits were purchased frozen from a local supermarket. Avocado skins and seeds were recovered from fresh avocado processing residue supplied by Fressure Foods, NZ, and processed in-house to generate the extracts used in the study. Reference and standard compounds (ascorbic acid, (+)-catechin, gallic acid, and Trolox), solvents, and reagents were of analytical grade and from Sigma-Aldrich except for acetone, which was from Merck, Darmstadt, Germany. Tannic acid (TaAc) was purchased from BDH Laboratory Supplies, Poole, C



Article

Figure 1. Total phenol content (TPC as gallic acid equivalents) and total flavonoid content (TFC as catechin equivalents) of extracts and catechin (external standard). See Table 2 for abbreviations. O157:H7, methicillin-resistant S. aureus, and B. thermosphacta were purchased from the Environmental Science & Research Centre, NZ. C. estertheticum and Serratia proteomaculans were kindly supplied by AgResearch, NZ. Bacterial strains were recovered in Brain Heart Infusion (BHI, Difco, NZ) and kept in blood agar plates (Fort Richard, NZ) at 4 °C for storage until the experimental day. Bacteria were grown in Mueller−Hinton broth (MHB, Difco) except for L. monocytogenes (grown in Trypticase Soya Broth supplemented with 0.6% of yeast extract, TSBYE, Difco) for 24−48 h prior to the experiments. Each bacterium was incubated at its optimal growth temperature to achieve 1 × 108 to 1 × 109 colony-forming units/mL (CFU/mL). Preparation of Compounds for Antibacterial Assays. Extracts were weighed in sterile vials and kept at −20 °C. On the experimental day appropriate dilutions were performed to achieve the final concentration desired. Most of the compounds (Table 2) were diluted in sterile water, while PiTa, gallic acid, catechin, and CaTa were diluted in 80% acetone (ECP, NZ) to facilitate their dissolution. Gel Diffusion Assays. The gel diffusion assays were carried out according to Eseifeka et al.30 with some modifications. Compounds tested were solubilized as described above and prepared at a final concentration of 10 mg/mL. Bacteria were grown as described above and then centrifuged for 5 min at 4,000 rpm, washed twice with phosphate buffered saline solution, and resuspended in 10 mL of 1% salt peptone water (SPW: 1% NaCl, 0.1% Difco peptone) to achieve a final concentration of 1 × 105 to 1 × 106 CFU/mL. An aliquot (1 mL) of each bacterium suspension was placed into a Petri dish and immediately mixed with 18 mL of melted Mueller−Hinton agar (MHA) or Trypticase soya agar supplemented with 0.6% yeast extract (TSAYE, Difco) for L. monocytogenes. The plate was then gently swirled and the agar left to solidify in the laminar flow for ca. 30 min. Using a sterile 6 mm core borer, wells were made into the agar, and 35 μL of test compound solution was deposited into them. Appropriate diluent controls were included in all plates. Each of the bacteria was incubated at its optimum growth temperature (from 20 to 37 °C) for 24 to 48 h. The zones of (growth) inhibition (ZIs) surrounding the wells were measured with a digital caliper. A control disk containing 35 μL of 80% acetone was included in each tested plate. Three independent experiments were conducted in triplicate. Minimum Inhibitory Concentration (MIC) (NCCLS, 1993).31 The determination of MIC was only performed for some of the compounds, the selection being mainly based on the results of the ZI assays. The bacterial strains were prepared as described above with minor modifications. Bacteria were grown in MHB or TSBYE for 24−

Dionex Ultimate 3000 Rapid Separation LC and a micrOTOF QII mass spectrometer (Bruker Daltonics, Bremen, Germany) fitted with an electrospray ion source. Component separation by LC was achieved using two UHPLC Zorbax SB-C18 2.1 × 100 mm, 1.8 μm columns (Agilent, Melbourne, Australia) connected in series. The columns were maintained at 60 °C with a mobile phase flow rate of 350 μL/min. The solvents were A, 100% acetonitrile, and B, 0.2% formic acid, with a solvent gradient of 10% A, 90% B, 0−0.5 min; linear gradient to 45% A, 45% B, 0.5−18 min; linear gradient to 100% A, 18−25 min; 100% A, 25−28 min; linear gradient to 10% A, 90% B, 28−28.2 min; 10% A, 90% B until the next sample injection at 31 min. Injection volumes were 1 μL. The micrOTOF QII parameters were 225 °C; drying N2 6 L/min; nebulizer N2 1.5 bar, end plate offset −500 V, mass range 100−1500 Da, acquisition rate 2 scans/s. Negative ion electrospray was used with a capillary voltage of +3500 V. Postacquisition internal mass calibration used sodium formate clusters with the sodium formate delivered by a syringe pump at the start of each chromatographic analysis. Each sample was analyzed in standard HRMS mode and then reanalyzed in HR/MS/MS mode to obtain information about the molecular fragments. mSigma values were calculated to compare measured isotope ratio matches with expected ratios. An automated MS/MS routine was used where the most intense ion present in a survey scan was isolated and then fragmented in the collision chamber in subsequent scans. Antioxidant Capacity Assays. The antioxidant capacity of samples was determined by the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging method. The steady state measurement variant of the method, based on different dilutions of the sample, was used as described by Buthkup et al.29 with some modifications: 80% acetone was used as the solvent for DPPH samples and as the blank, and the control was 3.9 mL of DPPH solution + 0.1 mL of 80% acetone. A Cary 300 Bio UV−visible spectrophotometer was used to measure the absorbance at 517 nm; the maximum time before absorption measurement was 120 min. The results were expressed as the effective concentration EC50, defined as the amount of antioxidant necessary to decrease DPPH radical absorbance by 50%. A more powerful antioxidant will thus have a lower EC50 value. The assays were performed in triplicate and the results averaged. Antibacterial Assays. Bacterial Strains. Listeria monocytogenes ScottA, Staphylococcus aureus ATCC 25923, P. f luorescens, Serratia liquefaciens, Salmonella Enteritidis, P. phosphoreum, Haf nia alvei, Salmonella Typhimurium, S. putrefaciens, Yersinia enterocolitica, and Lactobacillus sp. were obtained from The NZ Institute for Plant & Food Research Limited culture collection, NZ. Escherichia coli D



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Table 3. Compounds Identified in Extracts retention time (min)

peak area

measd mass to charge ratio (m/z)

formula (M − H)−

11.45 10.93 11.89 10.31 11.74 9.57 12.34 4.69 12.19 12.72 8.79

6375389 6263395 5411294 5243978 4814487 4253607 4157982 3322599 3270636 2607683 1973555

773.0755e 697.0688e 1132.7814 621.0628e 1132.7812 545.0557e 1234.4550 321.0251 1183.4513 1335.7923 939.1125

C69H46O42 C62H42O38 no match C55H38O34 no match C48H34O30 no match C14H9O9 no match no match C41H32O26

10.32 9.52 10.79 8.79 8.42 2.49 10.03 2.43 9.92 6.04

6161114 3025594 2946321 1759565 551348 450380 295294 287535 283156 282024

621.0630e 545.0557e 697.0688e 939.1114 300.9986 343.0669 387.1662 331.0665 545.0557 421.0759

9 4.9 13.54 4.61 9.46 4.11 4.76

1967402 751612 644914 469483 193960 185973 134849

6.61 4.32 7.68 5.68 6.8 5.33 5 4.9 5.45 6.72 2.63

δ (mDa)a

mSigmab

proposed compd IDc

ID leveld

TaAc 3.7 1.5

14.6 5.7

nonagalloyl glucose octagalloyl glucose

0.0

18.8

heptagalloyl glucose

−1.6

20.9

hexagalloyl glucose

−0.1

15.9

digallic acid

−1.6

17.5

pentagalloyl glucose

3 3 5 3 5 3 5 2 5 5 3

C55H38O34 C48H36O30 C62H42O38 C41H31O26 C14H5O8 C14H15O10 C18H27O9 C13H15O10 C48H34O30 C19H17O11

−0.2 1.6 0.5 0.5 −0.4 −0.2 −0.2 0.5 −1.6 −1.8

18.0 17.4 7.3 26.4 11.0 13.1 18.5 17.2 8.7 16.4

heptagalloyl glucose hexagalloyl glucose octagalloyl glucose pentagalloyl glucose ellagic acid galloylquinic acid unknown galloylglucose hexagalloyl glucose tetrahydroxyxanthone glucoside

3 3 3 3 3 3 4 3 4 3

303.0515 289.0713 301.0355 577.1330 243.0661 577.1330 865.1981

C15H11O7 C15H13O6 C15H9O7 C30H25O12 C14H11O4 C30H25O12 C45H37O18

−0.5 −0.5 −0.1 −2.2 0.2 2.1 0.4

2.4 13.4 16.1 13.6 9.9 28.8 41.8

dihydroquercetin catechin quercetin procyanidin dimer piceatannol procyanidin B1 procyanidin trimer

2 1 1 2 3 1 2

1770118 1755201 1225781 1090680 636396 615076 583533 451913 372797 361332 335443

881.1965 593.1303 865.2020 577.1358 577.1341 289.0715 511.1452 289.0713 577.1352 561.1403 339.0361

C45H37O19 C30H25O13 C45H37O18 C30H25O12 C30H25O12 C15H13O6 C23H27O13 C15H13O6 C30H25O12 C30H25O11 C14H11O10

3.0 −0.3 −3.5 0.6 1.0 0.3 0.5 0.4 0.1 0.0 0.3

11.7 16.8 14.6 14.4 34.4 13.4 23.9 14.6 8.2 11.3 11.5

catechin > catechin > gallocatechin catechin > gallocatechin prorobinetinidin trimer prorobinetinidin dimer prorobinetinidin dimer robinetinidin unknown catechin prorobinetinidin dimer fisetinidol-4 > 8-catechin unknown

2 1 1 2 3 1 2 2 1 3 4

4.92 6.13 5.47 4.14 4.76 4.62 6.41 2.63 6.33 6.72 13.57 2.43 6.04

3293050 2954710 907789 863849 476798 457356 384381 335443 316940 213246 146856 287535 282024

289.0733 289.0743 577.1394 577.1357 865.2017 577.1360 865.2063 169.013 291.0900 576.1303 301.0370 331.0665 421.0759

C15H13O6 C15H13O6 C30H25O12 C30H25O12 C45H37O18 C30H25O12 C45H37O18 C7H5O5 C15H15O6 C60H49O24 C15H9O7 C13H15O10 C19H17O11

1.5 2.5 −4.3 −0.6 3.2 −0.8 7.7 −0.6 2.6 −3.0 −1.5 0.5 −1.8

4.6 16.2 9.4 17.5 10.3 11.8 15.5 39.8 13.4 74.8 6.0 17.2 16.4

catechin epicatechin procyanidin B2 procyanidin B1 procyanidin trimer procyanidin dimer procyanidin trimer gallic acid unknown procyanidin tetramer quercetin galloylglucose tetrahydroxyxanthone glucoside

1 1 1 1 3 2 3 3 4 3 1 3 3

6.12 5.46

2490173 2074746

289.0722 577.1359

C15H13O6 C30H25O12

0.4 0.8

16.9 9.1

epicatechin procyanidin B2

1 1

MaSd

PiTa

MiTa

GrSd

AvSk

E



Article

Table 3. continued retention time (min)

peak area

measd mass to charge ratio (m/z)

formula (M − H)−

δ (mDa)a

mSigmab

6.4 5.14 6.72 4.98 5.66 8.4 5.37 7.52

1039617 953374 663401 424041 334857 289662 256002 228052

865.2019 353.0870 576.1260 443.1910 576.1251 577.1347 865.1987 595.1299

C45H37O18 C16H17O9 C60H49O24 C21H31O10 C60H49O24 C30H25O12 C45H37O18 C26H27O16

3.4 0.8 −1.3 −1.3 −2.2 0.5 0.2 0.6

10.2 15.4 14.8 22.8 52.5 11.9 10.6 28.8

procyanidin trimer chlorogenic acid procyanidin tetramer unknown procyanidin tetramer procyanidin dimer procyanidin trimer quercetin 3-arabinoglucoside

2 1 3 4 3 2 3 2

3.79 6.13 6.4 4.93 3.18 5.99 8.9 4.33 5.3 5.18

1445056 940333 765748 736750 633272 499408 472840 433427 293381 222293

353.0876 289.0722 863.1842 337.0931 315.1080 863.1821 441.1775 345.1189 575.1169 353.0869


−0.2 −0.4 1.3 0.2 0.5 −0.8 −0.8 0.2 2.6 −0.9

8.2 17.7 18.0 13.8 15.3 23.0 3.5 16.7 111.4 13.8

neochlorogenic acid epicatechin procyanidin trimer A-linkage coumaryl quinic acid unknown procyanidin trimer A-linkage unknown unknown procyanidin tetramer A2 chlorogenic acid

2 1 4 1 4 4 4 4 3 1

4.91 2.9 6.54 3.04 12.82 2.47 7.64 8.66 4.26 9.4

670167 631225 309894 264788 242912 199363 187079 186096 180061 178821

289.0718 783.0693 329.0872 305.0666 551.1022 466.0261 433.0386 463.0864 603.0672 491.0812


0.0 0.6 −0.6 0.1 −2.0 2.2 −2.6 −1.8 −3.2 −1.9

16.6 11.2 14.8 16.1 21.9 4.2 14.9 20.5 12.6 19.7

catechin bis(HHDP)glucose vanillic acid glucoside gallocatechin unknown castalagin (ET) ellagic acid pentoside quercetin 3-galactoside ChemSpider ID: 10197386 methylquercetin glucuronide

1 2 3 3 4 2 2 1 3 3

proposed compd IDc

ID leveld

AvSd

FeSk

a Delta (mDa) difference between the measured and calculated m/z values for the assigned formula. bmSigma: numerical expression for the difference in the measured and calculated isotope pattern of a compound; low values indicate a good match. cIdentification. d1. Positive identification: retention time, calculated elemental composition, mSigma value, and fragment ion masses all correspond to an authentic standard. 2. Tentative identification: elemental formula calculated with mSigma value support; possible structure identified in chemical databases (e.g., ChemSpider); fragment ion mass data supports the proposed molecular structure. 3. Possible identification: elemental formula calculated with mSigma value support; possible structure identified in chemical databases, but no support from fragment ion mass data. 4. Elemental identification: elemental formula calculated with mSigma values support but no suitable compound found in chemical database. 5. No identification: no reasonable calculated elemental formula fits the measured mass data. eDouble charged ion [2−].

48 h. The cultures were then centrifuged for 5 min at 4,000 rpm, washed twice with phosphate buffered saline solution, and resuspended in 10 mL of 1% SPW. The test compounds were prepared as described previously to obtain a final concentration of 20 mg/mL. In a 96-well plate, an aliquot (120 μL) of MHB or TSBYE broth was distributed. Each bacterium was inoculated (5 μL) in duplicate into the wells. After that, compounds were 2-fold serially diluted (10 to 0.078 mg/mL) and added to the inoculated wells (125 μL). Absorbance measurements (OD 595 nm) were taken once the dilution was completed (T0). Plates were incubated at the optimum growth temperatures of each bacterium. Final absorbance readings were taken after 24 h, and the level of growth inhibition was determined by comparing them with the readings at T0 and subtracting control backgrounds. The MIC values were taken as the lowest concentration of compounds that completely inhibited the growth of the microorganisms after 24 h of incubation. Four independent experiments were conducted in duplicate. If a given replicate did not prevent growth at 10 mg/mL, a value of 20 mg/mL for the MIC was used to calculate means and standard errors.

moisture (typically 5−10%), which was not considered in the TPC and TFC results. Anhydrous (+)-catechin, the external standard for checking the accuracy of the TPC assay, yielded the expected result of ∼1000 mg GAE/g. The extracts with high TPC values (>300 mg GAE/g) were either tannins or seed extracts, AvSk being the only exception. The mass data and identification attempts for the most abundant compounds (with retention times of 2−18 min) in each extract are shown in Table 3 (see Table 3 footnote for the definition of the different levels of identification). The phenols of AvSk and AvSd consisted mostly of flavonoids, as shown by their high TFC/TPC ratios. Their TPC values were much higher than those reported by ́ Rodriguez-Carpena et al.,32 who extracted fresh avocado skins and seeds with 70% acetone. This may be due to differences in material preparation and extraction methods. The base peak chromatographs (BPCs) of AvSk and AvSd showed that these extracts contained a large number of compounds. Many of the compounds detected in these extracts were identified at levels 1−3 as procyanidin monomers, dimers, or oligomers. Phenolic acids were also detected in the avocado extracts. These results

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RESULTS AND DISCUSSION Phenolic Composition of Extracts. The TPC and TFC of extracts are shown in Figure 1. The extracts contain some F



Article

́ are in agreement with the work of Rodriguez-Carpena et al.,32 who found that the phenolic compounds in Hass avocado skins and seeds were predominantly procyanidins. The condensed tannins (PiTa, MiTa, and CaTa) showed quite different TFC/TPC ratios. For PiTa it was high, which is supported by the LCMS results. Its BPC was relatively simple with only seven compounds. Dihydroquercetin and five procyanidins were identified with a high level of certainty (level 1 or 2). A much lower TPC of 363 mg GAE/g was reported for a commercial pine bark extract from Pinus maritima.37 Regarding MiTa, its tannin content of ca. 77% of the dry matter (according to its technical datasheet) is somewhat higher than expected based on its TPC, even allowing for a moisture content of 10%. Since condensed tannins are proanthocyanidins, the rather low TFC/TPC ratio of MiTa is also surprising. However, the LCMS analysis showed that its main components are flavonoids. Cationic tannin is produced from mimosa tannin by the Mannich reaction, which introduces quaternary amino groups into the tannin. Based on the TPC and TFC values of CaTa and MiTa, this reaction reduces both TPC and the TFC/TPC ratio. The high TPC and low TFC/TPC ratio of TaAc and MaSd are consistent with gallotannins. This is supported by the LCMS data showing the presence of polygalloyl glucoses containing 5−9 (TaAc) or 5−8 (MaSd) galloyl units, identified at level 3. The BPCs of the two extracts also had many similarities. Aragbo1 found ca. 75% of tannin-related substances in mango seed to be gallotannins and 25% related to condensed tannins, while Kabuki et al.34 found mango seeds to contain 79.5% of unspecified polyphenols. Engels et al.,35 who extracted mango seeds with 80% acetone, found the seed extracts to contain galloyl glucoses with 4−12 galloyl groups. Grape seed extracts have high TPC,5 which was also the case for the one used in this investigation (GrSd). According to Nakamura et al.,36 the phenolic substances in grape seed extracts are mainly oligomeric and polymeric proanthocyanidins with minor amounts of low molecular weight procyanidins and gallic acid. Our LCMS results gave similar results, although the most prominent components were identified as catechin and epicatechin at level 1. The BPC of GrSd resembled that of AvSk; some of their identified major components were also the same. The extracts from berry juicing residues and some of the fruit skin extracts had moderate TPCs (57−172 mg GAE/g), their TFC/TPC ratios ranging from 0.24 to 0.61. The LCMS data show that FeSk phenols comprised flavonoids, ellagitannins, and phenolic acids. They often contained a pentoside or hexoside moiety and were identified at level 1 or 2. Earlier reports have shown that commonly eaten berries such as those included in this study contain the same types of phenols.37 The skins of most other fruits and AvPu gave low TPC values ( gallic acid; pyrogallol was not investigated) are partly in line with Kim’s findings.26 In general, the extracts with a high or moderately high antioxidant capacity produced large ZIs with most bacteria. The monomeric reference compounds gallic acid and catechin, however, had much lower antibacterial activity than the extracts rich in polygalloyl glucoses (TaAc, MaSd) or procyanidins of different sizes and monomeric flavanols (PiTa, GrSd). Our findings agree with those of Chung et al.,41 who reported tannic acid to be a more potent antibacterial agent than gallic acid against food-borne bacteria. The MIC assays were only carried out for selected extracts (Table 6) based on their performance in the ZI assays and potential availability as byproducts. The best-performing extracts in the ZI and MIC assays were clearly MaSd and TaAc, followed by mTaAc and FeSk. According to Engels et al.,35,42 the antibacterial activity of mango seed extracts is due to the iron-chelating ability of the gallotannins with steric effects making the smaller gallotannins somewhat more effective than the larger ones. Other antibacterial chelating compounds such as mangiferin43 (a tetrahydroxyxanthone glucoside) worked synergistically with the gallotannins. A tetrahydroxyxanthone glucoside was also identified in MaSd at level 3. The fact that MaSd and TaAc exhibited similar antibacterial activities despite the large difference in their TPC could thus be largely due to the presence of mangiferin (and/or other chelating compounds). Their different gallotannin size distributions, resulting in different degrees of steric hindrance, could also affect their relative antibacterial activities.42 The weak antibacterial effects of gallic acid compared to TaAc and MaSd are likely due to the fact that the iron (Fe3+) chelates formed by the extracts are able to form lattices by coordinating with up to three odihydroxyphenyl moieties, possibly from different molecules, until the complexes get large enough to precipitate out of the solution.42 In general, the highest levels of antibacterial activity were seen against the Gram-positive bacteria: S. aureus, C. estertheticum, B. thermosphacta, and L. monocytogenes. The Gram-negative bacterium S. putrefaciens and Lactobacillus sp. (Gram-positive) were more difficult to inhibit than Grampositive bacteria as only AvSk and AvSd showed low MICs

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.23 3.26 2.70 3.48 1.43 0.13 0.30 3.26 2.96 3.58 1.38 1.53 2.70 2.81 0.12 3.38

AvSd 0.94 20.00 5.00 20.00 20.00 0.63 0.47 20.00 0.31 2.50 10.00 20.00 20.00 1.88 5.63 5.00

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

MiTa

0.14 2.11 1.25 3.07 2.67 0.74 0.34 0.00 0.08 0.21 1.83 1.25 1.83 3.09 3.04 1.30

12.50 12.50 11.25 1.88 20.00 3.75 0.47 20.00 20.00 7.50 7.50 6.25 20.00 1.56 1.88 20.00

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

3.39 3.16 3.37 3.45 0.00 3.75 0.75 2.67 2.65 3.01 2.63 2.71 1.64 3.08 3.01 2.19

GrSd 2.50 20.00 20.00 11.25 20.00 7.50 0.31 20.00 15.00 5.00 20.00 20.00 20.00 0.63 0.63 2.50

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2.50 2.78 3.15 3.43 0.00 3.09 3.23 2.19 2.66 3.46 2.75 0.00 0.00 0.10 0.05 3.37

AvSk 2.50 20.00 10.00 20.00 20.00 0.63 0.94 20.00 0.31 5.00 20.00 20.00 20.00 12.50 2.50 20.00

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2.32 0.00 2.10 3.07 1.25 1.21 2.39 0.00 0.08 2.62 0.00 0.00 0.00 3.34 2.97 2.20

against them. Gram-negative bacteria are usually more resistant to antibacterial agents than Gram-positive because of their protective polysaccharide layer,44 as it was also observed in the present study. This becomes important as many pathogenic and spoilage bacteria are Gram-negative. A comparison of the MIC values with the spoilage bacteria information in Table 1 shows that MaSd and TaAc in particular are able to provide significant inhibition of several major fish and lamb spoilage bacteria while also inhibiting many pathogenic bacteria. Kabuki et al.34 found that the MIC profile of ethanolic mango seed extract was different from those of flavonoid-based ethanolic extracts (tea catechins and theaflavins). Also FeSk and mTaAc gave relatively low MIC values against several of the bacteria tested. Based on the MIC and ZI results, these four extracts proved the most promising candidates for antibacterial packaging films able to extend the shelf life of chilled fish and lamb. As for the other compounds, their MIC values may be too high for such applications despite their ZI activity at the dose investigated. This is the case especially when activity against a broad range of bacteria is desired. Before any packaging materials containing these compounds may be used for food transport, however, the migrating compounds and the packaging material (at least its food contact layer) must comply with strict toxicity criteria. These depend on the type of compound and the expected level of migration. For any novel compounds or packaging materials, nontoxicity must be demonstrated unless exclusion from toxicity testing has been granted. Migration limits include compound-specific limits and maximum limits applying to all compounds (e.g., 60 mg/kg food in the EU) regardless of toxicity. Compliance with these issues should be an integral part of antimicrobial packaging design from the outset. Using compounds (of natural origin where possible) and packaging materials approved for food contact by European Food Safety Authority (EFSA), the US Food and Drug Administration (FDA), and other food safety authorities can significantly reduce testing requirements and improve the chances of finding feasible antibacterial packaging solutions. To sum up, based on our in vitro assays, the most promising natural extracts for antibacterial packaging films were MaSd and TaAc, i.e., the extracts rich in polygalloyl glucoses and with the highest antioxidant capacity. However, the TPC and LCMS I



Article

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analyses indicate that TaAc is higher in polygalloyl glucoses than MaSd, whose antibacterial activity was somewhat higher. This could be due to the different polygalloyl (penta- to nonagalloyl) glucose size distributions in the two extracts or other components in MaSd working synergistically with polygalloyl glucoses and/or showing high antibacterial activity on their own. Extracts rich in flavonoid type phenols showed a moderate level of antibacterial activity while those with low total phenol content displayed low activity. Further work should be carried out on the extracts based on polygalloyl glucoses, particularly relating to the economics of using them in supplement and packaging applications and in relation to their safety for human consumption.

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AUTHOR INFORMATION

Corresponding Author

*Tel: + 64 7 343 5899. Fax: +64 7 348 0952. E-mail: petri. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Fressure Foods, NZ, for supplying the avocado processing residue.

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REFERENCES

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Antibacterial Activity against Foodborne Bacteria and Antioxidant

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