Vacuum freeze-drying of biological materials is one of the best methods of water removal, with final products of highest quality. The solid state of water during freeze-drying protects the primary structure and the shape of the products with minimal volume reduction. In addition, the lower temperatures in the process allow maximal nutrient and bioactive compound retention. This technique has been successfully applied to diverse biological materials, such as meats, coffee, juices, dairy products, cells, and bacteria, and is standard practice for penicillin, hormones, blood plasma, vitamin preparations, etc. Despite its many advantages, having four to ten times more energy requirements than regular hot air drying, freeze-drying has always been recognized as the most expensive process for manufacturing a dehydrated product. The application of the freeze-drying process to plant-based foods has been traditionally dedicated to the production of space shuttle goods, military or extreme-sport foodstuffs, and specialty foods such as coffee or spices. Recently, the market for ‘natural’ and ‘organic’ products is, however, strongly growing as well as the consumer’s demand for foods with minimal processing and high quality. From this perspective, the market for freeze-dried plant-based foods is not only increasing but also diversifying. Freeze-dried fruits and vegetables chunks, pieces, or slices are nowadays majorly used in a wide range of food products such as confectionaries, morning cereals, soups, bakeries, meal boxes, etc. Instant drinks are prepared out of freeze-dried tea, coffee, or even from maple syrup enriched with polyphenol concentrated extracts from trees. The possibilities are endless. In this review, the application of freeze-drying to transform plant-based foods was analyzed, based on the recent research publications on the subject and personal unpublished data. The review is structured around the following related topics: latest applications of freeze-drying to plant-based foods, specific technological problems that could be found when freeze-drying such products (i.e., presence of cuticle; high sugar or lipid concentration), pretreatments and intensification technologies employed in freeze-drying of plant-based foods, and quality issues of these freeze-dried products.

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Sugar maple (Acer saccharum M.) and red maple (Acer rubrum L.) barks were treated with hot water to extract nutrients in order to explore, for the first time, its potential as safe dietary antioxidants. The organic and inorganic nutrients of these extracts, as well as their safety on human PLB-985 cells differentiated into neutrophils-like cells, were determined. Proximate analysis showed that both bark extracts were low in moisture and fat. Sugar maple bark extract (SM-BX) showed crude protein and ash content higher than those found in red maple bark extract (RM-BX). In addition, SM-BX had total sugars higher than those evaluated in RM-BX, while complex sugars (oligo- and/or poly-saccharides) were similarly abundant in both bark extracts. Furthermore, SM-BX demonstrated a wide array of vital minerals (K, Ca, Mg, P, Na, Fe and Cu) in quantity larger than that evaluated in RM-BX, whereas RM-BX have Zn and Mn levels higher than those found in SM-BX. Phytochemical analyses showed that RM-BX exhibited total phenolic and flavonoid contents higher than those measured in SM-BX. Consequently, RM-BX presented an antioxidant activity higher than that of SM-BX: 2.85-fold ABTS radical cation scavenging capacity and 1.9-fold oxygen radical absorbance capacity. Finally, RM-BX and SM-BX were greatly safe since, at concentration up to 100 μg/ml, they did not modify the viability of neutrophils as determined by flow-cytometry assay using Annexin V-FITC/Propidum Iodide as markers. In conclusion, our in vitro studies indicate that both red and sugar maple bark extracts have a real potential as food additives.

Background: We have determined previously that the water extract of sugar maple bark contained an important quantity of a complex sugar. In this study, we investigated the organosolv pulping of pre-extracted bark to follow the acid conversion of sugars into major products, furfural and 5-hydroxymethyl-2-furfural (HMF), while comparing the structures of organosolv lignins. Methods: The bark particles were pre-extracted with an ethanol–water mixture or water only. The extractives-free barks were then converted into cellulosic pulp and lignin by a patented organosolv process. The composition of residual liquor was determined by using HPLC-UV. Results: The pre-extraction with water was more efficient for complex sugars recovery than with the ethanol–water system. HMF was determined to be more abundant in residual liquor than furfural after ethanol–water pre-extraction while their quantities were comparable in the residual liquor after water pre-extraction. The higher yield of HMF from ethanol–water pre-extracted bark (1.18%) than from water pre-extracted (0.69%) could be related to the efficiency of complex sugar removal during the pre-extraction step. Conclusions: The pre-extraction before pulping affected, at least in part, the composition of residual liquor in terms of HMF production. These results demonstrate how the bark can be converted into valuable products and intermediates for organic synthesis.

The aim of this research was to develop a natural food ingredient based on maple sugar powders (MSP) enriched in polyphenols. By incorporating 0.01% (wt/vol) of hot water extract (hot water extraction: 90°C and 1 hr; bark/water, 1/10 wt/vol) from sugar and red maple bark into substandard quality maple syrup, the latter was enriched by 13–20% in total phenolic content (TPC). The mixtures (syrup and extracts) were dehydrated using freeze‐drying (FD: at −36°C for 15 hr and subsequently at 30°C for 10 hr) or vacuum double‐drum drying (VDD: at 80°C and 660 Torr) to obtain the powders. Influence of drying processes on TPC, antioxidant capacity, and physical properties of powders was studied. Both drying processes caused a decrease in TPC and antioxidant capacity of MSP. Nevertheless, bark extracts contributed to higher TPC (8–10%) and antioxidant capacity (>40%) compared to control MSP. The moisture content of VDD powders (0.63–0.71%, dry basis) was significantly lower than that of FD powders (4.10–4.40%, dry basis). MSP produced by FD were amorphous and those produced by VDD crystalline. FD powders had instant‐like properties (dissolution time of 12–13 s), whereas those produced by VDD were less cohesive (Hausner ratio, 1.13–1.21), with excellent flowability. Practical Applications Consumers are increasingly attracted by natural food products. Canada is the world major producer of maple syrup, a nutritious natural sweetener exclusively obtained from maple trees sap. Unfortunately, a “very dark” color syrup is accumulated as surplus in large quantity in Canada as it is considered of substandard quality. In this research, freeze‐drying (FD) and vacuum double‐drum drying (VDD) techniques were studied to produce maple sugar powders (MSP) from this substandard surplus syrup. This syrup was additionally enriched in polyphenols by adding hot water extracts from maple barks. The obtained polyphenol‐enriched MSP have shown interesting qualities for application as natural sweeteners, such as free flowing or instant‐like powder. Our results indicate that FD and VDD are suitable techniques for substandard syrup conversion into value‐added maple product. MSPs have a huge potential of application as natural food ingredients of instant drinks, cereal mix, cookies, and energy bars.