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Chinese dumplings

S. Huang

BRI Australia, P. O. Box 7, North Ryde, NSW 2113 Australia

Introduction

Apart from steamed bread and noodles, Chinese dumplings are another significant traditional wheat flour based food in China. They are popular not only throughout China, but also in Japan, Korea, and Southeast Asian countries. Chinese dumplings were traditionally eaten on special occasions: festivals and celebrations; however, they are becoming more widely consumed as living standards in China continue to improve.

Chinese dumplings are usually made from wheat flour dough sheeted to a thickness of 1.5 - 2.0 mm, this sheet forms a skin which is used to wrap a variety of delicious fillings. The fillings usually consist of mincemeat, vegetables, seafood or a mixture.

There are three ways of cooking dumplings: boiling, steaming and frying. Boiling is the most common way of cooking and boiled dumplings are called Shuijao. When dumplings are fried, they are first pan-fried on the bottom of the dumpling and then steamed with some water in the pan. They take on a crisp golden brown texture on the bottom and a soft one on top, bursting with juice inside. On the other hand, steamed dumplings can be made into different and often beautiful shapes before steaming to produce a soft texture, and shiny appearance for celebrations and banquets. Dumplings prepared using these different cooking techniques have different product quality requirements. The present paper is concentrated on boiled dumplings.

Dumplings were commonly made at home which was a time consuming process. Food manufacturers have responded to the consumer demand for greater convenience through the production of frozen dumplings that are available for sale in many retail outlets. These products may be sold in bulk or as pre-packed products. Good quality boiled dumplings have a bright, white skin which has a soft, smooth and elastic texture. These properties are highly dependent on the flour quality used and their processing.

One common problem observed in commercial samples of frozen dumplings is the occurrence of surface (skin) checking. This is a serious quality fault as it results in the dumplings breaking during cooking When the dumplings break the juices and flavours from the fillings are lost, and the dumplings are effectively ruined.

Cracks develop during freezing, warehouse storage, transport and supermarket storage management. Very small cracks, referred to as ‘crazing’, are very likely to heal during cooking. However, large cracks are more likely to result in the dumplings breaking during cooking. Small cracks in the dumpling skin formed during initial freezing can develop into larger cracks during poor cold storage and transport management.

The present study established an optimum laboratory freezing procedure for dumplings, to allow the key flour components critical for dumpling quality to be established, and to investigate the suitability of Australian wheat for this product.

Materials and methods

Flour

Two flours were used for dumpling freezing trials: Newcastle APH and Pt Kembla APH.

Thirty-seven Australian wheat varieties and one commercial flour (as control) were used to evaluate flour quality requirement.

Dumpling Manufacture

Dumplings were processed according to the laboratory method of Huang et al (2002).

Measuring Dumpling Firmness

Immediately after removal from the blast freezer, the firmness of frozen dumplings was measured on the upper surface of the dumplings with a round flat 12mm diameter probe, using a constant force of 100g. When the dumpling withstood puncture by this probe for more than 10 seconds, the blast freezing time was assessed to have been sufficient.

Dumpling Cooking

Four fresh dumpling samples were placed into 4 separate compartments of a cooking basket (one control dumpling sample was always included, each sample had 22-24 dumplings). The cooking basket was placed into boiling water and the timer was immediately started. During cooking a soft spatula was used to gently submerge any floating dumplings. At 10.5min the cooking basket was removed from the boiling water and immediately drained for 5 seconds before plunging into ice-water with continuous agitation by repeatedly removing and re-submerging 20 times to maximize heat transfer. The basket was drained for another 10 seconds before plunging into a second ice-water bath, with agitation repeated as for the previous bath. Finally the basket was drained for 1 minute followed by an assessment of dumpling breakage. A taste panel was carried out 15 minutes after cooking.

Dumpling Evaluation

Taste panel

A taste panel for descriptive sensory evaluation was trained extensively on the softness, smoothness and elasticity of cooked dumpling skins. To overcome the distraction from fillings, a special test procedure was developed.

Breakage counting

After cooking, the breakage of each dumpling sample was assessed and the extent of breakage was catalogued under three grades described as: bad, medium and slight. A breakage score was calculated as follows: 100 – a x 5 – b x 3 – c x 1.5 (a, b and c are the number of bad, medium and slight breakage respectively).

Colour measurement

Minolta colour measurement was carried out 0.5 hour and 24 hour on the dumpling sheet without dusting starch. L* a* b* data were recorded.

Results and discussion

1. Establishment of Optimum Freezing Procedure

(i) Supporting Materials for Dumplings in Blast Freezer

The thermal conductivity of supporting materials used during freezing can be quite different. The thermal conductivity of plastic material is approximately 0.15 W/(m.K), while for aluminum it is approximately 350 W/(m.K) (Jastrzebski 1997). This implies that aluminum transfers heat more than 2000 time faster than plastic. The thermal conductivity of stainless steel is 18 W/(m.K), which is between plastic and aluminum. When these materials are used as supporting tray, their effect on heat transfer is quite different. Plastic transfers heat very slowly, so there is not much heat transfer between the bottom of the dumplings and the plastic tray. In contrast, a lot of heat is transferred between the bottom of the dumplings and an aluminum tray. Consequently, dumplings were frozen slowly downwards from the top of dumplings on plastic tray, while dumplings on aluminum tray were frozen upward from the bottom at a much faster rate. Dumplings on a stainless steel tray tended to freeze more evenly from both the top and bottom. These experiments also established that the cracks formed more rapidly for dumplings on aluminum trays than on stainless steel and plastic tray. In addition, the cracking of the dumplings on the aluminum trays was more intense than on either the stainless steel or plastic tray. Dumplings on raised mesh trays or cooling wire froze faster than on the other types of tray because they allow a more efficient circulation of airflow in the blaster freezer. This corresponds to the situation for conveyor belts in commercial freezing situation.

(ii)Blast freezing

To establish the effect of temperature and air velocity on freezing time and cracking or checking, dumplings were frozen supported on cooling wires, which mimics commercial conveyer systems. The cooling wires enabled blast freezer air to flow below and above the dumpling surfaces and the wire support had minimal contact with dumplings compared to a solid flat tray.

With no air flow at -35C, it took 31 minutes for the dumplings to develop a hard outer shell, which had the minimum firmness necessary for packing without damage, while it took 36 minutes at -20oC with no air significant air flow (Figure 1). However, with 0.8 m/s airflow, the freezing time sharply decreased by six minutes at -35C. When the air speed was increased to 2.4m/s, the freezing time decreased even further four minutes at -35C.

Figure 1. Effect of air flow speed on freezing time of Chinese dumplings

Cracks and crazing (or checking) were grouped together for the purposes of analysis. The effect of air speed on the checking was clear (Figure 2). Without air flow, there was 40% cracking at -35C, but near zero cracks at -20C. However, with an air flow speed of 0.8m/s, the cracking rate increased rapidly, reaching 54% and 57% at -35C and -20C, respectively.

Figure 2. Effect of air velocity on Chinese dumpling crack / crazing formation

Dumplings were blast-frozen for different time periods to achieve the same minimum firmness for packing. Dumplings developed a higher crack/crazing rate at -35C (40%) than that at -20C (almost 0%) with no airflow. However, the crack/crazing rate was similar or better at -35C than -20C at higher air velocities (0.8 and 2.4m/s)(figure 2). This was a combination of the effects of temperature and time. It appears that at high airflow and low temperature, the surface of dumplings looses moisture very quickly, and, as a direct consequence, cracks and/or crazing develop within a short time.

In the freezing of dumplings, a compromise must be made between the time of the blast and storage freezing operations. In the current investigation, a quick blast freezing to firm the outside of dumpling was performed until a suitable level of surface firmness was achieved. The storage freezing was then performed to complete the process. This method reduced surface cracks significantly.

(iii)Packing

If the packing was performed after the dumplings had achieved a suitable firmness, then the subsequent packing did not cause any increase in the current status of the cracking and crazing.

(iv)Storage in a Chest Freezer for Final Freezing

During this subsequent freezing, which involved no airflow, no increase in the current status of the cracking and crazing was observed.

(v)Distribution and Supermarket Management

Examination of commercial products obtained from a supermarket showed a range of quality faults including cracks. It is expected that the cracking was due to poor initial freezing and subsequent cold chain management in transportation and storage.

An optimized laboratory freezing procedure for dumplings was developed at BRI. as follows: dumplings were blast frozen at -35C and 0.8m/s air flow velocity for 5 min on a stainless steel supporting tray; then packed and transferred to a storage freezer within one minute, freezing was complete in 60 min. This resulted in solid frozen dumplings of which 80% were perfect and 20% had very fine crazing. The fine crazing healed during cooking and did not result in rupture of the dumpling skins during cooking.

2. Flour Quality Requirements for Chinese boiled Dumplings

A high correlation between flour swelling volume (FSV) and dumpling eating quality was observed when the 37 flours were compared (r = 0.81***, p<0.001) (Table 1). There were also high correlations between RVA peak viscosity and dumpling texture (r = 0.58***, p<0.001), and between RVA break down and dumpling texture (r = 0.72***, p<0.001). However the correlation between RVA final viscosity and dumpling texture was negative (r = -0.52***, p<0.001). Protein content did not show a critical effect on eating quality of dumplings within the protein content range of this work (10.88% 0.09), which was close to the optimum range for dumplings. However, protein content did have a significant effect on the breakage score (r = 0.59***, p<0.001). Farinograph dough development time also showed a similar effect on the breakage score (r = 0.43***, p<0.001). This indicated that protein content and dough strength should reach a certain level to prevent dumpling breakage.

There was a very close relationship between dumpling whole eating quality and elasticity (r = 0.95***, p<0.001). The correlation between FSV and elasticity was also very high (r = 0.72***, p<0.001). This meant that FSV control highly on both elasticity and whole eating quality.

Since key dumpling quality requirements included several important aspects: colour, eating quality and resistant to cooking, good dumpling flours should have a good balance in these aspects.

Table 1. Correlation between floura and dumpling quality attributes

Flour quality attribute

Dumpling texture

Dumpling elasticity

No-breakage score

FSV (ml/g)

0.81***

0.72***

n.s.

Protein (%)

n.s.

n.s.

0.59***

Starch damageb (%)

n.s.

n.s.

n.s.

Water absorption (%)

n.s.

n.s.

n.s.

Dough develop time (min)

n.s.

n.s.

0.43**

Stabilityb (min)

n.s.

n.s.

n.s.

RVA peak viscosity (RVU)

0.58***

0.55***

n.s.

RVA break down (RVU)

0.72***

0.70***

n.s.

RVA final viscosity (RVU)

-0.52***

-0.49***

-0.35*

aFor all data sample number was 37 except b data where the sample number was 34.
***
p<0.001, **p<0.01, *p<0.05, n.s. = not significant

Conclusions

Higher airflow and lower temperatures sped up the freezing process for dumplings but resulted in more surface cracks. Of these, air velocity was found to have the most significant effect on cracking. Supporting tray design and material affect the heat transfer rate and hence, freezing speed of dumplings with flat stainless steel trays resulted in more even freezing.

An optimal freezing procedure was found to involve a short blast freezing period with low air flow to firm the outside of the dumplings. This resulted in minimal crazing and gave enough strength for packing and transferring to the storage freezer to complete the freezing process.

Flours with medium protein content (10.5 – 11.0%) and dough strength, high FSV, with white and bright colour were found to be most suitable for the production of frozen Chinese dumplings. The Australian wheat variety Bonnie Rock was found to produce high quality Chinese boiled dumplings (data not shown).

Acknowledgements

The Author would like to thank the Grain Research and Development Corporation and BRI Australia for financial support. Appreciation is also due to Ms Di Miskelly for supplying commercial flour.

Refrences

Sidi Huang, Wheat Products 2: Bread, Cakes, Cookies, Pastries and dumplings in ‘Asian Foods’ Technomic, Lancaster, Pennsylvania (1999).

Sidi Huang, Steven Zounis, Bob Anderssen and Ken Quail. Surface Crack Formation during the Freezing of Chinese Dumplings. In the Proceedings of 52nd RACI conference, September 9-12, 2002, Christchurch, New Zealand, pp. 301-304.

Sidi Huang, Ken Quail, Rachel Reed, Nerizza Sy, Graham Crosbi and Vicky Solah. Flour Quality Requirements for Chinese Dumplings. In the Proceedings of 52nd RACI conference, September 9-12, 2002, Christchurch, New Zealand, pp. 139-141.

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