Hallo Dear

Please find the following link about article in title

Shelf life and sensory attributes of a fruit smoothie-type beverage processed

with moderate heat and pulsed electric fields

http://ucanr.edu/datastoreFiles/608-718.pdf

You can use more metrials  with UHT and HPP e.g. Nisin , pediocin and sodiumbinzoate,  

ًWell. To increase the shelf life of such beverage with good sensory, You can add nisin ( 500 IU/mL   ) which is equivalent to 1mg /mL or ln case of low temp. add 0.5mg/mL. 

Also MAP could be possible but try the first

Flash pasteurisation is an option that's commonly used. OR add lemon juice to aid in adjusting the pH (depending on the pH of the final product already)

Continuous flow microwave sterilization followed by rapid cooling and aseptic filling can produce superior ambient temperature shelf stable smoothie products.

These smoothies packaged in Combibloc cartons (bottom of the linked page) have been produced using AseptiWave continuous flow microwave technology and have been tested by an independent trained taste panel as not significantly different from comparable market-leading refrigerated smoothies. Shelf life at ambient temperature 12 months. 

https://www.heb.com/static-page/article-template/H-E-B-Juice-Stand-Drinks-and-Recipes

the problem there is that we are just talking about shelf life, I think in terms of sensory characteristics after 4 months in ambient temperature no matter what processing you do the product will not be a smoothie anymore (in terms of characteristics evaluation) 

Laser sterilization can be used for a few doses for several seconds

Electric sterilization can be use without risk ingredient in smoothies