Chocolate, known as the "king of sweets", is loved by people for its pleasant flavor and taste. However, many quality problems, such as frosting, deterioration, and insects, directly affect the taste experience of consumers and even severely affect human health.
Packaging Requirements for Chocolate
Among the quality problems of chocolate, the most common ones are frosting, deterioration and insects. Frosting refers to the problem that during the production, storage and selling, the surface of the chocolate is covered with uneven white spots, and even the whole surface becomes off-white in color and loses its gloss. Moreover, the interior of the chocolate gradually becomes grit-like, losing the crispy and smooth taste. According to previous studies, chocolate hoarfrost is divided into sugar frost and fat frost. When the humidity around the chocolate is too high, the water vapor causes the granulated sugar crystals on the chocolate surface to melt. When the humidity is lowered, the sugar crystallizes again and forms a frost on the surface of the chocolate. Compared to sugar frost, the fat frost is more liable to occur and has more significant impact on the quality of chocolate. By observing microscopically, the chocolate is covered with a crystalline network composed of fat molecules and some liquid fat molecules and cocoa powder, sugar powder and other raw materials are evenly distributed in the crystalline network. When the ambient temperature rises, the high-melting point liquid fat molecules gradually migrate to the surface of the chocolate and then form hoarfrost.
Brownie is an important type of chocolate. It is made by blending protein-rich nuts such as hazelnuts, almonds, peanuts, and walnuts into the chocolate or evenly coating the surface of the chocolate. Brownie has a crispy taste in addition to the original mellow flavor of the chocolate, so much loved by people. But because the nuts are rich in oil, these oils are easily affected by light, temperature, oxygen and water vapor, causing hydrolysis and rancidity and oxidative rancidity, which emits bad halo and damages chocolate flavor. Compared to the above two problems, the impact of insects is more intuitive. On one hand, the insects are from the contamination of the chocolate raw materials and the eggs in the production process. On the other hand, the packaging is not tightly sealed, or there is a slight gap damage, the insects will sneak into the chocolate and grow on the chocolate.
All the quality problems such as frosting, insect or rancidity of the chocolate may be caused by many complicated factors, including raw material composition, processing, storage conditions and packaging methods. For chocolate products during production and storage period, the quality of chocolate can be controlled through adjusting raw material components, additives, temperature and cooling processing technique. Once the product is sold, the storage conditions will be out of control of the manufacturer and the impact of package will be magnified.
According to the above analysis, the package of chocolate should have the following properties to guarantee the quality of the products:
(1) Excellent water vapor barrier properties to prevent external water vapor from permeating into the package, which may cause the deterioration or frosting of chocolate. China has a vast territory, and the humidity in most Southern area is high, which has more requirements for high moisture barrier properties of packaging.
(2) For brownie, in addition to the moisture barrier properties, the package should also have good oxygen barrier property. By reducing the amount of oxygen permeating through the packaging material, the oxidative deterioration of the products can be inhibited during the storage period.
(3) Since fat frosting may occur under the high temperature and strong temperature fluctuations, the thermal insulation properties of the package shall not be ignored.
(4) In the packaging process, it is necessary to adopt a low-temperature sealing method, which can guarantee the seal performance to prevent the insects.
Current Situation of Chocolate Packaging
The evolution of chocolate packaging is always focused on the packaging materials and packaging forms that can meet the requirements mentioned above. The earliest chocolate packaging is double-layer tin foil paper. The thermal conductivity of tin is 67W/mK, very excellent heat insulation property. Meanwhile the tin foil paper can effectively block the permeation of water vapor and oxygen [3]. Due to the limited global reserves of tin, the cost of tin foil is rising year by year, and that's why it is gradually replaced by other materials e.g. aluminum foil.
Aluminum foil is a kind of soft metal material. Its biggest advantage is its high barrier property. It can effectively block the permeation of non-polar gases e.g. oxygen, and also has good moisture barrier property. However, the mechanical strength of the aluminum foil is poor, which cannot be used as packaging material alone, and its thermal conductivity is 203 W/mK, which is liable to cause the chocolate to melt. Therefore, aluminum-plastic composite packaging, aluminized composite packaging, and aluminized paper packaging are usually used to compensate the shortcomings of the aluminum foil in strength and heat insulation and meanwhile maintaining its inherent characteristics of the aluminum foil. However, it must be noted that the barrier properties of aluminum foil may be seriously reduced when being folded or flexing, which cannot be remedied once it occurs during storage or selling.
The requirement for high-barrier properties of chocolate packaging does not mean that excessively high-barrier materials are required in actual production, which may result in cost waste and environmental pressure. It is recommended for the manufacturers to select high barrier materials based on the raw material characteristics of product and quality guarantee requirements. In addition, optimization of the composite film structure and thickness, e.g. using composite films made of common film with good barrier properties and aluminum foil, and reducing the thickness of the aluminum foil layer, which can also obtain good barrier properties of the package.
Common Performance Indicators of Chocolate Packaging
We tested the oxygen transmission rate, water vapor transmission rate and seal performance of three types of chocolate packaging materials including PET/AL/CPP composite film, OPP/VMPET/PE composite film and vacuum aluminized paper.
Testing Instruments & Test Method
Oxygen Transmission Rate Test
Testing Instrument: OX2/230 Oxygen Transmission Rate Test System
Oxygen transmission rate test is performed according to ASTM D3985 (coulometric method). The test cell is divided into two parts. Oxygen flows in one side and carrier gas (nitrogen) flows in the other side. The carrier gas carries the oxygen that permeates through the film to the coulometric sensor and generates voltage after chemical reaction. The oxygen content can be obtained according to the voltage.
Water Vapor Transmission Rate Test
Water vapor transmission rate test is performed according to ISO 15106-3. The sample is placed in the test chamber of W3/330 Water Vapor Transmission Rate Test System. The nitrogen with stable relative humidity flows in one side of the sample, while dry nitrogen flows in the other side of the sample. Due to the humidity difference, water vapor permeates from the higher humidity side through the sample to the lower humidity side. In the lower humidity side, the permeated water vapor is carried by the dry nitrogen to the sensor, and generates the same proportion of electrical signals, by which the water vapor transmission rate can be calculated.
Seal Performance Test
Seal performance test is performed according to ASTM D3078 (bubble emission method). Testing instruments is MFY-01 Leak Tester.
Results & Analysis
| Sample | OTR cm3/(m2·24h·0.1MPa) | WVTR g/(m2·day) | Seal Performance kPa |
1# PET/AL/CPP | 0.215 | 1.371 | -90,NO LEAKAGE |
2# OPP/VMPET/PE | 29.219 | 2.013 | -39.3,LEAKAGE AT FOLDED LINE |
3# Aluminized Paper | 80.197 | 29.518 | N/A |
Note: Aluminized paper is usually used to wrap the chocolate, so we did not do seal performance test.
It can be found from Table 1 that the oxygen transmission rates of the samples 1#~3# are quite different. The oxygen transmission rate of 3# aluminized paper is 600 times of that of 1# aluminum plastic composite film.
As for water vapor transmission rate, 1# sample is almost the same as 2#, and 3# sample has the lowest water vapor barrier property, WVTR of which is 20 times higher than that of other samples.
In the seal performance test, the sealing of the 1# sample was good, which did not leak at the maximum negative pressure of -90 kPa, and the 2# sample leaked at a lower negative pressure. The above performance difference is related to the material structures of the samples. The 1# aluminum-plastic composite film is a composed of a layer of 7~12 μm aluminum foil and multi-layers of plastic film.
With the protection of plastic film, the aluminum foil can show its high barrier property. 2# aluminized composite film is a kind of film material with the deposits of aluminum vapor on its surface under high vacuum condition. The thickness of the aluminum coated layer is usually 40~70 nm, which is only 0.57% of the thickness of the aluminum layer of the aluminum-plastic composite film. The barrier properties are also reduced as the thickness decreases. The substrate of 3# aluminized paper is paper, which has lower strength and barrier property, compared to plastic film.
Due to various factors in the production process of aluminum foil, especially for the food package, if the thickness of aluminum foil is less than 20 μm, the pinholes will be inevitable. After flexing with external force, the pinholes of the aluminum foil become larger and larger or even broken to form visible pinholes or creases. In the seal performance tests of the two samples, 2# aluminized composite film leaked at the fold line of the bag, which indicates that the aluminum-coated layer of the material may have serious penetrating pinholes under the negative pressure, thus a rupture leak occurred. Studies have shown that appropriately increasing the thickness of the aluminum foil / aluminized layer can effectively reduce the number of pinholes [4]. Therefore, if the cost permits, the thickness of the aluminum foil layer or the aluminum coated layer can be appropriately increased to improve the barrier property and the flex durability of the aluminum composite film or aluminized film.
According to the test data, the three materials have different barrier properties and are suitable for products with different requirements. For big companies, the material of 1# sample structure can be used to pack chocolate with high quality and has requirements for long-term storage. The 2# and 3# materials are less expensive and have qualified barrier properties. They are ideal for small and medium companies to pack ordinary chocolate with less stringent storage requirements. Regardless of the type of packaging materials, excessive extrusion or squeezing should be avoided in storage and selling to avoid damage to the aluminum foil layer or the aluminized layer. It should be noted that, barrier properties are only one aspect that should be considered when selecting packaging materials, and chocolate manufacturers should take all aspects into consideration according to actual conditions of their own.
Conclusion
Chocolate is a high-end sweet, which represents beautifulness. If there are quality problems such as frosting, deterioration and insects, the sales and product image of chocolate will be adversely influenced. Through the analysis of the reasons for those quality problems, the package for the chocolate should have good barrier properties, heat insulation and seal performance to ensure the quality of chocolate during storage. Therefore, quantitative testing and data comparison analysis of the package is recommended to be carried out with professional testing technique to help the manufacturers to select proper cost-effective packaging materials and thus avoid more quality and safety problems with the chocolates.