Green asparagus were obtained from the community enterprise of Sublanga asparagus farmers in Kho Rung, Chai Badan district, Lop Buri province. Rice bran oil (Jaikao Rice Bran Oil) was ordered from Siam Crystal Rice Co., Ltd. (Bangkok, Thailand). Calcium chloride was purchased from KemAus™ (New South Wales, Australia) and seasonings were acquired from Adinop Company Ltd. (Bangkok, Thailand).

Green, undamaged asparagus were selected. Asparagus is graded into Grades A, B, AB, C, and undergrade by their stem periphery, straightness, and tip firmness, as shown in Figures 1A–E. In this work, all asparagus used were C-grade. Green asparagus were washed, trimmed and cut into three parts 7-8 cm in length, the tips, middle part, and bottom, with circumferences of 1.58 cm, 1.88 cm, and 2.22 cm, respectively (Figure 1F). The asparagus parts used in this study, called asparagus spears, were the tips and middle segments of the asparagus stem, while the bottom parts were discarded because they were tough. The yield of fresh asparagus was measured from the difference in the average weight of asparagus before and after discarding the bottom part. The results were reported as a percentage. A colourimeter (Minolta, model CR-10, Japan) was then gauged in triplicate to determine the colour of the raw asparagus. Proximate analyses of fresh asparagus were performed before further processing. The moisture content measurements were carried out using the oven drying method according to AOAC (2000). About 3 g of the samples were taken out and dried in the oven (Hot air oven, BINDER, FD, Germany) at 105°C for 3 h, then cooled in a desiccator until a constant weight was achieved (Tanita, KD-200, Thailand). Finally, the moisture content of the products was determined and reported as a percentage. The Kjeldahl method with Nx6.25 was used to quantify crude protein. Ash content was determined in a muffle furnace at 550°C for 3 h until white or light grey ash was obtained. The Soxhlet extraction method was used for the determination of fat content. The crude fibre was analysed by the loss on ignition of the dried residue remaining after digestion of the sample and determined by weight difference. Carbohydrate content was calculated by subtracting the crude protein, fat, fibre, and ash contents from the sample difference and presented as a percentage. The samples were soaked in 0.1% calcium chloride solution and blanched in water at 95–98°C for 45 s before being immediately cooled by soaking in water at room temperature (~30°C) for 2 min. The prepared asparagus samples were packed in the bags to freeze at −18°C before vacuum frying.

The vacuum frying equipment was fabricated and assembled by Phu Pha Farm Co., Ltd., Bangkok, Thailand (Figure 2). The machine consists of a vacuum chamber and frying basket, inverter and temperature controller, oil receiving tank with a filtering system for residue after frying, and a cooling system, vacuum pump, condenser, and cooling tower. The vacuum chamber was designed from stainless steel with a diameter of 670 mm, height of 640 mm, and wall thickness of 10 mm. The stainless steel of the frying basket had a thickness of 3 mm. The power of the rotary pump was 2.35 kW, and the electric power of the equipment was 15 kW. A study by Diamante et al. (2012) found that increasing the temperature and time during vacuum-frying of apricots at 230 mbar from 70 to 90°C and 35 to 65 min, respectively, led to a higher maximum braking force. In a similar study, Shyu and Hwang (2001) discovered that increasing the frying time of apple chips from 5 to 30 min at 80 mbar resulted in a crisper texture. However, Yamsaengsung et al. (2017) concluded that increasing the temperature from 100 to 120°C had no significant effect on the crispness of banana chips. Thus, for the quality analysis of vacuum-fried asparagus in this study, the main parameters adjusted were a temperature of 80°C and a frying time ranging from 38 to 42 min at a vacuum pressure of 986 mbar. Each bath consisted of 10 kg of the asparagus samples to 100 L of rice bran oil. The samples were fried at 80°C and 986 mbar for 38, 40, and 42 min. After the frying, the oil was recovered from the frying chamber to the reservoir oil tank while the fried product was further oil removing process. The excess frying oil on the surface of the vacuum-fried sample was removed by the centrifugal method up to 1,450 rpm for 5 min. The yield for the fried sample was calculated from the difference in the average weight of asparagus before and after vacuum frying each condition using an electronic digital balance (Tanita, KD-200, Thailand). Finally, these crisp asparagus snacks were stored in sealed plastic boxes for further study.

Scanning electron microscope (SEM: JEOL, JSM-IT300) was used to observe structural morphology within the asparagus after vacuum frying. The voltage was 10 kV, and a magnification of 30× was applied in this experiment. Vacuum-fried asparagus was prepared by defatting with petroleum ether for 12 h before monitoring via SEM to obtain clear images.

Sensory evaluation of the vacuum-fried asparagus with various flavours was conducted. Five seasoning powders at 6% (w/w) were mixed with the fried asparagus, with the suitability of each flavour determined using a sensory assessment. A kilogram of fried asparagus was mixed with 60 g of seasoning in a 5 L plastic box per batch, shaking for 5 min until fried asparagus was coated with the powder. Samples were served to 30 untrained panelists with a separate booth design in a sensory laboratory in the Food Science and Technology programme at Phranakhon Rajabhat University. 6–8 pieces of each seasoned vacuum-fried asparagus were served in a white plastic cup for them to sample, a total of 30–40 pieces were used. The panelists were required to rate their preference for five different seasonings of vacuum-fried asparagus samples. There were then informed about scaling each attribute, including appearance, colour, taste, flavour, crispiness, and overall liking. They used a seven-point hedonic scale ranging from 1, indicating they immensely disliked it, to 7, indicating they extremely liked it.

The nutritional content and microorganisms of the chosen vacuum-fried asparagus were finally determined by Central Laboratory (Thailand) Co., Ltd.

Three replicates of the asparagus were performed for each sample. The results were expressed through calculated mean and standard deviation. Statistical analysis was performed using SPSS software (SPSS Statistics 19.0). Data were analysed by analysis of variance with a one-way ANOVA, and Duncan’s test was used to determine the mean difference between treatments at the 5% significance level.

The proximate analysis results of the asparagus samples indicated that the moisture, protein, fat, crude fibre, ash, and carbohydrate content were 91.98, 2.33, 1.03, 1.06, 1.00, and 2.60%, respectively. However, it is an excellent source of essential nutrients such as protein, amino acids, and dietary fibre. The edible portion of asparagus contains up to 92% moisture content, and the primary nutrients in asparagus, such as fibre and protein, are not more than 3–4%. The asparagus parts used in this study were the tips and middle segments, while the bottom parts were discarded, thus, the yield of fresh asparagus was 45–47%. The colour of fresh asparagus was dark green according to the low L* value of 40.77, while the a* and b* values were also low at −1.87 and 20.98, respectively.

The asparagus were fried at the temperature of 80°C, vacuum pressure of 986 mbar and frying times of 38, 40, and 42 min. Each fried asparagus was passed to the centrifuge tank to remove excess oil for 5 min while the oil was economically reused in a vacuum fry process for 10–12 batches. The physicochemical properties of vacuum-fried asparagus were measured.

The moisture content, the water of activity (a_w), and the yield of the vacuum-fried asparagus are shown in Table 1. The moisture content, a_w, and yield ranged from 5.36–8.81%, 0.31–0.52, and 13.30–16.42%, respectively. It was found that the moisture content, a_w, and yield of vacuum-fried asparagus at 80°C under different frying times of 38, 40, and 42 min significantly decreased (p ≤ 0.05) with an increasing frying time. A similar decrease in moisture content was also observed in the vacuum frying of mango chips (Ayustaningwarno et al., 2020a). According to Ayustaningwarno et al. (2020b), the change in moisture content during vacuum frying can be described by an exponential model with a low correlation coefficient for its parameters. By applying thermodynamic theories, it is possible to calculate the temperature of the products during frying based on the boiling point of the moisture present in the sample. This boiling point increases as the water content of the sample decreases.

Moreover, the moisture content, a_w, and yield of the tip segment were slightly higher than the yield of the middle segment. This phenomenon is because heat reduces the moisture content of the sample, in this study from 97% to 5.36–8.81%, and a long frying process results in the evaporation of much water, which resulted in asparagus that was lightweight and with reduced yields. Generally, the a_w value for the dried product is below 0.6, resulting in the fried product can be stored over a prolonged period. The dried products are kept long because a_w correlates with microbial growth rates and many degradative reactions, indicating product stability and microbial safety.

The colour measurement L*, a*, and b* values of vacuum-fried asparagus are shown in Table 2. The L* value of vacuum-fried asparagus at 80°C under different frying times for 38, 40, and 42 min significantly increased (p ≤ 0.05). In contrast, the b* value decreased with increased frying times, and the negative a* values indicate a green undertone. Thus, the product colour was green-brown with brighter middle parts (Figure 3). The vacuum-treated samples became darker, less green, and more yellow compared to the fresh asparagus sample which was dark green according to the low L* value of 40.77, while the a* and b* values were also low at −1.87 and 20.98, respectively. Moreover, the brown colour of the vacuum-fried product increased with a longer vacuum frying time. A non-enzymatic browning reaction during heating could be the cause. The high temperature used in processed foods leads to non-enzymatic browning reactions, which may occur between reducing sugars and amino acids and between ascorbic acid and other oxidized ascorbic acids, which step into Millard-type browning reactions (Da Silva and Moreira, 2008; Dueik and Bouchon, 2011; Diamante et al., 2012). Diamante et al. (2011, 2012) reported that vacuum frying apricot and gold kiwi slices with different processes in terms of temperature and time resulted in products with higher browning indices. However, these reactions may be impeded under the vacuum frying process due to low temperatures and the absence of oxygen (Da Silva and Moreira, 2008; Dueik and Bouchon, 2011). In addition, the samples underwent milling and were then analyzed for colour using the Hunter Lab colourimeter. It is important to note that the colour of the milled material may differ from that of the surface due to the exposure of the inner material. Ayustaningwarno et al. (2021) reported that the homogenization process involves mixing the light and dark colour areas, as well as blending the surface material with the internal tissue and oil in vacuum-fried fruits.

The textural properties in vacuum-fried products result from chemical changes in the frying process. An indicator of crispiness was considered by the force required to break the fried asparagus. This value means that a lower breaking force indicates greater crispiness. As can be observed from Table 2, the crispiness value of vacuum-fried asparagus showed that it tended to decrease with longer frying times of 40 and 42 min for both the tip and middle part asparagus segments. This result could be due to the lower moisture content of the sample when the frying time was increased (Table 2), which consequently caused a crisper texture of the vacuum-fried asparagus. The moisture content in the product dramatically decreased, when water is removed from within the product toward the product surface, and the gas vapour expands inside the product causing a pressure build-up and the product puffing (Yamsaengsung et al., 2017), resulting in a crunchy texture. The texture of the vacuum-fried asparagus was affected by the chemical component differences among the asparagus part as well. The matrix of food refers to the chemical components of food and their molecular relationships, as well as the way these components are structurally organized, such as the role of pectin. Pectin is crucial in glueing adjacent cells, resulting in tissue rigidity and firmness (Huyskens-Keil et al., 2005). It is also essential in maintaining matrix cohesiveness during frying (Ayustaningwarno et al., 2018). In the case of asparagus, the pectin content of its shoot tissues, which are involved in growth and development, is lower compared to mature permanent tissues in the middle part of the plant (Huyskens-Keil et al., 2005). This can explain why the middle part of asparagus has a higher breaking value, as it contains more pectin, which helps form a crust. Yamsaengsung et al. (2017) found that the high starch content in vacuum-fried banana chips also helps form a crust. However, to obtain accurate information about the effects of pectin content in asparagus shoots compared to permanent tissues in the middle part of asparagus on quality attributes of fried asparagus should be investigated further.

In Figure 4, SEM images capture the inner structure of the vacuum-fried asparagus at 80°C and different frying times of 38, 40, and 42 min for each spear part. It was found that when the frying time was increased to 40 min, there was a gradual increase in the size of pores within the product, while they broke when the frying time increased to 42 min. The effects of vacuum frying on the structure of the vacuum-fried product using different parts of the asparagus were observed. The results indicate slight differences in the number and size of pores with different parts. Compared to the middle part, more extensive and fewer pores were observed in the tip of vacuum-fried asparagus.

Different characteristics of plant matrix could impact the quality of vacuum-fried products as mentioned. These factors include cell size, cell wall, flesh thickness, firmness, intracellular spaces, sugar and fibre content, and fibre type. It is possible that the pectin content in the middle part of the asparagus, which consists of permanent tissues, is higher compared to the actively dividing cells found in the shoot tissues at the tip of the asparagus (Huyskens-Keil et al., 2005). The crispiness value of the middle part of the asparagus is also higher than the tip (Table 2), indicating a crispy and porous matrix.

The analysis of the moisture content, a_w, yield, colour and especially textural properties also are essential characteristics. The main quality characteristic of snack products is their crispiness. The criteria for selecting fried products in this study focus on suitable product crunchy. The sample that was fried for 42 min had lower crispiness values compared to the one fried for 40 min, resulting in a crisper texture that made it more prone to collapsing. Therefore, the asparagus vacuum fried at 80°C for 40 min was chosen for further study, including the tip and middle parts. A further experiment was conducted to determine a suitable seasoning powder for the fried asparagus product, which is elaborated on in the next section.

This section presents the results of studying various seasonings on the vacuum-fried asparagus cooked at 80°C for 40 min. The moisture content and a_w ranged from 5.63–6.06% to 0.40–0.46, respectively. The moisture content and a_w of each flavour were not statistically different (p > 0.05).

The sensory attributes in terms of appearance, colour, flavour, taste, crispiness, and overall acceptability for vacuum-fried asparagus cooked at 80°C with a frying time of 40 min are presented in Table 3. Crispiness is the key quality characteristic of snack products for consumer sensory to determine the acceptance. Using various seasoning powders can affect the crispiness of the product in different ways. The results showed that there were no significant differences (p ≤ 0.05) in the appearance and crispiness of the seasoned vacuum-fried asparagus samples. In contrast, the liking scores of the sample ranged between 5.6 (like) – 6.05 (like very much) for appearance and 5.7 (like) – 6.45 (like very much) for crispiness, respectively. However, there were significant (p ≤ 0.05) differences in colour, flavour, taste, and overall acceptability of the vacuum-fried asparagus seasoned with Tom-Yum, Nori seaweed, truffle, salted egg, and cheese flavours. Moreover, the highest colour, flavour, taste, and overall acceptability score was for vacuum-fried asparagus seasoned with truffle. Thus, the nutrition values and microorganisms of the chosen vacuum-fried asparagus were determined. The sensory attributes in terms of appearance, colour, flavour, taste, crispiness, and overall acceptability for vacuum-fried asparagus cooked at 80°C with a frying time of 40 min are presented in Table 3. There were no significant differences (p ≤ 0.05) in the appearance and crispiness of the seasoned vacuum-fried asparagus samples. In contrast, the liking scores of the sample ranged between 5.6 (like) – 6.05 (like very much) for appearance and 5.7 (like) – 6.45 (like very much) for crispiness, respectively. However, there were significant (p ≤ 0.05) differences in colour, flavour, taste, and overall acceptability of the vacuum-fried asparagus seasoned with Tom-Yum, Nori seaweed, truffle, salted egg, and cheese flavours. Moreover, the highest colour, flavour, taste, and overall acceptability score was for vacuum-fried asparagus seasoned with truffle. Thus, the nutrition values and microorganisms of the chosen vacuum-fried asparagus were determined.

The nutritional value of the final product vacuum-fried asparagus seasoned with truffle is shown in Table 4. It was found that the truffle seasoned fried asparagus contained protein and fibre content of 14.21 and 15.55%, respectively, vitamin A and beta-carotene of 2.717 μg and 16.30 μg, respectively, and calcium and iron of 182.68 mg and 2.88 mg, respectively. The finding showed that vacuum-fried asparagus truffle flavour provides high nutrients, making them an excellent choice for consumers interested in eating healthy among plenty of crispy starchy snacks.

Crisp snacks are known for their low moisture content. However, even with low moisture, microorganisms can still affect the quality and safety of the product over time. Regulatory agencies often have specific microbiological standards and guidelines for food products, including crisp snacks. Meeting these standards is essential for complying with food safety regulations and ensuring consumer safety. Microorganisms in the vacuum-fried asparagus seasoned with truffle were analysed by Central Laboratory (Thailand) Co., Ltd., (2023). The study found that Bacillus cereus, Clostridium perfringens, Salmonella spp., and Staphylococcus aureus were 4.5 × 10 cfu/g, less than 10 cfu/g, non-detected, and less than 10 cfu/g, respectively, which is under the regulated amount according to the Notification of the Ministry of Public Health (No.416): Prescribing the quality or standard, principles, conditions, and methods of analysis for pathogenic microorganisms in foods.