Banana Microstructure Changes during Oven Drying Process

Fengying Xua, Zhen Chenb, Changyou Lic, Shengjie Deng,

Yongfeng Chen, Ce Xu and Jianyuan Liao

Key Laboratory of Key Technology on Agricultural Machine and Equipment, South China

Agricultural University, Ministry of Education, Guangzhou 510642, China

axu_fy@scau.edu.cn, bzhenchenid@scau.edu.cn, clichyx@scau.edu.cn

Keywords: Banana, Drying Process, Microstructure, Dehydration Performance, Quality.

Abstract. To learn about banana microstructure changes on affecting moisture dehydration

performance and its quality during oven drying process, this experiment chose scanning electron

microscopy to observe the microstructure changes on outer surface, inner surface and cross section

of banana peel and its flesh during drying process, in order to examine the relationship between

dehydration performance and its microstructure shape and microstructure pore structures changes.

The results showed that during drying process, all surfaces of banana peel and its flesh endured

shape deformation from regularly to irregularly. Microstructure shrank evenly in beginning, while

became conglomerate later, appeared to be significantly distortion and breakage. Microstructure

pore quantity and area showed great changes, those average diameter over 10um pore numbers

increased steadily, while pore area just reduced in beginning and then increased rapidly afterwards.

As banana microstructure shape and pore shrinkage and deformation increased greatly, which also

increased the resistance of moisture evaporating greatly. This may be an important reason that cause

banana drying speed and quality decreased, it is an urgent problem that needs to be solved during

fruit and vegetable drying process and storage.

Introduction

Drying is a process with material moisture evaporating by thermal action, this process is often

associated with material physical and chemical properties changes[1]. Banana (Musa paradisiaca )

drying process can reduce mechanical damage for fresh fruit, easy for storage and transportation.

Recent researches proved that banana drying dehydration performance and quality with relevant to

drying energy input method and amount[2-4], during drying process, as moisture evaporating,

banana appeared to shrink[5-6]. Some researchers studied on dehydration performance and quality

changes during banana drying process on temperatures, they found banana flesh microstructure

changed obviously during drying, all these researches mainly focus on fruit dry product

microstructure contrast [6-7], while pay less attention to microstructure changes affecting moisture

evaporating resistance or drying quality relationship research[8-9], so still needs to be further study.

This research observed banana microstructure changes during oven drying process, explored the

relevant relationship between these changes and its drying dehydration performance and quality

changes, in order to provide solutions to improve banana drying technology and its equipment

research.

Advanced Materials Research Vols. 524-527 (2012) pp 2259-2264Online available since 2012/May/14 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.524-527.2259

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Materials and Methods

Materials. Testing bananas from Panyu district of Guangzhou China, cultivated locally, fresh

without injury on 90% mature, average size, with potassium permanganate sterilization before

sampling. After sterilization, cut its peel and flesh from middle part, with sample size as peel at

1cm×1cm and flesh at 3mm thickness. The moisture content for peel is 92.71% and for flesh is

75.96%.

Instrument and Equipment. B101S-2 drying oven made by Shanghai KaiLang Company,

electronic weight indicator PT650F made by Guangzhou Zhida Company, scanning electron

microscopy XL-30ESEM made by Holland Philips-FEI and EDAX Company, vacuum spray

plating instrument HUS-5GB made by Japan Hitachi Company, IB-5 typed iron sputtering

instrument made by Japan EIKO Company.

Sampling Methods. Testing samples were divided into 4 groups, each group including 10pcs

banana peels, 3pcs banana flesh. All samples were drying in oven at 105ºC temperature, on 30min,

60min, 90min, 120min time respectively. After drying, weighed all peels and flesh samples

separately and made some selections, then started to prepare test slices, plated, then used

microscopy to scan slices, thus SEM pictures for peels and flesh in different drying time could be

obtained.

Results and Analysis

Analysis of Moisture Content in Banana Peel and Flesh. Fig. 1 showed that the moisture content

changes were similar for banana peel and flesh during oven drying process, with two stages

including moisture evaporating rapidly stage and evaporating slowly until absolutely dry stage[6].

On the moisture evaporating rapidly first stage, drying time was set 60mins, moisture content

changing rate in banana peels was faster than its flesh, so it could dry rapidly to near absolutely dry

status. In the second stage, moisture content in peels was close to absolutely dry state; so

moisture-evaporating speed was very slow, far less than that in its flesh.

0

20

40

60

80

100

0 30 60 90 120

Dry time/min

Moisture content/wb%

Peel Flesh

Fig. 1 The moisture content changes of banana peel and flesh during drying process

This experiment showed that during oven drying process there were lots free water among

microstructure or inside microstructure of peel and flesh. So the drying first stage was mainly

removing free water among microstructure, while the second stage was mainly for removing free

2260 Natural Resources and Sustainable Development II

water inside microstructure. As moisture inside microstructure evaporated and will increase the

density of organic solid solubility substance, which would lead to organic solid solubility substance

diffusion outward, so the organic solid solubility substance content in peels or flesh occurred

inconsistently changes, thus microstructure space changed and increased moisture evaporating

resistance, and drying dehydration performance and drying quality were affected.

Banana Peel and Flesh Microstructure Changes During Drying Process. Based on 1000X

SEM observation on banana peels outer surface, inner surface, cross section and flesh surface

microstructure, microstructure pictures were obtained, as shown in Fig. 2 to Fig. 5 respectively.

(1) Banana peel outer surface microstructure changes during oven drying process

30min 60min 90min 120min

Fig. 2 Peel outer surface microstructure changes at different time delays

Fig. 2 showed peel outer surface has microstructure changes greatly during oven drying process

at different time delays. In the initiate stage, free water among microstructure evaporated evenly,

peel microstructure connected to each other loosely and with smaller curve deformation, thus

microstructure pore remained evenly spreading. While as more and more moisture evaporated out,

microstructure became gathering and conglomerated, distortion became severely, microstructure

pores became smaller, thus some parts even cracked. When peels dried to near absolute dry status,

microstructure distorted greatly with deformation and bumps, some parts even diffused with

particles. Finally when dried to absolute dry stage, microstructure deformed and conglomerated

significantly, particles became more and more among microstructure and diffused evenly.

This experiment showed that during drying first stage when heat energy imported while free

water among microstructure evaporated, loose distribution peel microstructure still remained

microstructure without much damage. When thermal action became more intensively, free water

inside microstructure lost gradually, microstructure shrinkage and deformation increased, space

among microstructure reduced and even disappeared at last, thus microstructure became bounded

by each other and density of organic solid solubility substance inside cells became too high and

diffused outwards, as a result peel outer surface cells became more tight and increased the inhibition

of free water evaporating outwards.

(2)Banana peel inner surface microstructure changes during oven drying process

30min 60min 90min 120min

Fig. 3 Peel inner surface microstructure changes at different time delays

Advanced Materials Research Vols. 524-527 2261

Fig. 3 showed peel inner surface also had microstructure changes similar to outer surface during

oven drying process. On the beginning stage, free water among cells evaporated rapidly while peel

cells still remained its shape basically. When drying proceeded on, peel cells became dehydration

distorted and deformation on shape, moisture inside microstructure diffused outwards, peel inner

surface began to change its uniform microstructure and shrinkage appeared, some microstructure

started to mix together and connected to each other and formed some microstructure network

structure. When drying came to almost absolute dry status, pore space among microstructure

disappeared greatly, glucose substance inside microstructure diffused outward partially,

microstructure mixed and connected to each other in large amount. When drying came to absolute

dry status, peel inner surface became bumps structure, microstructure diffusion particles increased

greatly, while both distribution number and density were far less than its outer surface. As a result,

during dehydration process microstructure damage on inner surface may less than that of outer

surface, while pores for moisture evaporating basically vanished, thus moisture transportation was

severely resisted.

(3)Banana peel cross-section microstructure changes during oven drying process

30min 60min 90min 120min

Fig. 4 Peel cross-section microstructure changes at different time delays

Fig. 4 showed peel cross section also had microstructure changes significantly during oven

drying process. On the beginning stage, microstructure remained its shape well; kept in clear border

with each other, pore space distributed evenly among microstructure. While as drying proceeded on,

microstructure on banana peels had shrinkage significantly and formed obvious microstructure

network structure, pore space among microstructure disappeared almost, peel microstructure

became bumps structure. When free water evaporated more and more to almost absolute dry status,

peel cross section microstructure changed greatly and became conglomerated, microstructure

network structure disappeared, pore space had partial broken area arose as well. To absolute dry

period microstructure became conglomerated and cracked in connection more seriously,

microstructure network changed from close connection to broken state, peel cross section

microstructure occurred different sizes broken pores.

This experiment showed that during dehydration process free water among microstructure

evaporated will lead to microstructure shrinkage and pore among microstructure close gradually,

cross section microstructure closed tightly, resistance increased greatly to prevent moisture

transporting through cross section. Furthermore, as free water among microstructure or inside

microstructure dehydrated gradually, connection among microstructure lost its elastic property and

cracked under external force, as a result this partial broken lead to entire deformation.

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(4)Banana flesh outer surface microstructure changes during oven drying process

30min 60min 90min 120min

Fig. 5 Flesh outer surface microstructure changes at different time delays

Fig. 5 showed banana flesh microstructure changed significantly during oven drying process. On

the beginning stage, flesh microstructure became distorted along with free water removed, while its

microstructure remained in clear microstructure network structure and seldom interconnected with

each other. As drying continued on, flesh microstructure appeared shrinkage on large area, piled up

and polymerized together. The microstructure network structure could be seen clearly when began

to polymerized, while almost disappeared when drying to absolute dry status, some dented area also

appeared, thus flesh piled up and bumped significantly. This changes might be happened by

microstructure connection broken greatly under thermal action, its organic solid solubility substance

inside microstructure became pasting when microstructure connection broken[8-9], it was also an

important factor that lead to flesh drying quality changed.

(5)Banana Peel cross-section microstructure statistics during oven drying process

Different drying process SEM microstructure pictures for banana peel and flesh could be

obtained through SEM observation, with 500X setting and counted the numbers and area on all

pores with diameter larger than 10um within red-circle-marked area as shown in Fig. 6, the

red-rectangle-marked area was 100µm×100µm in size, statistics data as shown in Fig. 7.

30min 60min 90min 120min

Fig. 6 Peel cross-section 500X microstructure statistics at different time delays

0

1

2

3

4

5

30 60 90 120

Time/min

Number of max hole

0

200

400

600

800

1000

1200

1400

Max Area /um2

Number of max hole

Max hole area

Fig. 7 Flesh cross-section 500X microstructure statistics at different time delays

As Fig. 6 and Fig. 7 showed a correlation-ship among peel cross section microstructure changes

and its average-diameter-over-10um pore numbers and area, pore numbers increased steadily along

with drying time continuing, while average pore area reduced rapidly to almost zero in beginning

Advanced Materials Research Vols. 524-527 2263

stage and then increased rapidly until reaching stable value as shown in Fig. 7. This result showed

that on peel cross section moisture among microstructure evaporated greatly on initial drying stage,

pore structure was affected less and distributed evenly, while as drying proceeded on, free water

removed greatly and led to microstructure shrinkage and deformation significantly, thus

microstructure pores closed tightly. As free water kept evaporated under thermal action,

micorstructure membrane strength reduced greatly and started to break, this breakage developed

from initial partial area to entire area, thus more broken pore numbers increased while broken pores

area reduced a little bit, banana drying quality and dehydration performance affected significantly.

Conclusions

Banana oven drying process was a kind of energy coupling procedure among peel, flesh and its

microstructure, the microstructure changes affected microstructure moisture dehydration, this

influence might include two stages as moisture among microstructure dehydrating and moisture

inside microstructure dehydrating. When on the first stage, peels surface and flesh microstructure

distorted slightly, pore area among microstructure remained evenly in proper ratio. While as drying

continued, free water inside microstructure forced to remove under thermal action, thus

microstructure became shrinking and distorted seriously, deformation and then breaking, some

microstructure started to break and dissolute, thus pores among microstructure changed from evenly

distribution status to shrinking, close tightly and cracking continuing as an uneven status, thus

moisture transpiration resistance among microstructure increased significantly and prevented

further dehydration. This was mainly factor that lead to peel and flesh moisture dehydration

performance and quality decreased on drying. Further study still needed to discuss how to set proper

oven drying operation conditions to improve banana peel and flesh microstructure changes scope

along with optimizing drying quality and performance.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China

(31071583, 30900870), the Key Project of Natural Science Foundation of Guangdong Province

(9251064201000009).

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